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WITH THE AUTHORS COMPLIMENTS.
Education Department Bulletin
Published fortnightly by the University of the State of New York
Entered as second-class matter June 24, 1908, at the Post Office at Albany, N. Y., under
the act of July 16, 1894
No. 489 ALBANY, MEY: FEBRUARY 15, IQII
New York State MUSCU Dh
nn \{ Nn
Joun M. CrarKe, Director Z NS gonlon NStit,,
Museum Bulletin 146 fc. P
a
é
‘
GEOLOGY OF THE NEW YORK, CTX
(CATSKILL) AQUEDUCT ===
STUDIES IN APPLIED GEOLOGY COVERING PROBLEMS ENCOUNTERED
IN EXPLORATIONS ALONG THE LINE OF THE AQUEDUCT FROM
THE CATSKILL MOUNTAINS TO NEW YORK CITY
BY
CHARLES P. BERKEY , | *
PAGE | PAGE
Introduction and acknowledg- Ciecorr he Rondout valley sec-
PARTI 8 2s, AEN Braet 5 {EVO} CS ae ae) ee ae 125
Th allkill Valley sec-
PAtceneral features... os. 065.00. 9 7 on Rea hate i: ie 2 wey 149
Ch. 1 Catskill water supply 8 Ancient Moodna Valley.. 153
PEOIEGi pee po ieee 9 9 Rock condition of Foundry
2 Problems encountered in TOM kee wat ias (he Moana 163
the rOgectseng yy Rouen 17 10 Geology of Sprout brook. 171
3 Relative values of differ- | II Structure of Peekskill
ent sources of informa- Checkley yc. ae me 175
tion and stages of devel- 12 Croton lake crossing..... 183
GPM Ave katie ae oe 25 13 Geology of the Kensico
4 General geology of the Com See oh ge one IQI
TOOUGI gio Py eho ks 29 14 Stone of the Kensico
: GUIEERITCS RSNA a a Nae 195
II Geologic problems of the 15 The Bryn Mawr siphon.. 201
PREC MEE As aa ty hia 75 16 A study of shaft 13 and
SUE O8 (S610 A pea On as A 75 vicinity on the New Cro-
Ch. 1 General position of aque- ton aqueduct....... BAe oy
CIC Soe oe Sip 17 Geological conditions
2 Hudson river canyon.... 81 affecting the location of
3 Geological conditions delivery conduits in New
affecting the Hudson York city SS cbol ap areaecy ce coe PCLC 215
PIVET (CTOSSHIGs <4 Sere ee 97 18 Areal and _ structural
4 Geological features in- geology south of 59th
volved in selection of site street. ........ 8 Hae nisl etpic 231
for the Ashokan dam.... 109 19 Special exploration zones. 237
5 Character and quality of 20 The general question of
the bluestone for struc- postglacial faulting...... 271
Hird purposes....0. esas BUG Me Mere erate Lote ie Wak waco epdim: Ccs OF i |
“a ALBAN Y¥.
UNIVERSITY OF THE STATE OF NEW YORK
IQiIl
M232r-Apt1o-2500
} f %
; i
tS p 4
Be :
1913
IQI7
1919
IQI4
Igi2
1918
1922
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1920
1916
IQ21
STATE OF NEW YORK
EDUCATION DEPARTMENT
Regents of the University
With years when terms expire
WHITELAW Rerp M.A. LL.D. D.C.L. Chancellor New York
St CLrarrn McKetway M.A. LL.D.Vice Chancellor Brooklyn
DanieL Beacu Ph.D. LL.D. - =— — — = Watkins
Puiny T. Sexton LL.B. LL.D. — \-—- — — = Palmyrm
T. Guitrorp Smith M.A. C.E.LL.D. - - - Buffalo
WiiiiaM NottincHaM M.A. Ph.D. LL.D. - — Syracuse
CHESTER S. Lorp M.A. LL.D. - - - — -— New York ©
ALBERT VANDER VEER M.D. M.A. Ph.D. LL.D. Albany
EpwWARD LAUTERBACH M.A. LL.D. — -— — — New York
EuceENE A. Puitpin LL.B. LL.D. -—- - - -— New York
Lucian L. SHEDDEN LL.B. LL.D. - - - -— Plattsburg-
Francis M. CARPENTER - —- -—- -— — -— -— Mount Kisco
Commissioner of Education
ANDREW S. DrapeErR LL.B. LL.D.
Assistant Commissioners
Avucustus S. Downinec M.A. Pd.D. LL.D. First Assistant
CHARLES F. WHEELOCK B.S. LL.D. Second Assistant
Tuomas E. Finecan M.A. Pd.D. Third Assistant
Director of State Library
James I. Wyer, Jr, M.L.S.
Director of Science and State Museum
Joun M. Crarxke Ph.D. D.Sc. LL.D.
Chiefs of Divisions
Administration, GEorGE M. Witrey M.A.
Attendance, JAMES D. SULLIVAN
Educational Extension, WILLIAM R. Eastman M.A M.LSS.
Examinations, HARLAN H. HorRNER B.A.
Inspections, FRANK H. Woop M.A.
Law, FRANK B. GILBERT B.A.
School Libraries, CHARLES E. Fitcu L.H.D.
Statistics, Hiram C. CasE
Trades Schools, ARTHUR D. Dean B.S.
Visual Instruction, ALFRED W. ABRAMS Ph.B.
S
P
New York State Education Department
Science Division, April 6, 1910
Hon. Andrew S. Draper LL.D.
Commissioner of Education
str: The extraordinary engineering operations which have been
undertaken in the effort to provide the city of New York with an
adequate wat:r supply have illuminated in most unexpected manner
the geological structure and history of the region of the Hudson
valley south of the Catskill mountains. So broad has been the
scientific scope of this engineering problem and so direct its de-
pendence on geological structure that the Commissioners of the
New York City Board of Water Supply early found it of essential
moment to enlist in their service a corps of trained geologists.
In 1909 an agreement was effected between the Board of Water
Supply and the State Geologist, in pursuance of which the geolog-
ical data acquired in the preliminary and final surveys for the aque-_
duct were intrusted to Dr Charles P. Berkey, a member of the staff
of the board as well as of the geological survey, for summation and
presentation of their broader and more important bearings.
I transmit to you herewith Dr Berkey’s report thereupon, entitled
Geology of the New York City (Catskill) Aqueduct. It is a
document of high value not only in enlarging and perfecting our
knowledge of the geological structure of the commercial center of
the United States, but its data and conclusions must prove of pro-
found importance to all large engineering and architectural propo-
sitions concerned with the region of the lower Hudson valley.
[3]
4 NEW YORK STATE MUSEUM
I therefore submit this, subject to your approval, for immediate
publication as a bulletin of the State Museum.
Very respectfully
JoHN M. CLARKE
Director
State of New York
Education Department
COMMISSIONER'S ROOM
Approved for publication this 7th day of April 1910
BIO
Commissioner of Education
— a <2
SEATENST:
ATLANTIC . OCEAN
copyrighted ISN? by Brvedwey Mezstine
The Catskill and Croton water supply systems of New York city
Education Department Bulletin
Published fortnightly by the University of the State of New York
Entered as second-class matter June 24, 1908, at the Post Office at Albany, N. Y.,
under the act of July 16, 1894
No. 489 ALBANY, N.Y... FEBRUARY I5, IQII
New York State Museum
Joun M. Crarke, Director
Museum Bulletin 146
GEOLOGY OF THE NEW YORK CITY (CATSKILL)
AQUEDUCT
STUDIES IN APPLIED GEOLOGY COVERING PROBLEMS ENCOUNTERED IN
EXPLORATIONS ALONG THE LINE OF THE AQUEDUCT FROM THE
CATSKILL MOUNTAINS TO NEW YORK CITY :
BY
CE AKUES P BERKEY
EV EFRODUC TION AND ACKNOWLEDGMENT
It is the writer’s hope that the series of studies brought together
in this bulletin may help to effect a wider appreciation of the prac-
tical usefulness of geology. The volume contains a summary of
the local geologic facts and the general principles found helpful in
solving some of the problems encountered in a single great engineer-
ing enterprise. The summary is accompanied by brief discussions
of the methods employed and of the final results or conclusions
reached. It is therefore essentially a study in applied geology.
Seldom has so favorable an opportunity been afforded to follow
extensive exploratory work and check geologic hypothesis or theory
by subsequent proof. And still more seldom have engineers in
charge of similar works so fully appreciated the value of geologic
investigations and the extent to which they can be utilized as a
guide.
More credit is due to Mr J. Waldo Smith, chief engineer of the
Board of Water Supply of the City of New York, than to any one
else for appreciating the importance of the geologic complexity of
[5]
6 NEW YORK STATE MUSEUM
the Catskill Aqueduct problem. His exceptional insight into- its
nature led to the adoption of measures in this direction that are
now proved to have been fully justified. A staff of geologists has
been maintained. From time to time engineers of the regular staff
who have shown unusual aptitude in such investigations have been
assigned to special duty on geologic exploratory work. In the pre-
liminary investigations of the Northern Aqueduct, Division Engineer
James F. Sanborn was very intimately connected with the geologic
work. With him the writer worked out many field studies that
later formed the basis of advisory reports, covering locations, kinds
of explorations to be made, and interpretations of data. No one
has had a better grasp of both the geologic and the engineering
aspects than Mr Sanborn. It is with great pleasure that the writer
acknowledges many valuable suggestions and much help through
association with him. In the later exploratory work within the city
similar service has been rendered by Mr John R. Healey, who has
much to do with the geologic detail-of the delivery conduit data.
The consulting geologists employed by the board were Professors
James F. Kemp, W. O. Crosby and the writer.
A special debt is acknowledged to Prof. James F. Kemp, consult-
ing geologist of the board, whose confidence in the writer’s work
originally brought him into touch with these investigations as an
assistant, and with whom since that time many joint reports to the
board have been written.
Valuable advice and assistance in arranging for the issue of this
report has been given by Department Engineer Alfred D. Flinn of
Headquarters Department. For some of the corrections and sug-
gestions special ackiowledgment is made to Department Engineer
Thaddeus Merrimar ,
The department engineers, Robert Ridgway of the Northern
Aqueduct, Cariton E. Davis of the Reservoir, Merritt H. Smith, for-
merly of the Southern Aqueduct, Frank E. Winsor of the Southern
Aqueduct, William W. Brush and Walter E. Spear of the City De-
livery have given every facility for gathering geologic data within
their territory and have contributed largely to the better understand-
ing of their special fields.
The geologic matter relating to special problems has been. worked
out with the aid of the division engineers in direct charge in the
field. Among these must be mentioned L. White of the Esopus
division, William E. Swift of the Hudson river division, A. A.
Sproul of the Peekskill division, Lawrence C. Brink of the Wall-
GEOLOGY OF THE NEW YORK CITY AQUEDUCT i
kill division, J. S. Langthorn of the Ashokan reservoir, Wilson
Fitch Smith of the Kensico division, T. C. Atwood of the New
York city delivery division.
The data included in the tabulation of this bulletin have been
gathered largely by others. Many of the explanations and conclu-
sions are the outgrowth of the work of engineer and geologist,
together. A large number of associates are engaged on this public
work in such relations to one another that the individuality of each
is obscured in the common effort to reach an enviable efficiency and
success for the whole enterprise.
The combined efforts of many, unselfishly given, have thus
brought together a total far in excess of what any one individual
could accomplish. Acknowledgments should therefore be made to
those members of the staff of the Board of Water Supply who can
not in the nature of the case be mentioned by name. Were it not
for their cooperation the great mass of data here summarized could
not have been compiled.
CHARLES P. BERKEY
Special Geologist, New York State Geological Survey;
Consulting Geologist New York City Board of Water Supply
Columbia University, New York City November 1, 1910
I
GENERAL FEATURES
CHAPTER f
CATSKILL WATER SUPPLY PROJECT
New York city obtains its chief water supply from the Croton
river watershed. Other sources! now drawn upon are less important
although some of them, such as the Long Island underground
supply, are capable of considerable additional development. The
average daily consumption of Croton water was approximately
324,000,000" gallons for 1907. At the present rate of increase of
population the consequent daily increase in consumption of water
is 15,000,000 gallons in each succeeding year.
The entire daily flow of water in the Croton river for the 18
years from 1879 to 1897 averaged only 348,000,000 gallons. About
10,000,000 gallons per day is lost by evaporation and seepage
from existing reservoirs. The records for 40 years, from 1868 to
1907 make a somewhat better showing. Making no allowance for
evaporation the average flow amounts to 402,000,000 gallons. With
due allowance for evaporation,? however, this only increases the
daily supply as now planned by about 47,000,000 gallons. That is,
the possible total additional water within the Croton watershed
would suffice for only three years’ growth of the city. Much of
this additional water belongs to periods of excessive precipitation.
To save it would require additional storage facilities for 305,000,-
000,000 gallons, and, it is estimated, would probably cost $150,-
000,000.
1 Brooklyn is in part supplied by these additional sources which furnished
145,000,000 gallons daily in 1907.
2The figures used here as to consumption and capacity and available
supply are taken from the printed statements of the commissioners of the
New York City Board of Water Supply in a circular dated April 16, 1908,
and are based upon the investigation and reports of the corps of engineers
headed by J. Waldo Smith, chief engineer, John R. Freeman and William
H. Burr, consulting engineers. The reports of this commission and
various others that have had the responsibility of investigating the future
supplies for New York city have been drawn upon freely for such data.
3 The average rainfall for the past 40 years is about 49 inches per year.
Only about 48 per cent of this runs into the streams. The rest evap-
orates or is absorbed by the vegetation or joins underground supplies
that do not again appear at the surface in the district.
[9]
IO NEW YORK STATE MUSEUM
Taking into account the small relief possible in this direction and
the certainty that in less than five years the demands of the city
will be greater than the total capacity of the Croton watershed, it
is clear that some other source of large and permanent supply is an
absolute necessity. |
In the search for such additional sources, there has been much
careful work done by able commissioners.t. In the meantime, resi-
dents of certain districts where there are possible supplies have
taken steps by legislative action to effectually? prevent New York
city encroaching upon their territory. Criticisms? of all kinds
largely by those only partially informed as to the magnitude and
complexity of the problem and partly by those ignorant of the
simplest factors in its solution, have been kept perpetually before
the public. One needs only a slight acquaintance with such public
works to realize that it is much easier and more common to criticize
and raise the cry of corruption or incompetence than it is to give
really valuable advice or solve a real problem or carry an enterprise
of the most vital public importance to a successful issue.
It is sufficient here to observe that exhaustive studies of the whole
question of water supply by competent men have resulted in a
practically unanimous conclusion that the streams of the Catskill
mountains are the most satisfactory, economical, reliable, abundant
and available future source of water.
1 The Report of John R. Freeman C. E., 1899-1900; Report of the Burr-
Herring-Freeman Commission, 1902-4; the Studies of the Department of
Water Supply, Gas and Electricity, 1902-4; Investigations of the Board of
Water Supply, 1905 to the present time.
2 Acts of the Legislature of 1903-4.
3 The commonest suggestions neglect the question of permanence or
constancy of supply. The following sources are often mentioned, (a) Lake
George, forgetting that this beautiful lake has an abnormally small water-
shed and could never figure as a large permanent supply; (Db) artesian
wells, ignoring the fact that with the exception of certain portions of Long
Island there is almost no artesian capacity, and on Manhattan and the
mainland the crystalline rocks make such development useless; (c) Lake
Ontario, apparently overlooking the great distance (400 miles) and the
many other complications that this international water body involves;
(d) the Housatonic river, neglecting the difficulties of interstate origin;
(e) Dutchess county, where the city is prohibited by legislative enactment;
(f) the Hudson river, ignoring the fact that the Hudson is an estuary
of the sea with brackish water of a very impure quality and wholly unfit
for domestic uses. It is, however, worth while to note that Hudson river
water is sure to be used more and more extensively for fire protection and
similar purposes in the more densely populated portions of the city by
means of an entirely different system of conduits. This is one of the
most promising directions of relief looking to the more distant future.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT ‘ee XTE
———
The Catskill supply will furnish over 500,000,000 gallons of
water daily and was estimated to cost $161,857,000. That is, the
additional supplies from the Catskills as planned will, when com-
pleted, be sufficient for the increasing demands_of the growing city,
for the next 35 years. And some of it may be badly needed long
before it can possibly be delivered.
Parts of the Catskill system?
The chief sources within the Catskills now included in the plans
- of the board are:
I Esopus creek, to be taken at a point near Olive Bridge.
2 Rondout creek, to be taken at a point near Napanoch.
3 Three small streams tributary to the Rondout.
4 Schoharie creek, to be taken at a point near Prattsville.
5 Catskill creek, to be taken at a point about 1 mile northeast of
Durham.
6 Six small streams tributary to the aqueduct between Catskill
creek and Ashokan reservoir.
The comparative areas of watershed and their daily capacity are
estimated? by the corps of engineers as follows: _
| AREA |
| ages STORAGE IN DAILY SUPPLY
MILES GALLONS IN GALLONS
came | |
fe SOEs WeaketShed 0. 56/0'.:...:. ue 70 000 000 0008! 250 000 000
2 Rondout watershed....-...| 131 20 000 000 000 | 98 000 ooo
3 Three small tributaries...... | Paap pase Sota Ws eee Eee he | 27 000 000
4 Schoharie watershed........ | 228 45 ©00 000 000 | 136 000 ooo
5 Catskill watershed......... 163 30 000 000 000 | I00 000 000
Goat, stanlbetreams. 020%...) Oe ey eae ee Ge a 49 000 000
C170 [£2 sh ea Been a irae 904 165 000 000 000 | 660 oa0 000
1 The subdivisions and proposed locations given here are taken chiefly
from the Report of the Board of Water Supply of the City of New York
to the Board of Estimate and Apportionment, October 9, 1905.
2 Estimates are much more complete for the Esopus, which it is planned
to develop first, than for any other streams; and it must be understood
that the figures are subject to revision dependent upon modifications of
original plans to meet the conditions that develop upon more elaborate
investigation.
3 Preparations are to be made for storage of 120,000,000,000 gallons of
water on the Esopus, but a part of this capacity is intended to accommodate
supplies drawn from other sources than Esopus creek itself.
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT I3
The evident certainty that present supplies from the Croton and
Long Island will be very inadequate long before the Catskill system
can be completed has influenced the adoption of plans contemplating
the construction of certain parts in advance of the rest. To begin
with, only the Esopus watershed is to be developed by the con-
struction of the great Ashokan dam at Olive Bridge making the
reservoir of full capacity. At the same time that portion of the
aqueduct between the Ashokan dam and the present Croton reser-
voir is to be completed in advance of other parts so as to make it
possible to turn additional supplies into the Croton system, the
capacity of the present Croton aqueducts being somewhat in excess
of the Croton storage in dry years. It is furthermore desirable that
increased storage capacity should be secured much nearer to New
York city, and with that end in view Kensico reservoir is to be
greatly enlarged. It is estimated that this may be made to hold 50
days’ supply of 500,000,000 gallons daily.
The development of the Catskill system is being carried on by the
Board of Water Supply, which was appointed by Mayor McClellan,
as provided in chapter 724, of the laws of 1905. The present board
consists of John A. Bensel, president, Charles N. Chadwick and
Charles A. Shaw. The Engineering Bureau of the Board is in
charge of J. Waldo Smith, as chief engineer, Merritt H. Smith, as
deputy chief engineer and Thaddeus Merriman, assistant to chief
engineer.
Influenced doubtless in large part by the unity of certain portions
of the project, either because their essential engineering features
are distinct, or because their construction is more urgent, or in order
to facilitate the work of supervision of so great an undertaking, the
following departments have been created:
1 Headquarters department (executive). In charge of general
designs, plans of construction and preparation of contracts. Alfred
D. Flinn, department engineer.
2 Reservoir department. In charge of development of the Cats-
kill watershed and the construction of the various dams and res-
ervoirs. Carlton E. Davis, department engineer.
3 Northern aqueduct department. In charge of the construction
of full capacity aqueduct from the Ashokan dam (60 miles) to Hunt-
ers brook in the Croton system. Robert Ridgway, department engi-
neer.
4 Southern aqueduct department. In charge of the construction
of full capacity aqueduct from Hunters brook in the Croton system
I4 NEW YORK STATE MUSEUM
to Hill View reservoir on the northern limits of New York city
and of the storage reservoirs and filtration work. Merritt H. Smith,
and more recently F. E. Winsor, department engineer.
5 Long Island department. In charge of the development of the
underground water supply of Long Island. A plan looking toward ~
this end has been prepared and approved by the city authorities and
is now being reviewed by the State Water Supply Commission.
6 City aqueduct division. In charge of the delivery of water
from Hill View reservoir throughout Greater New York. Origi-
nally in charge of W. W. Brush, now under Walter E. Spear, as
department engineer.
Departments are further divided into “ divisions” each in charge
of a division engineer and a full corps of assistants. The subdi-
visions of these larger units, although primarily based upon con-
venience and efficiency of engineering supervision, coincides rather
closely with the larger geologic problems included in this bulletin.
'
Generalities of construction
The chief types of structure projected include (1) masonry dams,
(2) earth dikes with core walls, (3) “cut and cover” aqueduct
through country of about the elevation of hydraulic grade, (4)
tunnels through mountains or ridges that are too high, and (5)
pressure tunnels under valleys or gorges that are too low.
Some of these are oi record proportions. For some of the de-
tails and figures see the different special problems in part 2.
All items complete as planned involve a total of:
Io dams
IO impounding, storage and distributing reservoirs
4.5 miles of dikes
54-5 miles of “cut and cover” aqueduct
13.9 miles of tunnel at grade
17.3 miles of pressure tunnel below grade
34 shafts of aggregate depth of 14,723 feet.
6.3 miles of steel pipes making
92.5 miles of aqueduct compiete to Hill View equalizing
reservoir
1 filtration works
18.0 miles of delivery tunnel in New York city to the termina!
shafts in Brooklyn
16.3 miles of delivery pipe lines
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 15
Allowing for contingencies and costs for engineering supervision
the system is estimated to cost $176,000,000 and many years will
be required for its completion. The present plans, however, con-
template only the immediate development of the Esopus watershed.
the storage reservoirs near the city and the main aqueduct to the
various points of delivery within the city limits. It is expected
that part of this additional supply of water will be available by the
year 1913, or early in 1914.
GHAPTER IT
PROBLEMS ENCOUNTERED IN THE PROJECT
When the Ashokan reservoir is filled the surface of the stored
waters will stand 590 feet above the sea. Hill View reservoir on
the northern borders of New York city will have an elevation of
295 feet. The distance between these two points is nearly 75 miles
in direct line. The contour of the country and other exigencies
of construction will increase this to approximately 92 miles. A
main distributary conduit in New York city will add 18 miles more.
The destination of the water therefore before distribution begins
is 300 feet lower than its starting point. This is sufficient head to
permit gravitational flow and a self-delivering system. If the hy-
draulic gradient can be maintained it would evidently constitute a
decided advantage. The plans have therefore from the beginning
contemplated such construction. It means then that a flowing
grade must be maintained in all tunnels or channels or tubes and
that when a depression has to be crossed the pressure must be
maintained in some sort of a conduit so that the water may rise
again to a suitable level on the other side.
The difficulties of accomplishing this in a work of such magnitude
are not at first apparent. The full significance of the undertaking
can be realized only after a study of the country through which the
aqueduct must be carried. It then resolves itself into a series of
problems, each one having its own characteristics and peculiar
difficulties and methods of solution and each requiring a thorough
understanding of the topographic features of the vicinity and a
working knowledge of geologic conditions. 3
General questions
It is sufficient at this point to call attention to the facts of the
topographic map and point out only the most general physiographic
features that may at once be seen to materially modify the simplicity
of the line.
For example, one has scarcely left the great reservoir, with water
flowing at 580-90 feet above tide, before the broad Rondout
valley is reached, with a width of 4% miles nowhere at great
enough elevation to carry the aqueduct at grade. If it is to be
crossed at all, and it must be crossed to reach New York city, some
[17]
18 NEW YORK STATE MUSEUM
special means must be devised. If a trestle be proposed, one finds
that it would have to be 4%4 miles long (24,000 feet), and in some
places 300 feet high, and at all points large enough and strong
enough to carry a stream of water capable of delivering 500,000,000
gallons daily —a stream that if confined in a tube of cylindrical
form would have a diameter of about 15 feet. :
A steel tube might be laid to carry the water across and deliver
it again at flowing grade, but here one is met with the fact that it
would require a tube of unprecedented size and strength and if
divided into a number of smaller ones the cost would be greater than
that of a tunnel in solid rock.
The other alternative is to make a tunnel deep enough in bed
rock to lie beneath surface weaknesses and superficial gorges and
in it carry the water under pressure to the opposite side of the
valley. This is the plan that seems best suited to the magnitude
of the undertaking and would seem to promise most permanent con-
struction. But no sooner is this conclusion reached than it is
realized that there are now several hitherto unregarded features’
that assume immediate and controlling importance. Some of these,
for example, are (1) the possibility of old stream gorges that are
buried beneath the soil, (2) the position of these old channels and -
their depth, (3) the kinds of rock in the valley, (4) their character
for construction and permanence, (5) the possible interference of
underground water circulation, (6) the possible excessive losses of
water through porosity of strata, (7) the proper depth at which the
tunnel should be placed, (8) the kinds of strata, and their respective
amounts that will be cut at the chosen depth, (9) the position and
character of the weak spots with an estimate of their influence on
the practicability of the tunnel proposition. Then after these have
all been considered the whole situation must be interpreted -and-
translated into such practical engineering terms as whether or not
the tunnel method is practicable, and at what point and at what
depth it should cross the valley, and at what points still further
exploration would add data of value in correcting estimates and
governing construction and controlling contracts.
This is a general view of one case, the first one of any large
proportions in following down the aqueduct. - There. are. many
others. In nearly all of them the importance of geologic. questions
is prominent. Many of them, of course, are of the simplest sort,:
but, on the other hand, some are among the most obscure and
evasive problems of the science. And they do not become::any
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 19
easier simply to know that they must ultimately be stated in terms
precise enough for the use of engineers, and to know furthermore
that the real facts are to be laid bare when construction begins and
as it progresses. But from another viewpoint it may be regarded
as an exceptionally fine opportunity to study applied geology in its
best form and to see the intimate interrelationship between an
engineering enterprise of great public utility and a commonly con-
sidered more or less obscure science. The services of geology have
been seldom so consistently employed in earlier undertakings of
similar character. It is to be hoped that the accompanying illus-
trations of the practical application of geologic knowledge and facts
to engineering plans and practice may add to the appreciation of
the commonness and variety of such service in many everyday
affairs. Furthermore, this unique enterprise, the like of which for
magnitude and complexity has never before been attempted, has
given to those whose good fortune has brought them into working
relations with its problems, the opportunity of a generation in their
chosen field? The success stages from isolated observations,
inference, hypothesis, theory, conclusions, and fully proven facts
are all represented. The steps more or less fully coincide with the
degree of confidence observable in the tone of advisory reports to
the engineers in charge — representing suggestions, recommenda-
tions, or specific advice.
It is one of the cherished wishes of the writer of this bulletin
that some of these problems may be presented in such manner as
to serve a distinct educational purpose. For this reason in part,
deeming it even of greater importance than the mere enumeration
of newly discovered facts, the writer has chosen to treat the sub-
ject from the standpoint of an instructor illustrating the develop-
ment of working conclusions. - It is certain that not all readers have
the same degree of preparation or acquaintance with the subject-
matter, and it may therefore be useful to include many things that
some may well pass by. No excuse is offered except that such
method of treatment, in behalf of the general intelligent public that
it is hoped to reach, seems to the author to be advisable.
1 W. O. Crosby of the Massachusetts Institute of Technology, James
F. Kemp and Charles P. Berkey of Columbia University have constituted
the staff of consulting geologists throughout most of the exploratory work.
20 NEW YORK STATE MUSEUM
Other problems
The foregoing observations apply likewise to the other larger
problems of the aqueduct line. A list of the larger ones requiring
extensive exploration and illustrating geologic application in their
solution are given below:
1 Location of the Ashokan dam
2 Sources of material for construction
3 Crossing the Rondout valley
4 The Wallkill valley
5 Moodna buried valley
6 Pagenstechers gorge and Storm King mountain
7 The Hudson river crossing problem
8 The Storm King-Break Neck cross section
g Foundry brook
10 Sprout brook notch
11 Peekskill creek valley
12 Croton lake pressure tunnel
13 Bryn Mawr siphon
14 The new Kensico dam
15 Kensico quarries
16 New York city delivery tunnel
In addition to these there are several questions of general bear-
ing in which the chief lines of argument and the chief basis of con-
clusion are essentially geologic. Although little wholly new data is
yet available on these particular questions from any direct work of
the aqueduct, yet it will add materially to an appreciation of the
far-reaching influence of established geologic data and geologic rea-
soning to enumerate some of them:
17 Continental subsidence and elevation
18 Crustal warping
19 Postglacial and present faulting
20 Underground water circulation
21 Relative resistance of the different formations to corrosion by
aqueduct waters
22 Structural materials
Each of these problems or questions or topics is discussed sepa-
rately, so far as practicable. By adopting this plan, of course there
is a tendency to repetition but this to a certain extent is unavoid-
able. Some of it is overcome by suitable references to preceding
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 2!
discussions. Where such cross reference is too cumbersome, the
items are repeated in preference to leaving the case obscure. Thus
it is hoped to make each case a unit, and the whole series useful
and understandable.
Gathering data
In the accumulation of data all the members of the engineering
corps: as well as the men acting only in a consulting capacity have
taken part. Necessarily the bulk of the exact data has been
gathered by the men all the time on the ground and whose duty it
was to superintend explorations. The care and intelligence with
which this has been done is notable. A considerable proportion of
the labor of manipulating the accumulated data and interpreting it
so as to reach an explanation of conditions and formulate conclu-
sions has been assumed by the consulting men.
Too much credit can not be given to the heads of departments
and divisions for the open-handed way in which all needed facts
were held available at all times for comparison and guidance toward
sound conclusions. The information upon which investigations
have been initiated have been chiefly the following:
1 The geologic maps and reports of the New York State Survey
2 United States topographic maps
3 Geologic folio no. 83, New York city folio
4 Earlier engineering records and reports
5 Reports of special commissions on water supply
1 In this work, no group of men have had so direct responsibility as the
division engineers. The success with which so many complicated explora-
tions were carried out is chiefly due to their constant care and foresight
and perseverance and the able assistance of their staff. Those who have had
especially important divisions for the geological problems involved are given
due credit in the discussions of part 2, of this bulletin. It is easy, how-
ever, to neglect sufficiently full acknowledgment of their services in gather-
ing and formulating data of this kind. Among those having charge of the
most important exploratory work the following names should appear:
James F. Sanborn, for sometime assigned to geologic work on the North-
ern aqueduct.
William E. Swift. in charge of the Hudson river explorations.
William W. Brush, in charge of the early New York city explorations.
Lazarus White, in charge of the Rondout valley explorations.
Lawrence C. Brink, in charge of the Wallkill division explorations.
J. S. Langthorn, in charge of the exploratory work at the Ashokan Reser-
voir.
Wilson Fitch Smith, in charge of work at Kensico dam, and
A. A. Sproul, in charge of the Peekskill creek and Sprout brook explora-
tions.
22 NEW YORK STATE MUSEUM
Some of these are printed reports and records not directly con-
cerned with this enterprise, but whose information has been found
useful in this field. This is especially true of the first four sources
enumerated, I, 2, 3, 4. The last is a specific study with direct
reference to this project.
Investigations were begun from the above vantage point. The
methods employed and the explorations conducted constituting the
further sources of information and furnishing the complete data
upon which all conclusions have been based include the following:
6 Detailed topographic studies of the engineers of the Board of
Water Supply
Geologic field work making observations in detail of all geo-
logic factors that seem to bear onthe problem in hand
Wash borings for depth to bed rock
Chop drill holes through stony ground to bed rock
10 Shot drill holes in bed rock
Ir Diamond drill holes
12 Test pits and trenches for detail of drift structure
13 Test tunnels in rock for working quality
14 Deflection tests for holies that have swerved aside
15 Pumping tests for underground water supply
16 Pressure tests for rock porosity
17 Microscopic examinations of rock types
18 Laboratory tests of quality and behavior of materials.
a |
Oo ©
The mass of data accumulated from all these sources is surpris-
ing. For example, there are upward of 200 wash borings on the
different proposed Hudson river crossing lines alone; there are 69
drill borings and 177 wash borings on the site of Kensico dam:
there are 69 shot and diamond drill holes on the Rondout siphon
line aggregating 10.234 feet of rock core; there are 65 drill holes of
various sorts on the Moodna creek siphon aggregating in total pene-
tration of drift over 10,000 feet: there are 106 borings, besides
several pits and trenches at Ashokan dam location. At every point
explorations suitable to the particular problems in hand were con-
ducted. The whole mass of data is conveniently recorded, much of
it is tabulated, some of it is represented graphically, samples of
nearly all of the material are available for examination? and all
1The cores of all drillings and suitable samples of all borings in drift
have been saved and properly labeled and are to be permanently housed at
some convenient point on the aqueduct line when completed. At present
they are cared for at the different division offices.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 23
have been made use of in coming to a consistent understanding of
the conditions.
But the amount of accumulated data is no more remarkable than
the difficulties that have been encountered in obtaining it. For
example, in the Moodna valley it has taken three to four months’
time to put down a single hole to bed rock — the average time con-
sumed for each of the 15 holes exploring the deepest portion of
the valley was about 60 days. The chief trouble is caused by heavy
bouldery till. In one case a boulder was penetrated for 35 feet.
lying a hundred feet above bed rock.
The extreme of such difficulty is, of course, encountered in the
Hudson river itself, where the drill has to contend with: (1) the
rise and fall of the tides, (2) the river currents, (3) a maximum of
go feet of water, approximately 700 feet of silt, gravel, till, boulders,
etc., filling the old preglacial gorge. The heavy steamboat and
towing traffic has been a serious element in the problem. Probably
never anywhere have drillmen had to face so nearly insurmount-
able obstacles. In two years only two holes reached below a depth
of 600 feet below sea level. A third, now in progress, has pene-
trated a depth of 768 feet without entering rock.
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CHAPTER ill
RELATIVE VALUES OF DIFFERENT SOURCES OF INFORMA-
TION AND STAGES OF DEVELOPMENT
In the earlier stages of work topographic features were of most
concern, and they largely controlled the selection of reservoir sites
and possible lines for the aqueduct to follow. It was, however,
at once recognized that tunnels would be unavoidable and studies
as to the types of rock formations to be encountered were begun
It was also early appreciated that the soil or drift cover is very
unevenly distributed over the rock surface and that, especially in
the chief valleys requiring pressure tunnels, it would be necessary
to determine the profile of the rock floor. At this point wash bor-
ings were begun. But the natural limitations of the wash rig’ for
penetrating drift of all kinds left the information still too indefinite.
The wash rig can not penetrate hard rock. It can not wash up
anything but the finer matter, and a boulder of very moderate size
is almost as effectual a barrier as true rock ledge. By a combination
of washing and chopping or by the use of an explosive to break or
dislodge an obstruction some progress in unfavorable material may
be made, but the wash rig alone, in a drift-covered region, gives
only negative results. It is certain, for example, that bed rock lies
at least as deep as the wash rig has penetrated, but it is not certain
that it is bed rock instead of some other obstruction. Except in
areas of special drift conditions,? therefore, the wash rig was insuf-
ficient. To rely upon the process at random was clearly impossible,
and to determine whether or not the results of a particular locality
1A “wash rig” is a device composed essentially of two iron pipes, one
within the other, and so mounted that the inner one can be worked up
and down in sort of a churning fashion while water under considerable
pressure is forced through it to the bottom and out again by the larger
pipe to the surface, carrying up with the current the displaced sand and
clay. As progress is made with the inner pipe the outer one is from time
to time driven down and the process renewed and repeated till the hole is
finished.
2One of the most notable areas of special drift conditions is repre-
sented in the Walkhill valley lLsee discussion in pt 2] where there were
developed large deposits of modified drift, stratified gravel, sand and clays,
lying immediately upon the bed rock floor. In this the wash bore
process was eminently satisfactory, and the rapid progress made by it
together with its economy made this an especially attractive method of
exploration.
[25]
26 NEW YORK STATE MUSEUM
could be relied upon became involved at once with an interpretation
of local glacial phenomena, especially an interpretation of the char-
acter of the local drift. In order to see the limited application of
this method one needs only to point out that the majority of drift
deposits in this region are stony or even bouldery, forming thick
coverings in the valleys, and to call attention to the experience at
two or three points. For example, at Moodna creek, the prelimi-
nary wash borings were obstructed and bed rock reported at 5 to
15 feet below the surface where afterward, by other means, it was
proven to lie more than 300 feet down. Or again, in the pre-
liminary wash borings in the Hudson, the rigs were stopped and
rock bottom provisionally reported at from 25 to 200 feet below
sea level, but later explorations have proven at the same point that
rock bottom is more than 700 feet down.
Therefore, to the “ wash rig” was added the “chop drill” and
the “oil-well rig” and to these, or to modifications of them,’ the
success in reaching bed rock has been due. |
From independent field studies of a more strictly geologic nature
it became clear that many of the valleys, where pressure tunnels
were proposed, are of comparatively complex geologic structure and
exhibit considerable variety of rock quality and condition. This
then introduced and necessitated still more elaborate lines of ex-
ploration. It was not enough to know the profile of rock floor
alone, it became of equal importance to penetrate the rock and obtain
samples of it. So the shot drill? and the diamond drill* were
employed and the drill cores preserved for identification and
reference.
1 The essential features of the machines in most instances are, a high
tower or support, a heavy chisel-shaped plunger that can be raised by
a rope and dropped repeatedly in the hole, destroying or displacing
obstructions, and which can be followed by a casing driven down as
progress is made—a combination of washing, chopping and driving.
2 The shot drill, or calyx drill, is essentially a machine devised to rotate
a steel tube which is so adjusted and manipulated that a supply of small
chilled shot can be kept continually under the lower end as it bores into
the rock. The cutting is done by the shot immediately under the edge
of the tube. A core remains in the tube and may be recovered. Its best
position is vertical.
3 The diamond drill consists essentially of a bit or crown set with black
diamonds (bort) in such manner that when the bit is attached to a rotating
tube a circular groove is cut into the rock. By proper attachment to
jointed tubes and driving gear a hole may thus be bored at any angle and
to great depth and a core recovered.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 27
_ These preserved cores, now aggregating many thousands of feet
have been of great service in determining the precise limits of
formations and consequently the geologic structure or cross section,
by which detailed estimates may be guided.
Even these occasionally appeared to give insufficient data. The
peculiar behavior of certain holes, as, for example, one or more
at Foundry brook, led to the suspicion that the drill had swerved
from its course, following a particularly soft seam or zone, and
that the results secured by it without large corrections, were wholly
misleading. Tests proved that there had been a deflection.
At this and many other places it later became very desirable to
form some quantitative as well as qualitative opinion of the condi-
tions existing in the underlying strata. The percentage of core
saved, the rate of progress of the drill, the behavior of the drill, the
condition of the core recovered, the loss of water in the hole — all
these of course were considered.
For more definite evidence as to porosity and perviousness, a
series of carefully planned pressure tests? were made. By shutting
off connection with the walls of the hole above a certain stratum
and forcing water in under pressure, it was possible to demonstrate
that certain strata or certain portions were practically impervious
in their natural bed, while others were much less so, and:to get an
idea of their relative efficiency as water carriers. For the pressure
tunnels, especially, this test is a very suggestive line of investigation.
1 At Foundry brook [see discussion of this problem in pt 2], the remark-
able condition apparently shown was a reasonably substantial ledge of
granitic gneiss, so feet, followed below by 200 feet of apparently soft
sand and reported as such. No core could be recovered. So extensive
a zone or bed or layer or mass is hardly conceivable considering the
crystalline silicious character of the rock. It probably represents a steepiy
dipping crush zone along fault movement where the increased underground
circulation has been unusually effective in producing decay. After enter-
ing this zone the drill swerved from its initial course and kept within the
soft seam.
2 The pressure test is made by means of a force pump, fitted with a
gage on which the pressure is recorded, connected by a pipe to the por-
tion of the hole to be tested, and so adjusted to a device for blockading
or damming the hole that the water pressure is confined to those portions
of the walls of the hole below the dam, or between two dams if an upper
and lower one are used. In this way any portion of a hole, or stratum or
several beds together may be tested and the amount of water absorbed
per unit of time per unit of pressure determined. This is, of course,
directly related to the porosity of the rock and is approximately inversely
proportional to its presumed value as an aqueduct carrier.
28 NEW YORK STATE MUSEUM
Where the strata are especially porous and where underground or
permanent ground water supplies are very extensive and where at
the same time the largest or deepest pressure tunnels are projected
some uneasiness has been entertained as to the extent of interference
from inflowing water during construction. An attempt to form
some idea of the ease of such underground circulation has been
made by a systematic pumping of one or two critical holes. The
results leave many factors still too obscure to draw definite con-
clusions. The test will be taken up again in the discussion of the
Rondout siphon in part 2. |
Laboratory tests and experiments on materials complete the list
of lines of investigation with which this bulletin is concerned.
Although from the nature of the case these are elaborate and
unusually complete, the more important lines are not at all new.
_ All the methods of petrographic, chemical, and physical manipula-
tion that seem to promise practical results of value to the success
of the undertaking are followed and the data are organized and
interpreted and conclusions are formulated with as great definite-
ness for practical bearing as other lines of investigation.
CHAPTER IV
GENERAL GEOLOGY OF THE REGION
It will save much repetition and it is believed will altogether
serve a useful purpose in maintaining unity of treatment to give
an outline of the geologic features of the region in advance of the
discussion of special problems. It is intended only for those not
sufficiently familiar with the general geology to follow subsequent
matters.
The region includes some of the most complicated and obscure
sections of New York geology. It is simple in almost no one of the
larger branches of the subject. In physiography there is the long
and involved history and the results of long continued erosion of a
variable series of formations in different stages of modification as
to structure and metamorphism and attitude, modified still fur-
ther by subsidences and elevations, depositions and denudations,
peneplanations and rejuvenations, glaciation and recent erosion —
all together introducing as much complexity as can well be found in
a single area.
In stratigraphy the whole range of the eastern New York geologic
column is represented from the oldest known formation up to and
including the Middle Devonic —a succession of at least 25 distinct
formations which may for convenience be treated in groups that
have had similar history. Each of these formations has a constant
enough character to map and regard as a physical unit. Even this
classification ignores the great range of petrographic variability
shown in stich formations as the Highlands or Fordham gneisses.
All but two or three of these formations will be cut by the tunnels
of the aqueduct.
In petrography the range is even greater — so great, in fact, that
only an enumeration of the variations will be attempted. They
include clastics, metamorphics and igneous types; stratified and un-
assorted, coarse and fine, detrital and organic, marine and fresh
water, homogeneous and heterogeneous, argillaceous, calcareous and
silicious sediments, unmodified and thoroughly recrystallized strata ;
acid and basic and intermediate intrusions; massive and foliated
crystallines — of many varieties or variations in each group.
In tectonic geology an equal complexity prevails. There are regu-
lar stratifications, cross-beddings, disconformities, overlaps and un-
conformities ; interbeddings, lenses and wedges; flat, warped, tilted
[29]
30 NEW YORK STATE MUSEUM
and crumpled strata; monoclinal and isoclinal, open and closed,
anticlinal and synclinal, symmetrical and overturned, horizontal and
pitching folds; joints, crevices, caves, crush zones, shear zones, and
contacts; normal, thrust, dip, strike, large and small faults; veins,
segregations, inclusions, dikes, sills, bosses and bysmaliths.
With such variety of natural conditions it is not surprising that
the problems of the aqueduct are also of great variety. No two
have in all respects the same factors in control and no two can be
explored and interpreted upon exactly the same lines.
1 Geographic features or districts. (Physical geography’)
It will be convenient at this point to think of the surface topog-
raphy by districts — not wholly distinct from each other, but still
with essential differences of origin and form. From south to north
they are: (a) New York-Westchester county district. The area
of crystalline sediments. South of the Highlands. (6b) Highlands
of the Hudson (Putnam county). (c) Wallkill-Newburgh district.
From the Highlands to the Shawangunk range. (d) Shawangunk
range and Rondout valley. (e) Southern Catskills.
All have been sculptured by the same forces and with similar
vicissitudes, but the difference of history and structure and condi-
tion, already established when the physiographic forces began on the
work now seen, have caused the variety of surface features indi-
cated in the divisions made above. The more noticeable character-
istics of these five districts are here given.
a New York-Westchester district. The area south of the
Highlands proper is characterized by a comparatively regular suc-
cession of nearly parallel ridges separated by valleys of nearly equal
extent (% to 5 miles wide), making a surface of gently fluted
aspect and of moderate relief (0-500 feet) sloping endwise toward
the Hudson and the sea. The controlling factors in producing this
topography are involved in a series of folded, foliated, crystalline
sediments, of differing resistance to destructive agencies.
b The Highland region is one of rugged features, with a
range of elevation of o-1600 feet A. T., forming mountain masses
and ridges separated by very narrow valleys all having a general
northeast and southwest trend across which the Hudson cuts its
way in a narrow, angular gorge, forming the most constricted and
crooked portion of its lower course. The bed rock is all crystalline,
1 The physiographic history of a region is not understandable without a
comprehensive knowledge of its geologic features and structures and history.
It is therefore treated in a later paragraph.
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 31
oi massive and foliated types, metamorphosed sediments in part
with large masses of igneous intrusions and bosses.
c The Wallkill-Newburgh district lying immediately north
of the Highlands and extending to the Shawangunk range is a
region of gently rolling contour. Most of the area along the pro-
posed lines lies between 200 and 500 feet above the sea. ‘There are
only occasional rugged hills or short ridges, such as Snake hill and
Skunnemunk. The valleys are broad and smootn and the divides
are simply broad, hilly uplands. Bed rock is chiefly Hudson River
slates with occasional belts of Wappinger limestone. The larger
features, the trend of divides and valleys, are northeast and south-
west, although this regularity is not so marked as in the preceding
two districts. But the chief streams flow either northeast or south-
west to the Hudson along these general lines.
d The Shawangunk range and Rondout valley form a
transitional unit from the complicated structural and tectonic con-
ditions of the southerly districts to the uniform and almost undis-
turbed strata of the Catskills. Its southeasterly half is a mountain
ridge partaking of extensive faulting and folding and represented
by the Hudson River slates overlain unconformably by the thick
and very resistant Shawangunk conglomerate forming high east-
ward-facing cliffs. Toward the northwest these disturbances dimin-
ish, the strata gradually pass deeper beneath a great succession of
shales, limestones, and sandstones of the Helderbergian series, and
a broad valley is eroded in the softer portions. It is limited on the
northwest by the prominent and very persistent escarpment border-
ing the Hamilton series and forming the outer margin of the Cats-
kill mountains.
e The Catskill area is of simple structure. The strata are
well bedded and lie almost flat with a gentle dip northwest. The
surface features form a series of irregularly distributed escarp-
ments, hills, valleys, cliffs, gorges and mountains, rising rapidiv
toward the west, with moderate to strong relief and reaching ele-
vations of 2500 feet. The failure of the northeast-southwest trend
of feature that is so common in all of the other districts is a marked
difference. It is directly due to the flatness of the strata.
2 Stratigraphy
There are no strata of prominence in association with the main
aqueduct younger than Devonic age except the glacial drift. Imme-
diately adjacent areas, however,-some of which are covered by the
accompanying maps, and Long Island have later formations ex-
32 NEW YORK STATE MUSEUM
tensively developed. Such are the Triassic rocks of the New Jersey
side of the Hudson below the Highlands, and the Cretaceous and
Tertiary strata of the Atlantic margin on Long Island and Staten
Island. The development of underground water supply on Long
Island is especially concerned with these later formations, and with
the modified drift deposits of the continental margin.
The whole series of formations are more commonly considered
in groups that exhibit certain age or physical unity and that are
for the most part characteristic of certain regional belts and that
coincide somewhat roughly with the physiographic divisions already
noted. There is in the following description and tabulation no direct
attempt to unduly emphasize this relation or to belittle the divisions
recognized in the commonly adopted geologic column. It is, how-
ever, for the purpose in hand, more convenient and useful to keep
clear the physical groupings, because largely these groups, instead
of the more arbitrary subdivisions of age, are the units used in con-
sidering structural and applied problems. |
a Quaternary deposits. (1) Glacial drift. A loose mantle of.
soil and mixed rock matter covers the bed rock throughout the
whole region except (a) here and there where the rock sticks up
through (outcrops), and (b) at the most southerly margin along
the coast where the glaciers seem not to have reached.
Origin. This mantle is usually very different in lithologic charac-
ter from the underlying rock floor. There is almost always an
abrupt break between the rock floor and the overlying material.
The rock floor is grooved, smoothed, and scratched as if by the
moving of rock or gravel over it. The larger boulders are usually
of types of rock identical with ledges lying northward at greater or
‘less distance. Materials of exceedingly great variety both in
size and condition and lithologic character are often all piled to-
gether in the most hopelessly heterogeneous manner. These are
now commonly regarded as conclusive evidence of glacial origin.
There is no need of making the discussion exhaustive. It is almost
universally called the “ drift.”
Thickness. The thickness of the drift varies from almost 0 to ap-
proximately 500 feet. It is generally thickest in the valleys where it
has simply filled many of the original depressions and obliterated
much of the ruggedness of surface, the gorges and ravines and can-
yons of the preglacial time.
Sources. It appears from an examination of the grooves and
striae on bed rock, and the relationship of the different types of
drift to each other, and from a comparison of the types of boulders
*
*
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 33
with the ledges that may be regarded as their source, that the gen-
eral ice movement was from north to south swerving along the
southerly extension to east of south. Therefore it is not unusual
to find abundant boulders of Palisade trap stranded in New York
city or on Long Island, or boulders of the Cortlandt series, or of
the gneisses of the Highlands, or, in occasional instances, of sand
stones from the Catskills, or the limestones from the Helderbergs
or perhaps in rarer cases even rocks from greater distance, as the
Adirondack mountains.
Kinds of drift. There are in the region two fundamentally differ-
ent types of drift as to method of deposition. They are (a) unas-
sorted drift (till or hardpan), and (b) modified drift (stratified or
partially assorted gravels, sands, clay, etc.). The former (a) repre-
sents deposition directly from the ice sheet at its margin (terminal
Or marginal moraines) or beneath (“ground moraine”) without
enough water action to rework and assort the material. It there-
fore contains boulders, pebbles, sand and clay of a heterogeneous
mixture of the most complex sort both as to size and character. In
such deposits there is almost always sufficient intermixture of clay
and rock flour of the finest sort to make a very compact and dense
mass that is usually quite impervious to water. Such deposits are
distributed rather unevenly over the surface and where this uneven-
ness leaves hollows or basins, or obstructs the outlets of other de-
pressions, they may hold water and form small lakes or ponds or
swamps. This is almost universally the origin of the many thou-
sands of lakes of the northern lake region. It is evident that ma-
terial of this character, a type that commonly serves the purpose of
a natural dam or reservoir, would be especially important and useful
at certain places on the Catskill system. As a matter of fact, so far
as geologic features are concerned, it is the chief factor in choice of
location for the Ashokan dam [see discussion pt 2] and is a con-
trolling factor in the plans for the erection of the miles of dikes
at less critical margins of the reservoirs. Till is an extensively
developed type but frequently passes abruptly either laterally or
vertically into assorted materials of very different physical char-
acter.
(6) All materials associated in origin with the glacial occupation
that have been materially modified especially in the direction of an
assorting of material are referred to as “ modified drift ” deposits.
They include (1) deposits made by both water and ice together,
(2) those fermed by running water, (3) those laid down in stand-
34 NEW YORK STATE MUSEUM
ing water. Or again (1) those accumulated rapidly with very irreg-
ular supply of material at the margin of the ice-forming, hummocky
or hill and kettle surface (kames, eskers), (2) those carried along
valleys or general lines of drainage to a considerable distance beyond
the ice margin aggrading the valley with the overload of gravels
and sands (valley trains), (3) those washed out from the ice margin
iu more even distribution forming a gently sloping and thinning
extramarginal fringe (outwash or apron plains), (4) those fine
matters that are carried by glacial streams into the margins of more
quiet waters, either a temporary or a permanent lake or a larger
and slower stream or other body forming more perfectly assorted
and more evenly stratified deposits (delta deposits), (5) those
finer rock flours and clays that remain suspended longer and carry
out much farther settling only in the very quiet waters of lakes ov
estuaries or temporary water bodies of this character forming the
perfectly banded clays (glacial lacustrine clays).
It is evident then that modified drift has in the process of its
accumulation suffered chiefly a separation of fine from the coarse
particles and that in most cases the fineclay filling that makes the
till dense and impervious to water, has been washed out and de-
posited by itself in the more inaccessible deeper waters. As a re-
sult most modified drift deposits are pervious.and easy water
conductors, but poor or questionable ground for dikes or dams or
basins [see discussion of Ashokan dam, pt 2]. |
Some of them, the medium sands and gravels, furnish an excel-
lent and already cleaned structural material for concrete or mortar,
such as the Horton sand deposit, or coarser kinds may be crushed —
and sized before using as is done at Jones Point on the Hudson.
The finer silts and clays, usually overlain by assorted sands, are
abundant along the Hudson, having been deposited there at a time
when the water of this estuary stood 50 to 150 feet higher than
now. Recent erosive activity of the river has cut the greater pro- |
portion of the original deposits away but at many places large quan- ~
tities still remain above water level in the banks and still greater _
quantities extend beneath the river. These deposits are the support
of the brick industry of southeastern New York. The till deposits
are very difficult to penetrate in making borings because of the
boulders, the wash rig being almost useless. Modified drift of the
‘medium and finer sorts is easily and cheaply penetrated, and, if it —
i
lies on bed rock, such exploration gives reliable results.
Structure. But this is stating the actual conditions too simply. |
The glacial epoch was a complex one The continental ice sheet may
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 35
have advanced and retreated repeatedly, how many times in this
region is not clear. Wa5th each time of advance and retreat, the
work done by it partly destroyed, or disturbed or modified or cov-
ered the earlier ones in what appears now to be a most arbitrary
way (in reality, of course, in a very consistent way for the condi- _
tions that then existed). So one frequently finds a till beneath a de-
posit of stratified drift, or modified drift beneath till, or a succession
of a still greater number of changes in almost hopeless confusion.
In New York city, for example, at Manhattanville cross valley,
the exposed drift above street level includes (a) at the bottom,
water-marked stony till and assorted gravels, (b) in the middle per-
fectly horizontal, stratified rock flour and the finest sand, (c) top,
wholly unassorted bouldery till, covered by thin soil. It is evident
that the most careful and accurate identification of the surface type
without subsurface investigation would give, for such uses as are
now being considered, thoroughly unreliable evidence as to the
behavior of the whole body at this point. Therefore, a determina-
tion of the changes and quality forms an essential record. All of
these types are to be found in the region, but the different grades of
till and roughly modified material belonging to the kame type are
more common inland.
On Long Island the development of marginal modified types is
extensive and more or less obscured by the advance and retreat
noted above. The larger divisions recognized in deposits are (a)
an early accumulation of sands and gravels, strongly developed near
the western end of the island, known as the “ Jameco” gravel,
(6) an interglacial (retreatal) deposit of blue clays known as the
“Sankaty ” beds, (c) a later series of deposits, sands, clays, gravels
and till, belonging to the closing stages of the ice period correspond-
ing to the surface deposits of the larger portion of the whole region
(Tisbury and Wisconsin advances). Some of these sands and
gravels are important water-bearing sources for the new Brooklyn
additional supply. ;
The whole Long Island series according to Veatch?! includes:
Glacial 1 -
acial two lines of ter See pig cretite
Wisconsin stage [ minal moraines with out-
Ronkonkoma moraine
- ( wash plains
Great deposit of outwash sand and gravel (de-
Tisbury stage pression )
Gardiner interval with erosion (interglacial)
1 After PP 44, U. S. Geological Survey, p. 33.
2
36 NEW YORK STATE MUSEUM
¢ Folding (glacial folding)
Sankaty retreatal stage (interglacial) clay beds
Glacial — Jameco gravels
Postmannetto erosion (interglacial)
Mannetto stage Glacial — old gravels
Gay Head
Jameco
A radically different and in some respects a much simpler inter-
pretation’ of the Long Island deposits has been outlined by W. O.
Crosby. The essential feature of his classification is the unity and
simplicity of the glacial epoch. Only the moraines and associated
sands and gravels of outwash origin during advance and retreat are
regarded as glacial. All other deposits below and including the
sSankaty clay beds he regards as preglacial.
_ The Jameco gravels are interpreted as Miocene in age.
Certain persistent yellow gravels overlying the Jameco are classi-
fied as Pliocene.
6 Tertiary and Cretaceous deposits. (2) Tertiary outliers.
Deposits of Pliocene age are littoral in type [PP 44 U. S. G. S.
p. 28] and are not very well differentiated (Long Island, Staten
Island). Probably equivalent to the Bridgeton beds of New Jersey.
Certain “ fluffy” sands in thin beds are assigned by Mr Veatch
to the Miocene (Long Island, Staten Island). Probably equivalent
to the Beacon hill deposits of New Jersey. Crosby places the
Jameco gravels in the Miocene together with the Kirkwood lignitic
and pyritic clays and sands.
(3) Upper Cretaceous deposity? are extensively developed.
They form the chief bed rock of Long Island.
1 The writer offers both of these outlines of the glacial and associated
deposits in preference to either alone. Both Veatch and Crosby have given
immensely more time to the study of these questions than any one else.
It is hardly fitting for a newcomer in their field to reject either view. But
because of the very great difference between the two interpretations one
may be pardoned a preference. It is the writer’s opinion that the simpler
outline is the more tenable. It does not seem possible to establish a very
complex series of stages in the glacial epoch as represented in the deposits
of southeastern New York.
2 Crosby’s classification of the Cretaceous is as follows:
(a) Monmouth — slight development of marls. (Lower and middle
marl series.)
(b) Matawan — (clay marl series) probably present on Long Island.
(c) Magothy —an extensive series of variegated and micaceous sants
and clays. Heavy development on Long Island.
(d) Raritan — Plastic clay scales and the Lloyd sand.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 37
(a) A lignitiferous sand with occasional clay beds forming the
uppermost of the Cretaceous series is probably equivalent to the
marl series of New Jersey. But it lacks the prominent greens and
development characteristic of the region further south. Not clearly
separable from the underlying formation or Matawan beds.
(b) The Matawan beds. Gray sands and clays.
(c) Raritan formation. Clays and sands, plastic clays, the Lloyd
sand, an important water carrier lies about 200 feet below the top
of the formation. Occasional leaf impressions.
All of these formations, except where disturbed locally by glacial
ice, dip gently seaward. The sand beds of these strata are the chief
sources of underground water being developed by the new system.
c Jura-Trias formations. (4) Palisade diabase. This is a thick
intrusive sheet, or sill, of igneous rock of diabasic type. It is
700-1000 feet thick. It lies for the most part parallel to the bed-
ding of the surrounding, inclosing, sedimentary rocks, and, rising
gently eastward, forms a strong cliff continuously along the west
bank of the Hudson for 40 miles. It varies from very fine to very
coarse texture and is for the most part fresh, tough, durable, and
is the source of large quantities of the most satisfactory quality of
crushed stone now on the market for use in concrete.
(5) Newark series. This is a very great thickness of silicious
sediments, chiefly reddish conglomerates, red and brown quartzose
and feldspathic sandstones and shales. They dip gently westward
and northwestward at 10-20 degrees, and are confined, in this
region, to the west side of the Hudson south of the Highlands. The
formation supplies “ brownstone” for building purposes.
None of the Jura-Trias rocks, so far as known, will be cut by the
aqueduct.
d Devonic strata. (6) Catskill formation. This formation’ is of
continental type, chiefly a conglomerate. A white conglomeratic
sandstone forming the uppermost portion attains its greatest thick-
ness on Slide mountain (350 feet). It is a “ coarse grained, heavy
bedded, moderately hard sandstone containing disseminated pebbles
of quartz or light colored quartzite, and streaks of conglomerate.”
A red conglomeratic sandstone constitutes the much thicker por-
tion below (1375 feet). It is a “ coarse, heavy bedded sandstone of
dull brownish hue containing disseminated pebbles and conglom-
eratic streaks, differing from the overlying beds chiefly in color. In
1Grabau, A. W. N. Y. State Mus. Bul. 92. Geology and Paleontology
of the Schoharie Valley. .
28 NEW YORK STATE MUSEUM
both series the pebbles and conglomeratic streaks are scattered and
irregular, while the sands are often cross-bedded. Thin layers of
red shale occur, and locally gray sandstones.” The deposits prob-
ably represent flood plains, deltas, and alluvial fans accumulated
mostly above sea level.
(7) Oneonta sandstone (Upper flagstone). “Thin and thick
bedded sandstones from 20 to 200 feet thick with interbedded red
shales up to 30 feet thick.” Chiefly light gray to brown in color.
Abundant cross-bedding, occasional dark shale, frequent flagstone
beds. Capable of furnishing “ bluestone” flags and more massive
dimension stone. To be seen in the vicinity of West Shokan and
westward.
(8) Ithaca and Sherburne (lower flagstone “ bluestone”). “ Thin
bedded sandstone, with intercalated beds of dark shale. The sand-
stones are in masses from a few inches to 40 feet in thickness,
greenish gray to light bluish gray or dark gray in color, and are
extensively quarried as flagstones.” There are occasional conglom-
eratic streaks. Occurs in large development in the vicinity of the
Ashokan reservoir (500 feet). The heavier cross-bedded and
coarser grained beds are capable of furnishing an unusually good
quality of large dimension stone for heavy structural uses. The
beds of this formation near Olive Bridge will in all probability
furnish the greater proportion of stone of all kinds for the con-
struction of the great Ashokan dam [see discussion of bluestone
near Ashokan dam, pt 2]. The chief common fossil content is
impressions of plant remains. |
(9) Hamilton and Marcellus shales. “ Dark gray to black or
brown shales with thin arenaceous beds in the upper part.” Forms
the upper portion of the escarpment that follows the outer margin
of the Catskill foothills bordering the westerly side of the middle
Rondout and lower Esopus valleys. Occasionally beds are sub-
stantial enough for flagstone production (700 feet or more with the
Marcellus. ) |
The) chief ‘index “fossils are? Sipiiifex a epee eis
Athyris .spiriferoides,, "Clhonemeshee miosis tue
The Marcellus shale is not readily differentiated in the Esopus
valley. Characteristically it is a thin bedded shale of no great
thickness (180 feet in the Schoharie valley) lying between the
Onondaga limestone and the Hamilton and obscured by talus from
the escarpment (with the Hamilton 700 feet.)
Styliolana Zissurella, (Chometes imam cro mat ws;
Strophalesia trincata;, Lior hync nas angst.
(Ajddng 19}eAQ JO plvog Aq ydeisojoyd) ‘osplig aAyQ Ww ssvy ousnqioys ous,
<
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 39
The dividing lines between the different sandstones and shale
fcrmations, the Oneonta, Ithaca, Sherburne, Hamilton and Mar-
cellus, can not be sharply drawn in the Esopus region. ‘Together
they form in a large way a rather satisfactory field unit. For
specific purposes it is necessary to recognize that the lower por-
tions are prevailingly shales with thin bedded sandstones while the
upper portions are much more heavily bedded, the sandstones pre-
Fig.2 Spirifer mucronatus (Conrad), a characteristic and abundant index
fossil of the Hamilton shales of the Catskill margin
vailing. The five divisions may possibly be more satisfactorily
made on paleontologic characters than on physical, but in most of
the advisory reports on economic and practical problems involving
this district the subdivisions can not be emphasized. The whole
series is essentially conformable and is very little disturbed [see
report on bluestone quarries, pt 2].
(10) Onondaga limestone. A bluish gray, massive, thick bedded
cherty, somewhat crystalline limestone. It is strongly marked off
from the Hamilton and Marcellus above, and, because of its greater
resistance to erosion, usually forms a dip slope controlling stream
adjustment and ultimately inducing the development of unsymmet-
rical valleys with gentle easterly slopes and clifflike westerly borders
where the streams are sapping the overlying Marcellus and Ham-
ilton shales. It is not sharply separable from the Esopus below but
everywhere in this region graduates into it with increase of silicious
4O NEW YORK STATE MUSEUM
and argillaceous impurities. Estimating the formation from the drill
cores that have penetrated it, and placing the lower limit as nearly
as may be at the horizon of changes from predominant lime to pre-
dominant silicious content, the approximate thickness in this region
is placed at 200 feet. The rock where exposed exhibits considera-
ble joint development and these are considerably enlarged by the
solvent action of percolating waters. This factor is considered of
some importance in connection with the other limestones of the
district in aqueduct construction and permanence. The Onondaga
has been used as a building stone formerly sold as marble, some
grades of which are good stone. On the line of the aqueduct it is
confined to the Rondout and Esopus valleys. The chief fossils are:
Aitrypa- reticularis, Zaphrentts ~ peo meenea
Leptostrophia perplana, Platyceras dumosum
Leptaena rhomboidalis, Dalmanites selenurus.
(11) Esopus and Schoharie shales (a slaty grit). The Schoharie
as a distinct formation is not distinguishable in this region. The
very thick and comparatively uniform, gritty, black, dense, almost
structureless rock is a distinct unit. It is a silicious mud rock with
very obscure sedimentation markings, but showing independent
secondary cleavages induced by later dynamic factors, and, on long
exposed surfaces always exhibiting chiplike fragments as the result
of weathering. But it is not an easily destroyed rock. In so far
as the bedding is obscure and the induced structure predominates,
the rock is a slate; and in so far as it is distinctly gritty (sandy)
instead of argillaceous it is a grit. The formation might therefore
be more accurately designated as a slaty grit. The lack of plain
bedding structure makes it impossible to estimate its thickness,
since the foldings or other displacements can not be allowed for;
but the accumulated data of drill holes in more advantageous
position indicate an approximate thickness of 800 feet. The rock
is considered exceptionally good ground for the tunnel.
A few fossils occur the most characteristic being Taonurus
caudagalli. There are also in certain layers of limited extent,
Leptocoelia acutiplicata and Atrypa spinosa.
(12) Oriskany and Port Ewen transition (silicious shaly lime-
stone). There is no well defined and distinct separation here be-
tween the Oriskany and the underlying Port Ewen, but because of
the importance and persistence of the formation in other and re-
lated areas the name is held. The equivalent of the Oriskany is in
this district involved with a strongly developed transition zone
which in physical features is intimately associated with the Port
GEOLOGY OF THE NEW YORK CITY AQUEDUCT AI
Ewen as a single unit. If any distinct formation is to be recog-
nized it would be on the basis of transitional faunal character,
placing the fossiliferous upper 100 feet in the Oriskany transition
and confining the name Port Ewen to the rather unfossiliferous
and concretionary, shaly, argillaceous limestone of the lower 100
feet.
Fig.3 Spirifer arenoswus (Conrad), one of the characteristic index fossils of
the Oriskany occurring in the Port Ewen-Oriskany transition
This transition rock is strongly bedded, argillaceous and
silicious limestone, very quartzose in certain layers, but there
are no exposures in this area that would be called sandstones.
Fossils are abundant and show marked Oriskany peculiarities.
Those of most characteristic relations are: Hipparionyx
proximus, Leptostrophia magnifica, Spirifer
M_mrevisant. Spirtter)renosws,. Pilatyceras
modostm, Strophostylus expansus.
42 NEW YORK STATE MUSEUM
(13) Port Ewen shaly limestone. The beds below those noted
in the preceding paragraph are essentially argillaceous, shaly lime-
stones. They vary from rather massive to thin bedded, are dark
grayish in color, and have a peculiar nodular or concretionary de-
velopment along certain sedimentation lines. These spots have less
resistance to weather than the surrounding rock and therefore
develop rows of pits along the face of an outcrop. Their size,
6 to 18 inches or more across, together with their persistence makes
an easily recognized physical feature. The few fossils that are
found are not very characteristic. The following should be men-
tioned: Spiriter perlamellosie,
In the discussion and on the maps the Port Ewen and Oriskany
are treated together as a single unit as the Oriskany-Port Ewen
beds.
(14) Becraft limestone. Massive, heavy to thin bedded, light
colored, semicrystalline to thoroughly crystalline limestone. More
massive beds very pure, 94+%CaCO;. Shaly beds resemble the
New Scotland which they pass
into at the base. The most char-
acteristic features for field iden-
tification are (a) pink or light
colored spots, (0b) a more
coarsely crystalline condition
than any of the associated strata,
(c) occasional large calcite
cleavages to be seen wherever
a fossil crinoid base As pido-
Fig.<4 Sie bee cidlatne em demeie ne ef fll S)e Sete le 40 manus
eata_ Hall, the most characteristic index : Lt
poser eae Beacroft limestone of the Ron=- 1S broken, (d) the Mery charac-
Mit eaacre ater istic ye fOSsil, 4.75 dc euietaem ten
pseudogaleata, and (e) many crinoid stems.
The formation carries many fossils in addition to those given
above, among which are Spirifer concinnus, Uncin-
ulus’ camphellamnizs:
(15) New Scotland shaly limestone. Thin bedded, dark gray to
reddish sandy and shaly limestones. The rock breaks out in slabs
on weathering and develops red iron stains. It has especially
abundant fossils, the most characteristic of which are: Ortho-
thetes woolworthanus, Spirifer macropleura.
Other common ones are: Leptaena rhomboidalis,
Strophonella headleyama, Ripsdemella oblatas
Stropheodonta becki.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 43
(16) Coeymans limestone. Heavy bedded, dark gray, argillace-
ous and flinty limestone. The characteristic features for field
identification are (a2) abundant chert nodules, (b) the occurrence of
coral reef structure and heads of corals, Favosites helder-
Fig. 5 Spiriferf¥€macropleurfa (Conrad), the most characteristic index fossil
of the New Scoti:and beds in the Rondout region
beneia. 2the brachiopeds -Siteberella galéata and
Mpnypa metic ular is “are, very) common.
This formation has a thickness of about 80 feet and is rather
distinctly separated from the underlying Manlius. The Coeymans
is considered the base of the Devonic system of New York. It is
Fig.6 Sieberella galeata (Dalman), the most reliable index fossil of the
Coeymans limestone of the Rondout region
perfectly conformable upon the underlying series and it is evident
that in this region there was no important break in the progress of
deposition.
e Siluric strata. (17) Manlius limestone. Lime mud rock, fine
textured, dense, with plainly marked sedimentation lines, gray to
dark gray color. The most characteristic features in the field are
(a) fine texture, (D) sedimentation lines, as if laid down in quiet
waters as a lime mud, (c) solution joints sometimes enlarged to
4A NEW YORK STATE MUSEUM
cavelike form into which surface streams disappear (such as Pom-
peys cave near. High Falls), (d) mud crack surfaces (in lower
beds), (¢) occurrence of the fossil Leperditia alta.
Its abundant jointing and the tendency to develop solution cav-
ities from them is considered an objectionable character.
(18) Cobleskill and cement beds (limestone). It is not pos-
sible without the most painstaking, comparative, chemical and pale-
ontologic research to differentiate the cement layers from the
inclosing beds and to assign them all to the subdivisions that are
recognized in some previous publications,’ as the (a) Rondout
cement (0) Cobleskill limestone, (c) Rosendale cement, and (d)
Wilbur limestone. There are, however, two workable natudal ce-
ment beds, both at Rondout and at Rosendale, with a nonworkable
layer between each case, and also one between the lower
and the next underlying formation. Whether the two cement beds
at Rondout represent the Rondout and the Rosendale horizons
with the Cobleskill between, or whether they should both be re-
garded as Rondout with Cobleskill below, can not concern’ our
present problems. And again, whether or not the two cement beds
at Rondout are the same two that appear at Rosendale, or whether
they are equivalent only to the upper one with a new lower bed
(The Rosendale) added in this area and then with the Cobleskill
between these two as claimed by Grabau, does not alter the plain
fact that the whole series is a physical unit. It is a gray, rather
close texture limestone, resembling the Manlius proper, and con-
tains few fossils. It is perhaps even better yet to group all of
these limestone beds below the Coeymans into a single unit and
call it the Manlius series.
(19) Binnewater sandstone. Below the Manlius cement rock
series lies the 60-100 foot Binnewater. It is chiefly a well bedded
quartz sandstone, almost a quartzite in the upper beds with more
shale in its lower portion, in color varying from white to greenish
yellow and brown. The rock is rather porous in certain beds and
especially along the bedding planes and is not well recemented
where crushed by crustal movements. It is confined to the Rondout
valley.
(20) High Falls shale.2 Greenish to red argillaceous to sandy
shales. The exposures are often a brilliant red while the rock
1N: Y. State Mus. Bul. 92 (Grabau), p. 311-13; N. Y. State Mus. Bul. 80
(Hartnagel), p. 355-58; N. Y. State Mus. Bul. 69 (Van Ingen and Clark),
p. 1184, 1185.
2 The term given by Hartnagle. N. Y. State Mus. Bul. 80. p. 345.
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UOT
role)
I jJUSTUID Ye
(Ajddng 10}V\\ JO pleog oY} J0J
pussoy oy} JO spoeq ouUO0JSOUITT
et ee ae ene oe ee
(Ajddng s0}eA\\ JO prvog oy} JoZJ UMOIG “DO YL, Aq Ydeisojoyd) “suru jo poyjzott oy} pur
Speq Judd oY} JO 9UO JO dip pue ssouyoTYy} 9Y} SuLMoYs “A “N ‘oyVMOUUT “OUIM JUST UOJION OY} JO JOLIO}U]
9 a ee ae ee
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 45
from drill cores is seldom highly colored. The protected beds are
more commonly greenish in color and contain much iron sulphide.
Occasional thin limestone beds occur in the upper portion at High
falls —one of 4 feet forms the lip of the lower fall. The High
Falls shale is confined to the Rondout valley and on the line of
the aqueduct is 67-100 feet thick.
(21) Shawangunk conglomerate. The Shawangunk is a con-
glomerate and sandstone. The constituent pebbles are almost wholly
quartz, well worn, and varying in size from that of sand to pebbles
of several inches diameter. But for the most part the pebbles are
small, abundantly mixed with sand, bound together by a silicious ce-
ment. Rarely a true quartzite is developed and still more rarely a
shaly facies. The rock is therefore very hard, brittle, and in the un-
disturbed portions fairly impervious and resistant. But it suffers
from crushing along zones of disturbance in folding and faulting
and these zones are very imperfectly recemented. It is a durable
rock, very resistant to ordinary decay, but forms great talus slopes.
It is used for buhrstones (millstones), etc. It varies in thickness
on the lines of the aqueduct from 280-400 feet. The rock is lim-
ited in its northward extension to this district — southwestward
it is much more broadly exposed in the continuation of the Shaw-
angunk range.
The Shawangunk completes the conformable Siluro-Devonic
series down to the erosion interval at the close of the Ordovicic. ©
The series of conglomerates, sandstones, limestones, and shales
make an imposing column approximating 3000 feet of strata differ-
entiated with more or less ease into 15 separate and mapable
formations and a possible 5 or 6 more with careful paleontologic
work. The series begins with the capping beds of the Shawangunk
range and its northward extension toward the Hudson river at
Rondout and Kingston, and thence westward constitutes the rock
floor while its structures control the surface configurations far be-
yond the limits of the region under consideration. Immediately
to the north and partly within the area here treated is the famous
Rosendale cement region, the pioneer cement district of America
ands for" many) years ithe ybest » producer.) Ihe «strata used
are almost exclusively the upper members of the Siluric
(“cement beds”) closely associated with the Cobleskill between
the Manlius proper and the Binnewater sandstone. Rarely the Be-
craft from the Devonic series furnishes some cement rock.
f Cambro-Ordovicic formations. Between the Precambric
metamorphics of the Highlands beneath and the Siluro-Devonic
46 NEW YORK STATE MUSEUM
sediments of the Shawangunk range and the Catskills above, lies
a series of quartzites, limestones and slates less complexly dis-
turbed than the older and more disturbed than the younger series
—set off from both by unconformities representing time intervals
that cover both folding and erosion. They are of more than 4000
feet thickness —— how much more it is impossible to estimate be-
cause of the obscurity of data in the slates. There are very few
fossil forms preserved in them. The series is, however, readily
and sharply separable into three formations that may be mapped
upon lithologic characters alone. They are of most importance in
the Wallkill valley, Moodna creek, Newburgh, Fishkill, New Ham-
burg and Poughkeepsie districts. Their character, structure, and
conditions have required careful consideration in the decisions on
the Wallkill and Moodna siphons and in the discussions on the
proposed Hudson river crossings [see Hudson river crossings,
pt, 2]:
(22) Hudson River slates. The upper member of the Cambro-
Ordovicic series is in itself complex. Prevailingly it is a slaty
shale, occasionally it is a sandstone or shaly sandstone, or a simple
shale; still more rarely it is almost a true slate, and very rarely
a phyllite. The constituents vary from prevailing clay to quartz
sand repeatedly in almost every locality. It is probable that as a
rule the upper portions are the more heavily bedded and arena-
ceous. The rock is excessively affected by the dynamic movements
that have at least twice disturbed it. A slaty cleavage in the more
argillaceous members is most noticeable, but almost everywhere the
strata are strongly tilted, crumpled, broken, faulted, or crushed in a
most confusing way. This together with an original obscurity
in bedding, and the obliteration by subsequent shearing of much
that did exist, makes it impossible to reconstruct the complicated
structure or compute the thickness of the formation. It is of such
physical character as to absorb within its own limits much of the
disturbing movements, and neither the formations above nor imme-
diately below are so extensively and intimately affected. The
formation is widely exposed and forms the bed rock over very large
areas. Almost everywhere it is impervious to water, easy to pene-
trate by drill or tunnel, and resistant to decay. A few Ordovicic
fossils may be found, the most characteristic being Dalmanella
testudinaria.
(23). Wappinger limestone.1 (In part Cambric, and in part
ipa Rome, © Aloe see alien a) vik ee ca a
1The Wappinger Valley limestone of Dwight (1879) and Dana. The
Wappinger limestone of Darton and others.
nooijuog
‘OJVIOWO[SUOD YyuNnsur
(Ajddng 10}¥ A, jo pivog Aq ydessojoyq) ‘yu1od siyz t
LYS St AI0L SU, “osuer yunsuemeys oy Jo syvod
[}Wousq sassed jouuny
dU} fo IUQ) “ORION hoojuog
(A[ddng 19}e\\ fo pavog Aq ydersojoyg) ‘WorIstArp
YSINGMON 24} UO UOTJONAJSUOD JONpoNnbe 19A09-puv-jnd OJ Souo}spuRS PUL sSozVIS JOATY UOSpNyT Yysnoryy Youss, y
Q 31PT
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 47
Ordovicic). The formation is prevailingly of a compact, fine
texture, dark gray, either massive or strongly bedded limestone.
Where the stratification is very plain there are light and dark layers
and an abundant silicious intermixture. In many outcrops the rock
is sO massive that even the dip and strike are obscure. Some places
the rock is fine crystalline, almost a micromarble. On weathered
surfaces it almost always exhibits a crisscross etching which marks
the traces of rehealed cracks. From these it is seen that many of
the apparently massive compact beds have at one time been exten-
sively crushed. In many places there is scarcely a square inch
wholly free from these evidences. The formation is best exposed
in the wide belt that extends southwestward from the vicinity of
Poughkeepsie and crosses the Hudson at New Hamburg into the
Newburgh district. It undoubtedly underlies the slates in the rest
of the adjacent area. There are few fossils and they are rarely
found.
(24) Poughquag quartzite. Below the Wappinger limestone and
upon the upturned and eroded edges of the Highlands gneisses lies
a quartzite of variable thickness but which reaches at least 600 feet.
It is a strongly silicified quartz sandstone —a quartzite by indura-
tion. It is strongly bedded but seldom shaly. Traces of schistosity
may appear in certain zones and this is somewhat strongly developed
outside of the area at the type locality (Poughquag, N. Y.).
Only fragments of trilobite spines have been found in this forma-
tion within the district.
g Later crystallines south of the Highlands. South of the
Highlands proper except at one locality (Peekskill creek valley and
its southwestward continuation through Tompkins Cove and Stony
Point) the rocks are all much more thoroughly crystalline. There
are two formations, and in places traces of a third, above the Gren-
ville gneisses (Fordham gneisses and associates). These are known
locally as Manhattan schist, Inwood limestone, and Lowerre quartz-
ite. In Westchester and New York counties the quartzite
is rarely found, and in a considerable proportion of those places
where it does occur its relations are more consistent with the
gneisses below than with the limestone-schist series above. This
is true indeed of the type locality (Lowerre). There are, however,
at least two points where the occurrence favors the reverse inter-
pretation, so far as any is shown, and therefore a quartzite may be
regarded as finishing the series, and making uncertain but probably
unconformable contact with the underlying gneisses.
48 NEW YORK STATE MUSEUM
This series together with the gneisses below constitutes the bed
rock and controls the underground conditions for all of the line
south of the Moodna valley, 50 miles above New York. All of the
southern aqueduct, and the New York city distribution conduits are
wholly concerned with these rocks, and two divisions of the northern
aqueduct have a large proportion of their work in them.
It is not wholly clear what age these crystallines represent. It
is certain that the underlying gneisses are Grenville and that the
metamorphic quartzite, Inwood, Manhattan series, is Post-gren-
ville. It is possible that these latter are also Precambric. But
usage following the correlations of Dana’ and in the absence of
as good evidence from any other source has regarded them as the
Cambro-Ordovicic crystalline equivalents of the Poughquag-Wap-
pinger-Hudson River series of the north side of the Highlands.
The writer has elsewhere shown? that the evidence and arguments
are not all on one side and that considerable doubt may still be
entertained on that point. There is no object in following that
argument here or in modifying the treatment here followed of mak-
ing them a distinct series. Even if they should prove to be the
exact equivalents of the Hudson River-Wappinger-Poughquag
series the formations are physically so different and require so
different treatment in discussion that they must for our present
purpose be regarded as an essentially distinct series. From that
standpoint alone the usage here followed is justified. The Man-
hattan schist of Westchester county as a type differs as much
petrographically from the Hudson River formation of the New-
burgh district as the Catskill formation of Slide mountain differs
from the Jameco gravels of Long Island. In a discussion where
physical or petrographic character is in control there is no doubt
about the advisability of treating the two separately.
(1) Manhattan schist.2 This is primarily a recrystallized sedi-
ment of silicious type. It occurs as a nearly black or streaked,
micaceous, coarsely crystalline, strongly foliated rock. The chief
constituents are biotite, muscovite and quartz. Quartz, feldspar,
1Dana, J. D. On the Geological Relations of the Limestone belts of
Westchester county, N. Y. Am. Jour. Sci. 20:21-32, 194-220, 359-75, 450-56
(1880) ; 21 :425-43; 22:103-I9, 313-15, 327-35 (1881).
2 Berkey, Charles P. “Structural and Stratigraphic Features of the
Basal Gneisses of the Highlands.’ N. Y. State Mus. Bul. 107 (1907),
p. 361-78.
3 Manhattan schist of Merrill. N. Y. State Mus. 50th An. Rep’t, 1:287.
Same as “ Hudson schist,” of N. Y. city folio no. 83.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 49
garnet, fibrolite and epidote also occur in large quantity. Occa-
sional streaks or masses are hornblendic instead of micaceous.
These are interpreted as igneous injections. They are especially
abundant on Croton lake and near White Plains.
It is essentially a quartz-mica schist. But it is almost everywhere
very coarse textured and hardly ever exhibits the fine grained, uni-
form structure of typical schist. Its abnormal make-up — the pre-
dominance of biotite and quartz —is the best defense for its petro-
graphic classification. The abundance of mica makes it a tough rock
but not very hard. The joints and fractures formed in later move-
ments are not healed and zones of bad shattering are susceptible to
considerable decay. These crushings are sufficiently common to en-
courage borings to tap their content of water for small family use
throughout Westchester county; but they do not represent large
circulation in any case. On the whole, the rock if fresh is good
and durable. It may, though rarely, carry considerable sulphide.
Practically all of the strictly original sedimentation marks are de-
stroyed by metamorphism. The formation has great thickness, but
because of the destruction of original bedding lines by recrystalli-
zation and additional complication by most complex folding, shear-
ing, crushing and faulting, the structure can not fully be unraveled
and the thickness can not be estimated with any approach to ac-
curacy of detail. But there is probably a thickness represented of
several thousand feet.
(2) Inwood limestone or dolomite. This formation lies beneath
the Manhattan. It is everywhere coarsely crystalline either massive
or strongly bedded, often very impure with development of second-
ary (recrystallized) mica (phlogopite) and other silicates, espe-
cially tremolite. It is essentially a magnesian limestone or dolomite
in composition. There is an occasional quartzose bed in the midst
of the limestone as at East View. The upper beds are most charged
with mica and occasionally beds attacked by alteration have much
green, flaky chlorite. There are occasional interbeddings of lime-
stone and schist as a transition facies.
The coarser grades upon exposure to weathering readily vield by
disintegration to a lime (calcite) sand resembling rouchlv an ordi-
nary sand in general appearance. At Inwood, the type locality, this
disintegration is so pronounced that great quantities are readily
shoveled up and used for various structural purposes in the place
of other sand. This dolomite is especially liable, as now shown by
extensive explorations, to serious decay to great depth. The under-
ground circulation seems to attack the micaceous beds with great
50 NEW YORK STATE MUSEUM
success and in some places the residue after this solvent action is
of the consistency of mud. A nearly vertical attitude of the beds
accentuates the opportunity. The most troublesome piece of ground
encountered on the whole line of the New Croton aqueduct, con-
structed in 1885, was in a weak zone and crevice in the Inwood near
the village of Woodland on the margin of the Sawmill valley [see
discussions of Bryn Mawr siphon and New York city distributions
in part 2].
The thickness probably varies but in many places where there is
only a narrow limestone belt it is due more to shearing or faulting
out than to original thinning. The most satisfactory estimates are
based on the explorations at Kensico dam and the field observations
at 152d street. They indicate an approximate thickness of 700 feet.
But in all cases either the margins are obscured or there is possibility
of faulting to modify measurements. There are no fossils. Weath-
ering and erosion has almost everywhere developed valleys or de-
pressions especially small tributary valleys in all formations, but as
pointed out years ago by Professor Dana the principal valleys pre-
vailingly coincide with the limestone belts.
(3) Lowerre quartzite. At Hastings-on-Hudson and again
near Croton lake, there is a quartzite that appears to be
conformable with the Inwood above. There is possibly more than
50 feet. It is a simple, clean quartzite. The other quartzites of
Westchester and New York county have a more distinct relation-
ship to the underlying gneisses with which they are conformable.
The Lowerre of the type locality is of this second class. In the
great majority of places where this bed would be expected to occur
there is not a trace of it.
h Older metamorphic crystallines (Grenville series).' “ The
lowest and oldest, as well as the most complex in structure and rock
variety, of all the formations of the Highlands region of south-
eastern New York is essentially a series of gneisses.” Cutting
these gneisses as intrusions of various forms are a great number
and variety of more or less distinctly igneous types. In form they
vary from small dikes or stringers to great batholithic masses ; in
1This interpretation of the larger relations of the complex gneisses
constituting the basis of the series, lying below the Manhattan-Inwood-
Lowerre series, was presented by the writer under the title: Structural
and Stratigraphic Features of the Basal Gneisses of the Highlands. N. Y.
State Mus. Bul. 107 (1907). p. 361-78. The accompanying description is
largely an abstract of this paper.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 5I
the Cortlandt-series to the very acid granites of Storm King moun-
tain or the granophyric pegmatites of North White Plains; and in
relative age they likewise vary from a period antedating the chief
early metamorphic transformation of the Grenville to Postman-
hattan time. But these clearly igneous types attain a considerable
prominence as separable units in the practical consideration of the
problems of the project and on that account the chief ones will be
more fully described under the next group.
The older portion — the varicus schists, banded gneisses, quartz-
ites, quartzose gneisses, graphitic schists, and serpentinous and
tremolitic limestone, forming the complex through which and into
which the igneous masses have been injected — form together an
interbedded series that was originally a sedimentary group. There
is nothing known that is older in this region. Its characteristics and
relations mark it as in all probability the equivalent of the “ Gren-
ville” of the Adirondacks and Canada.
No single type and no single characteristic can be given as a
simple guide to the identification of this formation. The prevalence
of certain varieties or groups of these and the strongly banded
structure give a certain degree of character that forms a reason-
able working base. The formation includes banded granitic, horn-
blendic, micaceous and quartzose gneisses; mica, hornblende,
chlorite, quartz and epidote schists; garnetiferous, pyritiferous,
graphitic, pyroxenic, tremolitic, and magnetitic schists and gneisses ;
crystalline, tremolitic, and serpentinous limestones, aphi-dolomites,
serpentines and quartzites; pyrite, pyrohitite and magnetite de-
posits. This is the basal series. But it is complicated by a multi-
tude of bands of granitic and dioritic gneisses that represent
injections of igneous material at a time sufficiently remote to be
subjected to most of the early metamorphic modifications. The
equally abundant occurrences of quartz stringers and pegmatite
lenses though of later origin can nct be separated from this com-
plex mass and the whole must be regarded as a physical unit. The
occurrence of interbedded limestones and quartzites together with
a variety of conformable schists and banded rocks, marks the
formation as essentially an old recrystallized sediment.
No member of this older unit of the basal complex is sufficiently
prominent to indicate a great break or change up to the time of
the first great dynamic movements and igneous outbreaks. The
following comparatively constant members are sometimes persistent
enough to be considered formational units, but even more commonly
52 NEW YORK STATE MUSEUM
are obscure as to boundaries or are of too small development to
map separately.
(4) Interbedded quartzite. Always a quartzite schist and
always exhibiting conformity with the banded gneisses and schists.
This is regarded as the uppermost member.
(5) Fordham gneiss (Banded gneiss). Granitic and quartzose
black and white banded gneisses and schists of very complex com-
position and structure.
(6) Interbedded limestones. Crystalline. Interbedded, very
impure, serpentinous and tremolitic, granular dolomites, usually 2
to 50 feet thick, possibly reaching a thickness of more than 100
feet in a few cases.
(7) Older intrusive gneisses. Variable types, mostly granites or
diorites, strongly foliated sills.
Many are of very obscure relations. The line of close distinction
between recrystallized sediment, segregations accompanying that
change, and true igneous injection can not be drawn.
1 Special additional igneous types. Under this teenie are
included the massive or little modified, not at all or only moderately
foliated, igneous masses of later origin and local rather than re-
gional development. In some cases, however, they are of decidedly
controlling importance in the local geology and rise to the status
of definite formations. The most noteworthy of these within reach
of the aqueduct explorations are:
(8) The Storm King Mountain gneissoid granite
(9) The Cat Hill gneissoid granite (central Highlands)
(10) The Cortlandt series of gabbro-diorites (near Peekskill)
(11)9 The Peekskill sranite (east of | Peeksiall)
(12) The Ravenswood granodiorite (Long Island City)
(13) The pegmatite dikes and lenses (segregational aqueo-
igneous type)
(8) The Storm King gneissoid granite is one of the largest of
the clearly igneous and less completely foliated types. It constt-
tutes the whole of Storm King mountain and the larger part of
Crows Nest on the west side of the Hudson, and, crossing the river,
forms the chief rock of Bull hill and Breakneck ridge. It is a
rather acid, coarse grained, reddish granite with considerable
gneissoid structure in a large way [sce Hudson river crossings,
pt 2]. !
(9) The Cat Hill gneissoid granite is not essentially different
from the Storm King type as a physical unit. Its occurrence at a
(Ajddng 19}7eAA Jo pseog Aq ydeis0}0 :
GO 9UO O} SuIsuOjoq < rH) : oT Nh Ho Teen EEO Ree
J } fouo]od O}TUvIG poyulof A[peq & SI YOI oY] “UOISTAIp [[LySyo0g oY} UO JOUUN} UOSTIIvrL) OF [v}10d Ree,
GEGLOGY (OF THE NEW “YORK CIty) AQUEDUCT 53
different point (Cat hill), widely separated by other types from
the Storm King locality, and in rather large development, is worthy
of separate note. It is cut, of course, in the long tunne! through
Cat hill.
(10) The Cortlandt series of gabbro-diorites occupies an area
of about 20 square miles between Peekskill and the Croton river,
nearly all on the east side of the Hudson. It includes a very com-
plete range of coarse grained, massive, igneous rocks from soda
granites, grano-diorites and quartz-diorites to true diorites, norites,
gabbros, pyroxenites, and peridotites. They doubtless represent
stages or portions in the differentiation of a magma. The inter-
relations are only partially determinable, and the petrographic dis-
tinctions in detail are not useful here. The area occupied by the
Cortlandt series has an uneven hilly surface with no structural
trend, and makes the most striking contrast to the ridge and longi-
tudinal valley structure of the rest of the region of the crystallines.
(11) The Peekskill gramte, a white, or pink massive, very coarse
grained, soda granite, occupying approximately 4 square miles 1m-
mediately north of the Cortlandt area 2 miles east of Peekskill,
is believed to be genetically related to the Cortlandt series. The
evidence in favor of such a relationship has been gathered in the
prosecution of this work and has not been published. But it may
be said that the textures, structure, age, relationship to older crys-
tallines, interrelations with the Cortlandt series, consanguinity of
mineralogy, and composition all point toward the above relation-
ship. In essential relations, therefore, it is the acid extreme of the
Cortlandt series. Its economic features, however, are of sufficient
importance and its easy differentiation from the regular Cortlandt
types require that it should have separate treatment.
(12) The Rravenswood grano-diorite occurs chiefly in Brooklyn.
It is a slightly foliated mass intrusive in the Fordham gneiss and
is doubtless connected in origin with the sources of many of the
hornblendic intrusive bands in the Fordham and Manhattan forma-
tions in the district. It covers a known area of about 5 or 6 square
miles and may be more extensive. The rock is suitable for struc-
tural material and has required consideration in the study of “ Dis-
tributary conduits ” [see pt 2 East River section].
(13) Pegmatites. The pegmatites and pegmatitic granophyric
masses of all kinds are of almost universal distribution in the
foliated crystallines. They vary from quartz bunches or stringers
to pegmatitic lenses and irregular masses, and to definite granitic
54 NEW YORK STATE MUSEUM
or pegmatic dikes. In many places they constitute a large propor-
tion of the formation in which they occur. They doubtless vary
in age, but for the most part seem to belong to the later period of
_metamorphism. Many of them are massive and largely free from
foliation. They no doubt have a complex origin between simple
aqueous segregation on the one side and true igneous intrusion on
the other.
Summary of formations
Group a Quaternary deposits
(1) Glacial drift Occurs as a_ surface
Till and modified drift, extra mantle over nearly all
marginal outwash, sands and of the region under
gravels, etc. discussion, except the
immediate sea margin
UNCON FORMITY
Group b Tertiary and Cretaceous deposits
(2) Tertiary outliers
(a) Pliocene littoral deposits
(Bridgetons?) C
es . onfined to Long Is-
2 eciie Buy eae eae land, Staten Island
i
and the New Jersey
(3) Upper Cretaceous beds are
(a) Lignitiferous sand (marl series)
(b) Matawan beds (clay ae
(c) Raritan (clays and sands)
UNCONFORMITY
Group c Jura-Trias formations
(4) Palisade diabase intrusion Confined to the west
(5) Newark series of conglomerates, side of the Hudson
sandstones and shales south of the High-
lands
GEOLOGY OF THE NEW YORK CITY AQUEDUCT
UNCON FORMITY
56
Group d Devome strata
(6) Catskill, white and red conglom-
erate (1725 feet)
(7) Oneonta (upper flagstone) (3000
feet)
(8) Ithaca and Sherburne (lower flag-
stone) (500 feet)
(9) Hamilton and Marcellus shales
(flagstone and shales) (700
feet)
(10) Onondaga limestone (200 feet)
(11) Esopus and Schoharie — shales
(silicious) (800 feet)
(12) Oriskany and Port Ewen transi-
tion (100 feet)
(13) Port Ewen limestone and shale
(150 feet)
(14) Becraft limestone (75 feet)
(15) New Scotland shaly limestone
(100 feet)
(16) Coeymans cherty limestone (75
feet)
|
Confined to the Cats-
kills, the Esopus and
Rondout valleys, the
northern extension of
the Shawangunk
range, and Skunne-
munk mountain near
Cornwall
Group e Siluric strata
(17) Manlius limestone (70 feet)
(18) Cobleskill limestone and cement
beds (30 feet)
(19) Binnewater sandstone (50 feet)
(20) High Falls shale, including small
limestone beds (75-80 feet)
(21) Shawangunk conglomerate (250-
350 feet)
:
Confined to the Rondout
and Esopus valleys
and the northerly ex-
tension of the Shaw-
ano wmk) ran se,
through the cement
region of Rosendale,
Binnewater, Rondout
and Kingston, and a
small outlier at Skun-
nemunk mountain
56
NEW YORK STATE MUSEUM
UNCONFORMITY
Group f ‘Cambro-Ordovicic formations
(22) Hudson River slates, shales, and. Especially prominent as
sandstones (very thick) (Or-
dovicic) more than 2000 feet
(23) Wappinger limestone (1000 feet)
(in. part ‘Cambric and injipart
Ordovicic)
(24) Poughquag quartzite (600 feet)
(1) The Manhattan
(Cambric)
surface formations in
the Shawangunk
range, the Wallkill
valley, and the region
eastward and_ south-
ward to the High-
lands, on both sides
of the Hudson
Group g Later crystallines (South of the Highlands)
(Uncertain age)
schist, a thor-
oughly and coarsely crystalline
sediment of uncertain age —
generally supposed to be equiva-
lent to the Hudson River slates,
(Ordovicic) but here separated
without necessarily raising that
question because of their very
different physical and _ petro-
graphic character
(2) Inwood limestone (or dolomite),
a magnesian crystalline lime-
stone of uncertain age, generally
supposed to be the equivalent of
the Wappinger (Cambro-Ordo-
vicic), but here enumerated sep-
arately without necessarily rais-
ing that question because of
their very different lithologic
character and associates
(3) Lowerre quartzite, an occasional
quartzite of uncertain relations
and very limited development
]
|
|
|
|
|
|
|
|
|
|
|
|
|
Confined to the region
east of the Hudson
river and * sottht "or
the Highlands proper,
occupying the region
from the Highlands
to Long Island
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 57
UNCONFORMITY
Group h Older crystallines (Highlands gneisses)
(Grenville series of metamorphics and intrusives — Precambric)
(4) Interbedded quartzite. A Formations © character-
quartzose schist istic of the High-
(5) Fordham gneiss (chiefly lands and some of
sedimentary). Granitic larger ridges extend-
and quartzose banded ing southward to
gneisses and schists of ener Neways Mot icity A
very complex develop- we series, which in petro-
ment Bene graphic variety, is as
(6) Interbedded limestones | complex as all of the
(Grenville) associated | rest of the forma-
with the Fordham | tions of the region
gneisses together
vo Old intrusions. Large and varia- |
ble masses of granitic gneisses
of igneous origin cutting the
Grenville series, such as Storm
King granite, Cat Hull granite,
Che:
!
| Postgrenville in age
Group 1 Special additional igneous types
(8) Storm King gneissoid granite, These are masses of
Storm King-Breakneck district strictly igneous origin
(9) Cat Hill gneissoid granite. Garri- (except the pegma-
son district tite) > cand, vot Glarger
(10) Cortlandt series of gabbro-diorites. development = which
Peekskill-Croton district citer yy becatise yy (01
(11) Peekskill granite. A boss, related their abundance (peg-
to the Cortlandt series. Peeks- matites) or large area
kill district (Cortlandt) or eco-
(12) Ravenswood _ grano-diorite. A MO WiC yy ireare Ww hie’s
boss. Brooklyn, Long Island (Peekskill) or im-
City and Southern Manhattan portant bearing upon
(13) Pegmatites. Dikes, lenses, segre- the plans of the aque-
gations of general distribution duct (Storm King)
are worthy of sepa-
rate note.
58 NEW YORK STATE MUSEUM
3 Major structural features
In addition to the simpler structural characters of the strata,
already sufficiently emphasized in the individual descriptions, there
are numerous others of more general relation whose value and in-
fluence it is necessary to consider in many of the practical problems.
Those of most importance are the unconformities, folds and faults.
They are directly related to continental elevation and subsidence,
to mountain forming movements and denudation processes, to meta-
morphism and to igneous intrusion.
a Sedimentation structures. In the younger strata the prin-
cipal structures are those of bedding, stratification, conformable
succession, etc., characteristic of all sediments of such variety of
type. These are prominent in the older groups of formations down
to the crystallines, but the earlier Paleozoics are also affected so
profoundly by folding and faulting that attention is more concerned
with these induced or secondary structures.
b Unconformities. Time breaks, with more or less disturb-
ance of strata and accompanied by erosion, are numerous.
(1) That between the glacial drift and the rock floor is the most
profound. It causes the glacial drift to lie in contact with every
formation of the region from the oldest gneisses of the Grenville
series of the Highlands to the traces of Miocene beds of Long
Island:
(2) The interval between the Pliocene and the Upper Creta-
ceous beds is more obscure and hardly reaches the importance of
an unconformity. It is probably more nearly of the value of a
disconformity or of an overlap, and the very limited development
of the overlying beds in the region gives little chance for determin-
ing relations in much detail.
(3) The overlap and unconformity between the Cretaceous and
Triassic. A condition determinable only on the New Jersey side
of the Hudson river.
(4) The unconformity between the Triassic and underlying
formations of different ages. An interval representing mountain
development and extensive erosion, in which the chief movement
probably belongs to the close of Paleozoic time and includes the
Appalachian folding.
(5) Unconformity between Siluric and the Ordovicic strata. An
interval representing mountain development, folding and erosion,
in which the movement known as the Green Mountain folding took
place.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 59
(6) Unconformity between the Poughquag (Cambric) quartzite
and the underlying crystallines. An interval-in all observable cases
of great length and profound changes involving mountain folding,
metamorphism of the profoundest sort, and extensive erosion.
(7) Among the crystallines of the south side of the Highlands
there is one break of similar importance, between the Inwood lime-
stone and the underlying gneisses. Whether or not it is the same
as no. 7 above is not clear, but even if it represents the same break
the relations are somewhat different in degree and character because
of the lack of quartzite in almost all cases.
Within the gneisses of the Grenville series and their associates
of all kinds there are no breaks of the unconformity type known.
The contacts are eruptive in character, or are displacements
instead.
c Folds and mountain-forming movements. All of the forma-
tions from the oldest up to and including the Lower Devonic strata
are folded. Many of the smaller (minor) folds exhibit complete
form in the stream gorges of the district, but all of the larger ones,
the main folds, have in earlier time been eroded to such extent
that the series is beveled off and only the truncated edges are to
be seen, exhibiting strata standing more or less perfectly on edge,
and making restoration of the form a very difficult or impossible
task. This is only partially accomplished in the Siluro-Devonic
margin along the Shawangunk range; it is more complete in the
Cambro-Ordovicic north of the Highlands, and it reaches its most
perfect development in the crystallines of the Highlands and New
York and Westchester counties. These differences correspond
roughly to the differences in age of the strata, and, taken together
with the evidence of the profound unconformities, indicate that
mountain-forming movements of far-reaching importance visited
the region no less than three times. Each time of such disturbance,
of course, the underlying older series was affected by the move-
ments of that epoch in addition to any previous ones, and as a con-
sequence the older is to be expected to show more complexity of
such structures. Each succeeding series separated by such activity
is therefore one degree simpler in structure. |
Of these three epochs of great disturbance, one is (1) Precambric
and corresponds to the time interval marked by the unconformity
between the Poughquag quartzite and the gneisses: a second (2) is
Postordovicic and corresponds to the time interval marked by the
unconformity between the Hudson River slates and the Shawan-
60 NEW YORK STATE MUSEUM
gunk conglomerates, and the last (3) is Postdevonic (probably
Postcarbonic, judging from neighboring regions of similar history)
and has left as its most important evidence in this district, the
excessively complicated sharp foldings and thrusts of the Shawan-
gunk range and its extension in the Rosendale cement district.
Kinds. As to forms produced there are no usually described
types that are not to be found here. The simpler forms of anti-
clines and synclines, both open and closed, symmetrical and unsym-
metrical and overturned, are all common. The isoclinal is common
in the gneisses. In each epoch of folding the compression forces
were effective chiefly in a northwest-southeast direction producing
arches and troughs whose axes trend northeast-southwest. This is
the trend of the main structures throughout the region.
The extent of crustal shortening accomplished by this series of
compressions is undetermined, but that it -amounts to a total of
many miles is indicated by the fact that over broad areas the
strata stand almost on edge. Furthermore, in the older Highlands
and in portions of the Hudson river districts the folds have been
slightly overturned so that commonly the strata on both limbs dip
in the same direction (toward the southeast). This seems to indi-
cate a strong thrust from the southeast. All stages between the
gentlest warping to strongly overturned folds, and from minute
crumbling to folds of great extent and persistence are to be seen.
The effect of all the folding is chiefly to present a series of up-
turned strata to erosion and encourage a subsequent development
of valleys along the softer beds bordered by ridges of the more
resistant types.
As the axes of the folds lie in a northeast-southwest direction,
this gives a marked physiographic development of ridges and val-
leys of the same trend, a most conspicuous topographic feature of
southeastern New York.
d Faults. Accompanying the folding in each epoch, and
especially the stronger overthrust movements there has been a
tendency to rupture and displacement. These breaks are known
as faults. Multitudes of them are of minute prcportions and prac-
tically neglectable in a broad view, but many also are of large
extent, traceable across country for many miles and indicating dis-
placements in some cases of many hundreds of feet. For the most
part these faults are of the thrust type and wholly consistent with
the folds in origin. They run generally in a northeast-southwest
direction, especially the larger ones, and frequently form the sep-
aration planes between different formations. Occasional cross
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 61
faults occur (with northwest-southeast direction across the strike),
but so far as is known they are always of minor consequence. In
rare instances, the trace of a fault-line on the surface describes
curious curves, such as that at Cronomer hill above Newburgh,
apparently inconsistent with the chief structural trend, but a study
of the whole geologic relation in such cases shows them to be con-
nected with the projecting spurs of underlying formations which
in any large thrust movement plow their way with some success
through the younger overlying, less resistant, strata. They differ
in no material way from the cther more simple !ooking lines.
Both normal and thrust faults occur, but the thrust type appears
to be most common.
The amount of displacement cr throw is extremely variable. The
larger faults represent movements of several hundred feet. In
rare cases the movement may be as much as 2000 feet.
The effects may be grouped as follows: (1) the appearance of
formations out of their normal order, i. e. contacts between forma-
tions that do not normally lie next to each other; (2) the produc-
tion of escarpments, 1. e. steep cliff-bordered ridges; (3) the de-
velopment of zones of more or less extensively crushed rock along
the principal plane of movement; (4) the determination of loca-
tion for stream courses and gulches and valleys that cross the
formations.
All of these effects are more noticeable and better preserved for
the later movements than for the earlier ones. Many of those
dating back to the earliest epoch, affecting only the crystalline rocks
of the Highlands, are not readily detected. Most of the breaks
have been healed by recrystallization and the contacts are often
as close and sound as any other part of the formation.
But this is not so true of the later epochs — and in them a good
deal depends upon the type of rock affected. The more brittle and
hard and insoluble types are more likely to still have open seams
and unhealed fractures than the softer and more easily molded
formations. In some of these, recent water circulation has still
further injured the fault zones by introducing rock decay to con-
siderable depth. Because of the more ready circulation in them, it
is noticeable that some of the extensive decay effects are produced
in crystalline rocks that otherwise very successfully resist destruc-
tion. On the whole the softer clay shales and slates are less likely
to preserve open water channels of this sort than any other forma-
tion of the region.
62 NEW YORK STATE MUSEUM
No part of the region is wholly free from faulting effects, except
perhaps a part of Long Island. The Catskills also are very little
affected — so little that this type of structure has not require con-
sideration in the vicinity of Ashokan reservoir. But all parts of
both the northern and southern aqueduct system have had this
feature to consider.
Further discussion of the specific local problems introduced by
faulting and folding is given under the problems of part 2. A con-
siderably more extended comment on the age of fault movement
is given under the heading “ Postglacial faulting.”
4 Outline of geologic history
Most of the general features of geologic history have been
involved more or less in the foregoing discussion. It is im-
-possible to wholly separate matters that are so intimately inter-
related even though it 1s convenient to think of or consider one
phase at a time. But it may serve a useful purpose to summarize
the steps of progress as illustrated by local geology from the earliest
geologic time to the present.
a Earliest time. (Prepaleozoic, Agnotozoic, Proterozoic, or
Azoic Era). There is little doubt that the oldest rocks known in
this region are representatives of a time of regular sedimentation.
Conditions favored the deposition of silicious detritus of variable
composition with an occasional deposition of lime, nearly always in
very thin beds. What these sediments were laid down upon or
where they came from are unsolved questions. The remnants of
them that are still preserved are the basis of the “ Grenville series ”’
as interpreted in this area, and are the basal (oldest) members of
the “ Fordham” or “ Highlands gneisses.”
How long ago this series was deposited is not known. It can be
stated only approximately even in the rather flexible terms used in
historical geology. It is older than any Paleozoic strata (Pre-
cambric), probably very much older. It is even possible that this
series is as much older than the Cambric as that period is compared
to the present. In short, it is not known, and there is apparently
little immediate likelihood of finding out even to which of the sev-
eral subdivisions of the Prepaleozoic this series belongs. It is cer-
tain that before the Cambric sandstones of the Paleozoic era had
begun to form, this older series was disturbed by crustal move-
ments, folded, metamorphosed, intruded by igneous injections, ele-
vated above the water (sea) level of that time and eroded by sur-
face agencies. These movements and steps there is no doubt of.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 63
When subsidence! again depressed the area beneath the sea the
deposition of sands that we now call Cambric (Poughquag) quartz-
ite began. |
b Early Paleozoic time. With the sedimentation upon this old
crystalline rock floor a long time of apparently continuous deposi-
tion began which ultimately resulted in the accumulation of several
thousand feet of sandstones, limestones, and sandy or clayey shales
that are now known as the Cambro-Ordovicic series (Poughquag-
Wappinger-Hudson River series). But at the close of Ordovicic
time or late in that period another crustal revolution began. The
whole region was again compressed into mountain folds, faulted,
sheared, metamorphosed, elevated above sea level, and subjected to
erosion. This corresponds to the Green mountains folding of
Vermont.
With the next subsidence and a return of sedimentation a new
series began to form. The break marking the occurrence of all
these changes, known locally as the Postordovicic unconformity,
represents a considerable portion of Siluric time.
c Middle Paleozoic time. ‘The earliest deposits of this series,
which continued to accumulate through late Siluric and all of De-
vonic time, were heavy conglomerates very unevenly distributed
over the new rock floor. “These are the so called Shawangunk con-
glomerates, a formation that within the boundaries of this imme-
diate area and within a distance of 20 miles varies from a thickness
cf more than 300 feet to almost nothing. But for the most part,
sedimentation was regular and fairly continuous and of immense
volume. The whole series of conglomerates, sandstones, shales,
grits and limestones belonging to the later Siluric and the Devonic
are included. Not all are believed to be marine however. The
Catskill and Shawangunk conglomerates may well be of continental
type.
Long after the deposition of all of these strata another crustal
disturbance, for at least the third time, repeated the process of
mountain-folding and erosion. This was the time of the Appalach-
ian mountain-folding. In this region it caused a wonderfully com-
plex development of folds and faults that are especially important
and determinable as to type and age in the Rondout cement region.
The movement, of course, affected all of the older formations as
1 There may possibly be an intermediate stage, practically a duplication
of the whole as given above, between the very oldest and the Cambric,
represented in the “later crystallines,” but this may as well be neglected
for the present.
64 NEW YORK STATE MUSEUM
well, but on them, already disturbed by earlier displacements, the
features chargeable to the disturbance can not always be distin-
guished from older ones. All three of the mountain-forming com-
pressions seem to have been controlled by the same relationship
of forces and adjustments of movement, for the results are in each
case the production of folds or faults of similar orientation and a
final structure of uniform trend.
Deposition had been going on for ages, chiefly on the west and
north side of the older crystallines ; but with a return of sedimenta-
tion a decided reversal is noted. The Atlantic border is depressed
and much of the interior region seems not to have been subjected
to further deposition from that time even to the present.
d Mesozoic time. Again conglomerates, sandstones and
shales were laid down upon an eroded floor. From their condition
and lithology it is believed that they are partly of continental, flood
plain, origin. The series is thick, generally assigned to the Triassic
period and is extensively developed. During the time of accumula-
tion and to some extent subsequent to it, there was extensive
igneous activity pouring out and intruding basic basaltic matter in
large amount. The Palisade diabase sill, and the Watchung Moun-
tain basalt flows are the best examples.
At a later time small faulting occurred making frequent dis-
placements in this series. But mountain-folding has not again
visited the region. Such breaks as there are, are of the nature of
overlaps and disconformities rather than of the revolutionary his-
tory indicated by a true unconformity. One of these intervals
occurs in the Mesozoic between the Triassic and Cretaceous. Above
it the thick series of Cretaceous shales, marls, sands and clays are
developed. Succeeding this series a similar interval represents the
earliest Cenozoic time.
c Early Cenozoic time. The earliest Cenozoic (Eocene and
Oligocene) has no sedimentary record within this region.
There are small remnants of deposition representing Miocene and
Pliocene time. Above these again the record is blank up to the
time of the glacial invasion.
f Late Cenozoic time — glacial period. By some combina-
tion of conditions not very well understood, the chief features of
which no doubt are,—(1) continental elevation and (2) shifting
of centers of precipitation and (3) modification in the composition
of the atmosphere, a period of excessive ice accumulation was
inaugurated. Ice finally covered immense continental areas and
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 65
from its own weight by continuous accumulation spread out
(flowed) from great central areas toward the margins. There is
clear evidence of interruptions or advances and retreats of this
general movement many times. But the same type of work and
similar results were attained in each case. The chief features of
this work was the moving of rock material frozen in the ice to long
distances and the deposition of it again, more or less modified by
its contact with the ice or by the effect of water upon its release,
at other places and with entirely new associations. The tendency
to ice accumulation was finally overcome to sufficient extent for the
inauguration of the present condition of things. Whether it is a
permanent change or only an interglacial interval is not clear.
But the ice has withdrawn to the mountains and the polar north
at the present time. It has not occupied the surface of this region
probably within the last 40,000 years, and perhaps for a much
longer time.
5 Outline of geographic history — physiography
The surface features of a country are the result of the working
out of a long and complex series of processes with and upon the
materials of the rock floor or bed rock. The relationship of surface
features to the formations that occur in the rock floor and their
stages of development, in short, an interpretation of their origin and
meaning, constitutes geographic history or physiography. — It differs
little in essential character from geologic history, of which it is only
a special branch, i. e. the history of surface configuration. And it
can not be appreciated or understood except in the light of a
thorough knowledge of stratigraphic and structural geology. In
individual cases or particular regions the geologic knowledge must
also he specific.
a Early stages. Occasional glimpses of surface features, and
some scattered facts about their development are to be gathered of
older continental existence. Surface features characteristic of their
time were developed in the great intervals between each successive
period of continuous deposition. Traces of them are involved in the
unconformities of the geologic column already shown in the discus-
sion of geologic history. Hills, valleys, streams, shores and all the
appropriate assortment of forms must have existed. But they
could not have been like those of the present in many minor fea-
tures — especially in arrangement and distribution— because the
bed rock of those times had only in part reached the complexity of
66 NEW YORK STATE MUSEUM
structure and composition now belonging to it. Many items of im-
portance are indicated in some of these early periods. For ex-
ample, the sea encroached on the land borders repeatedly from the
westward — especially throughout Palezoic times, while in Meso-
zoic and Cenozoic times the evidence of shiftings of sea margins
is confined to the east and southeast borders, and likewise probably
no near by place has been continuously beneath the sea.
But the unraveling of these conditions is obscured by subsequent
events. Land surfaces that once were, became covered by later
sediments. The physiography of those times, Paleophysiography, as
well as paleogeography, is therefore a difficult and intricate line of
investigation. With these ancient surfaces the dicussion of present
features has little to do. Here and there the present surface cuts
across and exposes the edges of an older one giving traces of the
old profile; but in most cases it is so distorted by the foldings and
other displacements belonging to a later period that a restoration
of the original continental features is a task fit for the most highly
trained specialist.
The surface as it now exists, and the rock floor modified only by
the inequalities of the loose soil mantle, yields more readily to in-
vestigations of origin and history. |
b History of present surface configuration. On some por-
tions of the region there seems to have been no deposition since the
close of Paleozoic time. Throughout most of Mesozoic and Ceno-
zoic times, therefore, those regions probably have been continuously
land areas (continental) and have been subjected to the agencies
of erosion. This applies particularly to the Highlands region and
the Catskills and the Shawangunk range and intervening country.
What the surface configuration was like in the early stages is
wholly unknown. In the beginning, mountain-folding — the Appa-
lachian folding — was in progress and the features were probably
those of partially dissected anticlinal folds. With the progress of
erosion the Triassic deposits were accumulated along the eastern
border, probably on the continental slopes. Subsequently, further
elevation extended erosion over the Triassic areas also and the
Cretaceous beds were laid down on the margin. The general lines
of development have been the same from that time to the present.
Each successive important formation less heavily developed and
forming a band outside of and upon the older one — the whole now
constituting a series of successive belts the oldest of which is far
inland and the newest at the sea margin.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 67
Therefore, when long periods of denudation are referred to, it is
well to appreciate that this is especially applicable to the interior,
that the sea margins are comparatively new, and that certain of the
inland areas were suffering erosion long before the rock forma-
tions that lie beneath and form the rock floor of the sea border
districts were in existence.
Cretaceous peneplain. It appears from studies of these problems
in a broad way, and, drawing upon generalizations from continental
features of a much larger field than that of the present study, that
the continental region of which this forms a part must, in the
earlier periods, have remained in comparatively stable equilibrium
for an extraordinarily long time. So long a time elapsed that most
of the area was reduced by erosion to a monotonous plain (pene-
plain) at a very low altitude, probably not much above the sea
(base level). Only here and there were there areas resistant enough
or remote enough to withstand the denuding forces and stand out
upon the general plain as remnants of mountain groups (Monad-
nocks). Possibly the Catskill mountains of that day had such
relation.
This reduction of surface feature it is believed was reached in
late Cretaceous time. The continent stood much lower than now.
Portions that are now mountain tops and the crests of ridges were
then constituent parts of the rock floor of the peneplain not much
above sea level. This rock floor was probably thickly covered with
alluvial deposits (flood plain) not very different in character from
the alluvial matter of portions of the lower Mississippi valley of
today.
Upon such a surface the principal rivers of that time flowed,
sluggishly meandering over alluvial sands and taking their courses
toward the sea (the Atlantic) in large part free from influence by
the underlying rock structure. The ridges and valleys, the hills,
mountains and gorges of the present were not in existence, except
potentially in the hidden differences of hardness or rock structure.
Such conditions prevailed over a very large region — certainly all
of the eastern portion of the United States. This so called ‘Creta-
ceous peneplain is the starting point in development of the geo-
graphic features of the present.
Continental elevation. Following upon this period of stability
and extensive denudation came one of continental elevation. How
much above sea level this raised the areas under present discussion
may not be determined, but that it was a sufficient amount to
3
68 NEW YORK STATE MUSEUM
rejuvenate the streams and permit them to begin the sculpturing
of the land in a new cycle of erosion is perfectly clear. As soon
as the elevation and warping of the continental border made its
influence felt in the increased activity and efficiency of the streams
(rejuvenation) they began transporting the alluvium of their flood
plains and to sink their courses through this loose material to bed
rock. ‘The final result of long continued denudation under these
conditions in early Tertiary time was the removal of the loose
mantle and the beginning of attack on bed rock (superimposed
drainage). The streams formerly flowing on alluvium that had
now cut down to rock found themselves superimposed upon a
rock structure not at all consistent with their former courses.
With the progress of erosion on this rock floor all these differ-
ences of structure, such as the differences in hardness of beds,
the trend of the folds, the strike of the faults, the igneous masses,
etc., were discovered and the streams began to adjust their courses
to them. Valleys were carved out where belts of softer rock
occur, ridges were left as residuary remnants where belts of harder
rock exist, and the surface (relief) took on some of the char-
acter of present day lines. That is, the principal mountain ranges
of that time were the same as those of today in position and
trend; but they had not so great apparent hight because the in-
tervening valleys had not yet been cut so deep. The principal
escarpments of that time were due to the same structural lines
as those of today, only they have shifted somewhat along with
the general retreat of all prominences by the forces of weathering
and erosion. |
In the course of this work of sculpturing and the shifting of
valleys and divides and escarpments and barriers into constantly
greater and greater conformity with rock structure, it came about
by and by that practically all of the smaller and tributary streams
had so completely adjusted themselves to their geologic environ-
ment that their valleys almost ‘everywhere followed along the
softer beds (subsequent streams), the divides were chiefly of
harder beds, the trend of both were almost everywhere parailel to
the strike of the rock folds and other structures (adjusted drainage).
This undoubtedly involved in many cases a very radical change of
stream course, and in some cases an ultimate reversal of drainage
to such extent that tributaries were deflected inland against the
course of the master streams and in some cases actually flowed
many miles in this reversed direction before finding an accordant
junction (retrograde streams). At least three of the streams of
‘
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 69
southeastern New York are still of this type—the Wallkill, the
Rondout and the lower portion of the Esopus.
But the larger rivers, the great master streams, of the super-
imposed drainage system, in some cases were so efficient in the
corrasion of their channels that the discovery of discordant struc-
tures has not been of sufficient influence to displace them, or re-
verse them, or even to shift them very far from their original direct
course to the sea. They cut directly across mountain ridges be-
cause they flowed over the plain out of which these ridges have
been carved and because their own erosive and transporting power
have exceeded those of any of their tributaries or their neighbors.
They are superimposed streams (not antecedent), they have, with
their tributaries, settled down in the ancient plain, and, by their
own erosive activity, have carved the valleys deeper and deeper,
cutting the upland divides narrower and narrower until now only
here and there a ridge or a mountain remnant stands with its crest
cr summit almost reaching up to the level of the ancient pene-
plain on which the work began. If the transported matter could
all be brought back and replaced in these valleys the old plain
might be restored, but the work would immediately begin al! over
again.
Of these great master streams the Hudson is the only local rep-
resentative [see Study of the Hudson River gorge in part 2].
Tertiary incomplete peneplanation. Such processes, ii allowed ta
continue on a stable continental region. would ultimately reduce
the land for a second time to a monotonous plain (complete cycle
of erosion). The beginnings of such a plain would be made in
the principal stream valleys upon reaching graded condition. Their
lateral planation and the development of flat-bottomed valleys
would begin at about the level that the plain would stand in the
final completed stage. The difference of elevation between the
ridge crests or hilltops and these flat valleys, 1. e. between the old
peneplain and the new unfinished one would be an approximate
measure of the amount of the continental elevation that instituted
the new cycle. :
But judging from such remnants of this later plain as are to
be seen, the two, i. e. the old Cretaceous peneplain and the new
Tertiary peneplain are not parallel. Toward the southeast, toward
the sea, the oider plain descends more rapidly than the younger
and intersects it. Both pass beneath sea level in that direction.
The difference between them therefore varies with locality from
7O NEW YORK STATE MUSEUM
o feet to perhaps 2000 feet within the borders of the area (con-
tinental tilting or warping).
Late Tertiary reclevation. Traces of such an intermediate and
incomplete peneplain are to be seen in the compound nature of the
large valleys of the present day. Most of them are essentially broad
valleys into the bottoms of which narrower valleys and gorges are
cut. The tops of the minor hills and ridges of the broad valleys
represent the intermediate Tertiary peneplain that was interrupted
in its development before completion (interrupted erosion cycle).
The inner narrow valleys indicate that for the second time a re-
gional elevation rejuvenated the streams and they began their
work of cutting to a new grade. They have made a good begin-
ning at this task, and as a consequence have carved some reliet
in the old valley bottoms. These new streams have not yet reached
a graded condition. .
When the glacial ice began to invade this region all of the surface
features had had such a history. Leaving out of account minor
fluctuations of elevation and depression, of which there may have
been several of too transient character to make a lasting impres-
sion on the topography, the stages become comparatively few and
the general tendencies are easily understood.
The measurable differences of elevation between the Cretaceous
and Tertiary peneplains give some reasonable conception of the
amount of the first continental or regional elevation. Concerning
the altitude reached in subsequent regional elevation there is less
certainty. None of the streams, not even the master streams such
as the Hudson, reached grade, for it exhibits strictly a gorge type
not only within’ the present land borders, but it is now known to
show gorge development far beyond the present coast line. Judg-
ing from the Hudson, therefore, it seems necessary to conclude
that this continental region stood at a much greater elevation in
some portions of the later period than had formerly prevailed.
Probably the maximum elevation immediately preceded the glacial
invasion.
Conservative estimates as to the amount of elevation of that
time in excess of the present would place it at not less than 2000
feet. Much more than that is believed to be indicated, possibly
5000 feet or more.
In the meantime, the master stream, the Hudson and several
of the tributaries cut into their valley bottoms to such extent as
to make typical gorges so deep that their beds now, since the sub-
GEOLOGY OF THE NEW YORK: CITY AQUEDUCT 71
sidence, lie much below sea level. The Hudson bed is of this
character throughout its course from Albany to the Atlantic, and
in the Highlands, 60 miles inland, the known rock bed at one point
is more than 700 feet below sea level.
In late glacial time there was still greater subsidence (50-100
feet) than the present as is indicated by terraces above present
water level and the deltas formed at the mouths of tributary
streams.
Such in general outline is the history of successive conditions
governing the topographic development of the rock floor. The suc-
cession of periods of stability, elevation, stability again, reelevation
and subsidence have had an effect on all sorts of formations, but
the extent of the impress and its permanence varies greatly in
the different districts. It is not possible to study these differences
in detail here. They are the minor and special local characters that
are in control at particular localities. In discussions of special
problems some of these are taken up in more detail. But in each
case the general history as outlined above, together with the modi-
fying influence of known local structure and stratigraphic char-
acter are the foundations of a working understanding [see Hudson
River crossings, Moodna creek, Rondout valley, etc., pt 2].
Pleistocene glaciation. An additional modification and one largely
independent of and largely inconsistent with the distribution of the
smaller features of the rock floor is introduced by the glacial drift.
It covers almost everything, but so unevenly as to largely destroy
some of the detail. It is in places more than 350 feet thick (as
in the Moodna and Rondout valleys) and in others it amounts to
nothing. It covers the narrow ravines and gorges heaviest and
has altered the courses of many of the smaller streams, the original
channels being hopelessly buried. The result has been chiefly one
of reducing the ruggedness of outline that prevailed along the
newer gorges of late preglacial time.
Besides this the usual surface forms characteristic of glacial de-
posits, occur — the kame, the drumlin, the esker, the hill and ket-
tle topography of the terminal moraine, the overwash plain, the
delta, the lake deposit and the gentle undulations of the ground
moraine. These are superimposed on the rock floor features. Both
are equally important to understand in the problems that have been
encountered. Which set of factors is to be most regarded in a
given case depends wholly upon the locality and the kind of en-
terprise or work it is proposed to undertake.
72 NEW YORK STATE MUSEUM
c Physiographic interpretation. Rock floor contour is an ex-
pression of the differences in character and structure of the bed
rock formations themselves, brought about by ordinary surface
weathering and transporting agencies, varied in their action and
effects only by certain differences in elevation above the sea. It
is apparent therefore that it would be possible by careful observa-
tion of surface features to gather data sufficiently definite to fur-
nish a basis for suggestions about hidden and hitherto unknown
or undiscovered structural and stratigraphic characters. But the
application of it to practical engineering problems is a complicated
and difficult matter. And this difficulty is nowise simplified by
the occurrence of a drift soil that tends to obscure many of the
more delicate features. For example, the later narrow stream
gorges marking the stage of extreme regional elevation are com-
pletely buried. Only an occasional stream like the Hudson has
maintained its course unchanged and has begun excavating the
channel again. But even in this case, as will be shown under a
separate head, the work of reexcavation is only just begun and
the amount yet to be done and the corresponding original depth
of the gorge are wholly unknown.
Certain surface features, however, are readable and, considered
with due regard for all possible causal factors, give very useful
suggestions. From them one obtains clews as to (1) the attitude
or relations of the hard and soft beds and the weak zones, (2)
the dip and strike of strata, (3) the persistence of a formation,
(4) the occurrence of faults, (5) the direction of the chief dis-
turbances, (6) the resistance and durability of local rock types —
in short the structural characters of all kinds because differences
in the distribution of these characters have given the different topo-
graphic forms and geographic areas. They have made the features
of the Highlands look different from those of the Catskills, and
those of Wallkill valley different from the Croton. Because of
the long train of conditions with which these surface features are
each involved and the structures that they indicate they become
easily the chief factors in preliminary judgment of comparative
practicability of rival locations, and are the most reliable guide to
direction and character and extent of exploratory investigation for
many engineering enterprises.
d Physiographic zones. In summarizing the physiographic
data it appears that the following belts or zones may be regarded
as fairly distinct units:
GEOLOGIC
FORMATIONS —
Catskill and
Oneonta sand-
stone conglom-
erates
Sherburne flags
Hamilton and
Marcellus shales
Onondaga lime-
stone
Esopus grit
The Helderberg
series
Shawangunk
conglomerate
Hudson River
shales, sand-
stones and slates
Wappinger lime-
stone
Poughquag
quartzite
Storm King = --_
“~granite ,
The Highlands
gneisses
Plate 12
The Catskill |
mountains
Ashokan [reservoir
Hamilton escarp-
ment
Esopus creek
High Falls
Rondout creek
Shawangunk
mountains
Wallkill river
Hudson river
New Hamburg
Wappinger creek
Fishkill creek
Newburgh
Breakneck
Mountain
Storm King
mountain
Bull mountain
Crows Nest
Foundry brook
Cold Spring
West Point
Relief map of the region from the Catskill mountains to the Highlands ©
showing the principal physiographic features.
also the areal and structural geology.) | Ie
physiographic laboratory of Columbia University by Messrs Billingsley,
Grimes and Baragwanath)
(The original model shows
(Taken from model made in the
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 73
(r) Coastal plain. A district underlain by Cretaceous and later
recks and confined to a part of Staten Island and Long Island,
not exceeding 400 feet relief. This zone is characterized by den-
dritic drainage, except a narrow belt on its inner margin which
is a longitudinal valley of the “inner lowland” type. Long Island
sound occupies the position of this old adjusted valley.
(2) Piedmont belt. A district lying between the coastal plain
and the Highlands. It is underlain chiefly by crystalline rocks and
metamorphosed sediments. Not exceeding 800 feet relief. It 1s
characterized by adjusted drainage obscured only by drift. The
ridges and valleys trend northeast and southwest close together
and with very little variation on the east side of the Hudson,
while on the west side the gentle dips of the Triassic give broader
and more unsymmetrical forms with dip slopes and escarpments
wholly independent of the opposite side. The zone is essentially
transitional between the simple forms of the coastal plain and the
complex mountainous character of the Highlands.
. (3) Highlands. The rugged elevated zone formed by the crys-
talline gneisses. Reaching elevations of 1600 feet. It is character-
ized by irregular mountain masses and lofty ridges of a general
northeast trend but with many prominent irregularities both: of
form and of drainage. The valleys are deep and narrow. There
are many steep escarpments. It is a mountainous zone in which
complex structures and rocks have led to the development of com-
plex forms. The zone forms a sort of barrier 20 miles wide across
the Hudson river which exhibits its most zigzag and narrow and
gorgelike development in this district.
(4) Appalachian folds. Characterized by folded Paleozoic rocks
north of the Highlands. Reaching elevations of 1500 feet rarely
— general relief 400-800 feet. North of the Highlands the relief
is much less pronounced. The softer rocks of the early Paleozoic
formations permitted the development of a broad valley with almost
perfectly adjusted tributaries, most of which on the west side of
the Hudson are reversed. The topographic forms give expression
to the universal folding and faulting of the formations. It is
essentially a transition from the complex mountain zone of the
Highlands to the much simpler Catskill area.
(5) Catskill Monadnock group. Characterized by undisturbed
Paleozoic strata and very strong relief — reaching elevations of
3500 feet. The eastern margin is an escarpment facing the Esopus
and Rondout valleys which are adjusted to the gently dipping
strata of that side. Over the rest of the district the beds lie so
74 NEW YORK STATE MUSEUM
flat that drainage is essentially dendritic modified slightly by joint-
ing. The great relief of the Catskills is due wholly to erosion
of flat but very resistant strata that withstood the destructive ero-
sion of Cretaceous peneplanation and stand as residuary rem-
nants even to the present time. The Catskills are therefore essen-
tially a Monadnock group. In structure they are almost as simple
as the higher portions of the cuesta of Long Island, and they hold
the same relation to the forms developed by erosion out of the
old Paleozoic coastal plain of the interior.
Summary
Physiographically the most complex zone is midway in the region
under discussion —1i. e. The Highlands. This belt is bordered on
both sides by less complicated zones of less relief, of more regular
topographic forms and less obscure history—— the Piedmont zone
on the south and the Paleozoic folds on the north. The outer mar-
gins are both simple, essentially eroded coastal plains with strata
dipping away from the central belts and on which forms and drain-
age lines characteristic of such history are developed. These outer
zones are the coastal plain of Long Island on the south and the
Catskill Monadnock group on the north. It matters little that they
differ in age by almost half of the known geologic column.
IOAIOSOI
ueyoysy JO jivd & WIIOF OF ST YOIYM Use [][FIOAvOg oY} SSOJOv BuTyOO] suIeJUNOW []PIS}VD IY} JO MIA V
CI 93eIg
Il
GCEOLOGIC PROBLEMS OF THE AQUEDUCT
INTRODUCTION
The group of studies assembled in this part are chiefly those that
have required considerable exploratory investigation in connec-
tion with the proposed Catskill aqueduct and that have furnished
new data of a geologic character. In some cases the additional
investigations have discovered new and wholly unknown structures
or conditions and in all cases the features as now established are
much more accurately known than would otherwise have been
possible.
The benefits of the studies have been twofold and reciprocal.
On the one side the practical planning of the enterprise has con-
stantly required an interpretation of geologic conditions as a guide
to locations and methods and on the other the extensive investi-
gations carried on have given an opportunity for practical appli-
cation of geologic principles under conditions seldom offered and
the data secured in additional explorations serve to make the detail
of some of these complex features now among the most fully
known of their kind. Examples of such cases are (a) the series
of buried preglacial gorges (as in the Esopus, and Rondout and
Wallkill and Moodna valleys) and (0b) the completed geologic
cross sections (such as the Rondout valley, the Peekskill valley,
Bryn Mawr, etc.) and (c) the numerous additions to the knowledge
of local rock conditions (such as that at Foundry brook, Rondout
creek, Coxing kill, Pagenstechers gorge, Sprout brook, and others).
Almost every locality has its own specific problem and its own
peculiar differences of treatment and interpretation of features.
Nearly all of the studies here presented came to the attention of
the writer and others! in the form of definite problems or questions
involving an interpretation of geologic factors and an application to
some engineering requirement. Some of these questions, as is
pointed out more fully in part 1, chapter 2, are (a) the location of
1 Professor James F. Kemp of Columbia University and W. O. Crosby
of the Massachusetts Institute of Technology and the writer constituted the
regular staff of consulting geologists.
[75]
70 NEW YORK STATE MUSEUM
buried channels beneath the drift, (b) the character and depth of
the drift, (c) the kind of bed rock, (d) the condition of bed rock
for construction and permanence of tunnel, (¢) the underground
water circulation, (f) the occurrence of folds and faults, (g) the
position of weak zones, (/) the depth required for substantial con-
ditions, and many other similar problems.
These need not be treated in their original form. Indeed many
of them have now ceased to be problems in any real sense, for sub-
sequent provings have made them simple facts, and wholly new
questions came to take their places. In some of the larger prob-
lems, however, it 1s believed that a treatment which involves a dis-
cussion of the original problem and the method of solving it, to-
gether with the data thus secured and the final interpretation of
geologic features as now understood or established will be more
instructive than a mere enumeration of the collected results.
So far as possible each problem is treated as a unit and fully
enough to be understood by itself. But a general knowledge of
Iccal geology as outlined in part I is assumed.
CHARTER I
GENERAL POSITION OF AQUEDUCT LINE
Surface topography constitutes the chief factor in determining
the general course of the aqueduct. It is planned to control the
water so that it will flow to New York city. There is therefore a
gradual descent of aqueduct grade from 510 feet A. T. at Ashokan
dam to 295 feet at Hill View reservoir. Wherever the surface of
the country is approximately the same as the aqueduct grade for
that district it permits of the so called “cut and cover” type of
construction which is much cheaper than any other. Therefore,
other things being equal, the position that will permit the greatest
proportion of cut and cover work would have a decided advantage.
Se it is possible from any series of good topographic maps to lay
out trial lines that are sure to be worthy of consideration. The
topographic sheets of the United States Geological Survey and the
niaps of the New York Geological Survey are of great usefulness
i such preliminary work.
But a little field examination shows that there are many other
features and conditions that materially modify even comparative
cost and are still more important factors in consideration of per-
manence and safety. Sometimes it is not apparent that a course
has any objectionable features till considerable exploratory work
has been done. Likewise a serious difficulty at one point may more
than counterbalance advantages at some other, so that considerable
portions of the line are finally shifted to a better average position.
In the course of these preliminary explorations much valuable data
have been secured that now relate to points a considerable distance
off the present line. The information has, however, been necessary
and useful.
One of the cases of this kind where geologic conditions have
had an almost controlling influence is involved in the choice of
place of crossing of the Hudson river. It has involved a shift of
the whole line between the reservoir and the Highlands. Diff-
culties encountered in finding a crossing of the Esopus also con-
tributed to the argument favoring a shift of the line [see map of
trial lines west of the Hudson]. One of the points where explora-
tory work had reached definite results before the more southerly
line was finally adopted is near West Hurley. Here wash borings
[77]
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‘ig. 7 Geologic cross section of the Esopus valley at West Hurley as indicated by a series of wash borings
STATE MUSEUM
were successfully put down through
the fine sands and silts of the
lower Esopus valley so as to give
a fairly acceptable profile of the
rock floor [see fig. 7]. Esopus creek
in this portion of its course follows
the Hamilton shales escarpment
which forms a steep border on the
west side, while the east border of
the valley and floor are formed
by the underlying Onondaga lime-
stone. Gentle westerly dips prevail
for both formations, so that in the
perfect adjustment reached before
the glacial invasion a cross section
would have shown a typical unsym-
metrical valley—one side a gentle
dip slope and the other a bluff de-
veloped by the undercutting of the
stream as it shifted against the
edges of the shales.
Results of exploration show that
the valley is filled to a depth of
more than 200 feet with silts and
sands that are essentially overwash
and glacial lake deposits. The flat
surface further favors this explana-
tion as had been pointed out before
any explorations were made. Later
observations in that portion of the
Rondout valley which is a continu-
ation oi this structural feature indi-
cate similar deposits as far south
as the new line at Kripplebush, Io
miles away.
In this instance at West Hurley
by careful measurement of dips on
the Onondaga limestone and the
Hamilton shales it was possible to
estimate the approximate depth to
which the Onondaga floor rock
would pass by the time the base of
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 79
the escarpment is reached. It was further believed that the cov-
ered portion is wholly drift-filled down to the Onondaga. It was
easy therefore to estimate the approximate profile and suggest the
point of greatest probable depth. The accompanying figure illus-
trates the form and structure of this valley. Each valley has had
in a smaller way a similar study and adjustment of location of line.
The final result is shown on the accompanying map which indi-
cates the course of the aqueduct as now being constructed.
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CHAPTER II
HUDSON RIVER CANYON
This is a special study of the Hudson river gorge’ based upon
explorations by borings at the several proposed crossings. Alto-
gether 226 preliminary borings were made on I4 cross sections.
The most important lines of borings are located at seven different
points on the Hudson [see location map]. Four of them are in the
vicinity of New Hamburg, lying not more than a couple of miles
north and south of that village, while three others are located within
the Highlands. [See comparative geologic study in following
chapter.] The chief basis of information on all but one of these
lines is the wash rig, a contrivance as already pointed out that gives
rather incomplete data [see Relative Values of Data, pt 1]. On
this account it is not possible to give the true bed rock profiles of
the river canyon even approximately except at one location, 1. e.
the Storm King—Breakneck mountain line. An occasional diamond
drill hole has been put down on some of the others and this has
_ been done systematically at the Storm King location in a persistent
effort to determine the gorge profile and bed rock condition.
The work already done has proven that in the Hudson at least
the wash rig borings give wholly unsatisfactory profiles. The holes
de not penetrate the boulders and heavy glacial drift that is now
known to fill the canyon. The profiles, however, that were drawn
from this sort of data have some value. They indicate that bed
rock is still lower and that the finer silts extend down to these
depths. In some places there is a heavier filling of 400 to 500 feet
below them before the rock floor is reached.
Wherever the diamond drill has succeeded in reaching rock the
formational identification has been made and the geological cross
section is a little more complete. As a matter of fact, however, at
almost every locality the structural relations are so complex or so
obscure that they are still not fully known. The accompanying
profiles and cross sections summarize the mass of accumulated data:
1 Kemp, Prof. J. F. Buried Channels beneath the Hudson and its Tribu-
taries. Am. Jour. Sci. Oct. 1908. 26:301-23. Some of the accompanying de-
scriptions of river crossings follow closely this excellent summary of Hud-
son river explorations from Professor Kemp.
[81]
82
NEW YORK STATE MUSEUM
Fig. 9 Key map showing the locations of lines of wash borings
forming the basis of the accompanying cross sections of the Hud-
son above the Highlands
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 83
1 Points of exploration’
a Tuff crossing. This line is a half mile above Peggs point.
Wappinger limestone forms the east bank of the river and Hudson
river slates the western bank. There seems to be no abnormal
structural relation of the formations. All data are from wash
borings. The accompanying section gives the results.
b Peggs point line. Peggs point is 2 miles north of New
Hamburg. At this location Wappinger limestone forms the east
bank and Hudson river slates the west bank of the river as in the
previous case. The limestone dips gently westerly while the slates
have a variable attitude. This is a normal relation and there is no
direct evidence of any great structural break. A large number of
wash borings have been made and five diamond drill holes were
driven, three of them in the river. None indicate a greater depth
than 223 feet, although there is a wide stretch, 1040 feet, not ex-
plored by the diamond drill. This space must contain the deeper
gorge if one exists here. From the known conditions at the
entrance to the Highlands, 10 miles further down stream, where
the channel is known to be more than 500 feet deeper, it may be
rather confidently asserted that a deeper inner channel does exist at
this point.
c New Hamburg line. This line crosses the Hudson from
Cedarcliff to the village of New Hamburg. The river is narrow —
only 2300 feet. There are no drill borings within the river channel,
but there is one on each bank. Both penetrate Wappinger lime-
stone first and then pass into Hudson river slates beneath. How
much of a gorge exists here is wholly unknown except in so far as
may be judged from the wash boring. There are the same reasons
for believing that a gorge exists as those noted for the Peggs point
line.
Structurally this line is probably the one of greatest complexity.
t is however perfectly clear that the abnormal position of the
slates and limestone on the east side of the river is caused by a
thrust fault. A similar relation of the slates and limestone on the
west side must be due to a like movement, but whether they are
separated portions of the same structural unit or of two adjacent
ones is not clear, although they are probably distinct
1 All of these explorations on the Hudson river have been under the direct
supervision of Mr William E. Swift, division engineer, in charge of the
Hudson River division.
84
Fig 10
NEW YORK STATE MUSEUM
g “y
¥ $
fe) Section LT" Section AVL’. Section MN!
a S Wappinger Creek
EN « 2 : +
ak . OG Sa RK Sv Scoala me Be
based upon wash borings. [For locations see key map, fig. 9]
Cross sections of the Hudson river north of New Hamburg and of Wappinger
creek
GEOLOGY OF THE NEW ,YORK CITY AQUEDUCT 85
_ Five lines of wash borings were followed, and the results of these
are indicated in the accompanying figures. A maximum depth of
263.5 feet is shown by these wash borings.
d Danskammer line. This line is about a mile south of New
Hamburg. Two lines of wash borings were made, reaching a
maximum depth of 268.5 feet. In this case slates standing almost
vertical form the east bank and limestone dipping gently eastward
the west bank of the river. Whether there is a deeper gorge or a
more complex structure here is wholly unknown.
Of the three remaining lines, all of which are within the High-
lands, that one projected between Storm King mountain on the
west and Breakneck ridge on the east has been much the most
thoroughly explored. It is known as the Storm King line. The
other two have seemed to merit less attention. One crosses the
river from Crows Nest mountain to Little Stony point and Bull
mountain just north of Cold Spring, and is known as the Little
Stony point line. The other crosses at Arden point about a mile
south of West Point and Garrison.
_e Arden point line. Only wash borings were made. A
maximum depth indicated by this method is 220 feet. Structurally
this location appeared to have disadvantages, and although the
evidence as to bed rock conditions is confined to the natural out-
crops, there is no doubt but that it has objectionable features of this
sort.
The Hudson follows closely the structural control in this portion
of its course. These structural elements include the foliation, the
bedding of the original sediments, the subsequent shearing zones,
and the strike of folds and faults. Crushed and sheared zones are
nowhere in the Highlands seen so extensively developed as on the
islands and the east bank of the Hudson in this, the central portion
of its Highlands course. The river is very narrow, being only 2120
feet on this line.
f Little Stony point line. The river here is 2360 feet wide.
The rocks on each side are similar and give no clue to possible
depths of channel. Less than 200 feet was reached by the lines of
wash borings. Three drill borings penetrated the stony or bouldery
river filling somewhat deeper — one near the center reaching 322
feet. None, however, reached bed rock.
g Storm King crossing. Extensive exploratory work has
been carried on at this point, both on the banks and in the river.
Wash borings as usual have given poor results. Two diamond drill
holes were run at an angle toward and beneath the margins of the
86 NEW YORK STATE MUSEUM
LIME -AILN SovTH RANGE Section ZT
3 Water
Sand
/
Soft Clay
=
LIME AItN NoRTH FANEE Seclion PIN.
Peces Point 2evTrH RANGE Section 0.R
Mean High Water
an 8 =
Fig. 11 Cross sections of the Hudson river near New Hamburg based on wash borings
L[For locations see key map, fig 9]
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 87
river, and in addition a working shaft suitable for permanent use
has been started on each side of the river. These have thoroughly
explored the rock character to-a depth of about 800 feet. It has
proven to be of constant type, a gneissoid granite, affected by
moderate amount of jointing, shear movements and occasional dike
intrusion. The two sides are alike, the rock in depth is com-
paratively free from water, nearly all coming from the adjacent
surface drainage.
Persistent efforts have been made to use the drill in the river to
explore the rock channel, but with meager results. The difficulties
to be overcome in drilling in this tidal river to the necessary depth
are probably greater than have even been encountered in any
similar undertaking. The disturbance presented by the current, the
tide, the depth of water, the drift filling above the rock channel,
and the traffic in the river are a constant menace. The complex
character of drift filling in this gorge, especially the occasional
heavy bouldery structure, makes it necessary to reduce the size and
recase the holes repeatedly. But in this regard the work has
suffered less actual loss than by the menace of river traffic.
Several times after the greatest efforts had been put forth in
pushing the drills deep into the gorge a helpless or unmanageable or
carelessly guided steamer or scow has wrecked the work. In this
way some of the most critical locations have been lost together with
many months of labor.
The results are shown on the accompanying drawings.
It is worth noting that of those holes located far out in the river
channel only two have reached bed rock. Even these two have
penetrated the rock so little distance that there might be still some
doubt of permanent bed rock. The fact, however, that the rock
found is of the right type, 1. e. like the walls of the gorge, leads to
the conclusion that the bottom was actually penetrated. Neither of
these holes are in the middle of the river, and, although the
maximum depth of 608 feet was reached by one of them, the central
portion of the buried channel proves to be still deeper. One hole
located near the middle was able to penetrate to a depth of 626 feet
without striking bed rock. But it was finally lost.. The latest
results are from a boring that has reached a total deptht (January
I, 1910) of 703 feet, the last 8 feet of which was believed by the
drillers may be in bed rock. All above is drift and silt.
1 Subsequent exploration has proven that the bottom of the old channel
lies still deeper. This boring has been pushed to a depth of 751 feet with-
out yet touching bed rock (Oct. 8, I9I0).
NEW YORK STATE MUSEUM
Van Sk aterm ¢
Ligne
Gravel
Dock RANGE Secttan Gl.
Mean High Frater
Cross sections of the Hudson river at four points between Danskammer Light and
Fig. 12
New Hamburg [see key map, fig. 9, for locations]
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 89
2 Discussion
The present facts therefore indicate that the buried Hudson
channel is more than 700 feet deep between Storm King and
Breakneck ridge. Furthermore this is more than twice as great
depth as has been found (so far as yet tested) at any other point
either above or below this place. Although data of this kind are
scarce yet there are two other borings that have given surprising
results —(a@) at Peggs point and (b>) the Pennsylvania borings at
New York city.
Peggs point. At this place, where studies were made for a
possible crossing, a hole 700 feet from shore struck rock at 223
feet and the unknown space or interval within which it is possible
for a channel to lie is less than 1040 feet wide. This is about Io
miles above the Storm King crossing and in much softer rock
(Hudson River slates). Yet the Storm King gorge in granite is
deeper than that (deeper than 223 feet) for a width of nearly 2500
feet. Of course, there may be, and probably there is, a much
deeper channel at Peggs point within the 1040 feet unexplored
space. But even so there is a remarkable discrepancy in width of
gorge at these two points that must be accounted for in some other
way than simple stream erosion.
The Pennsylvania borings opposite 33d st., New York city.
The data gathered by the Engineers of the Pennsylvania Tunnel
Company in their explorations for tunnel from 33d street, Man-
hattan, to Jersey City, have recently been made public. There are
six holes into rock. Their positions and depth to rock bottom are
given below:
a 800’ from New York bulkhead 190’ to bed rock = aplite
b 1000’ from New York bulkhead 290’ to bed rock = hornblende
schist
c 2180’ from New York bulkhead 300’ to bed rock = chloritic
and serpentinous rock.
d 2350’ from New York bulkhead 260'(?) to probable boulder =
jasper breccia
€ 3300’ from New York bulkhead 270’ to bed rock = arkose
sandstone
f 13700’ from New York bulkhead 225’ to rock—brown sand-
stone
There are other shallower borings on both sides of the river.
Those on the Manhattan side are represented by several different
facies of Manhattan mica schist and granite and pegmatite in-
go NEW YORK STATE MUSEUM
trusives, while the New Jersey side is represented by different
varieties of arkose and gray and brown sandstone belonging to the
Newark series.
It should be noted that although only one hole marks rock bottom
as low as 300’ (that one situated 2180’ from the New York bulk-
head about the middle of the river), yet there is at least a 1100 foot
space on each side which is essentially unexplored, and within one
of these spaces there may be a deeper gorge.
The cores taken from the east side of this middle zone belong to
facies of the Manhattan schist formation, while those on the west
side belong to the Newark series. The middle one, however, is
essentially a soapstone or serpentine and may be a continuation of
the Hoboken serpentine belt. In any case, it belongs in age to the
older series of formations.
It is certain that here again, 50 miles below Storm King locality,
a very deep gorge, if one exists, must be comparatively narrow.
Submarine channel. It is worth noting in this same connec-
tion that a submerged gorge has been mapped by the Coast and
Geodetic Survey on the continental shelf from the vicinity of
Sandy Hook to the deep sea margin, a distance of more than a
hundred miles. This is interpreted by Spencer? and others with
apparently sound argument as the lower portion of the old pre-
glacial Hudson gorge formed during an epoch of great continental
elevation. The outer portion of this submerged gorge is very deep.
That section near shore is shallow and obscure. It has been
assumed that this obscurity and shallowness is due to offshore and
iver deposition, filling the channel with silt. No better explanation
is yet forthcoming. But even here the width of the submerged
gorge is suggestive. In very much softer sediments than any en-
countered in its whole course on present land, and in a part of its
course from 50 to 100 miles below the other sections, the river has
cut a gorge only 4000 feet wide at top and 2000 feet deep within
a broader valley 5 miles wide. In its deepest known part the
proportions are 10,000 feet in width at top to 3800 feet in depth.
From this it would appear that the inner gorge type of develop-
ment is characteristic of the Hudson, and that it was originally an
exceedingly narrow one compared to the present river width, indi-
cating rapid erosion during a brief and comparatively recent epoch.
This submerged continental margin condition is favorable to the
1 Spencer, J. W. The Submerged Great Canyon of the Hudson River.
Aim. Jour Seeh1e05, Vv. 10.
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT OI
assumption that there are narrower, still deeper channels within the
unexplored spaces ‘both at New York city and at Peggs point.
The only known exception and the one really surprising section
is the Storm King crossing. It is too wide, considering the profiles
at Peggs point and at New York city for simple normal stream
erosion. Thatis clear enough. But a still more difficult question is
whether it is also too deep. It is much deeper than any known
section above or below for a distance of 50 miles.
There appears to be only one satisfactory explanation of this
abnormal width of the deeper section and that is by glacial erosion.
Just above Storm King is the wide bay opposite Cornwall and
Newburgh. The few glacial scratches observed trend about s. 15° e.
The ice therefore moved to the east of south, and it is noted that the
course of the river is about the same. The northern tront of Storm
King mountain is steep and trends east and west while the northern
front of Breakneck mountain trends southwest. It would appear
therefore that these slightly converging mountain fronts served as
sort of a funnel into which the ice was forced from the wide gather-
ing ground immediately above, and through which there may have
been a tongue or stream of ice of more than average power and
efficiency moving almost in direct line of the present course of the
river. It is reasonable to expect that these conditions would favor
more than average glacial erosion.
3 Storm King—Breakneck mountain profile
It is practically impossible to draw a complete profile for the
Hudson river gorge at any point in its lower course. Even at Storm
King mountain or New York city or at Peggs point, at each of
which places considerable exploratory work has been done, only the
broadest features are known. Nevertheless, several things have
been proven and they are worth considering in this question. They
may be summarized as follows:
a If there is a very deep gorge at Peggs point (deeper than 250
feet) it can not be over 1000 feet wide.
b lf there is a very deep gorge at New York city (deeper than
300 feet) it can not be over 1200 feet wide.
c At Storm King, located between the other two and in harder
rock than either of them, a gorge at least 400 feet deep is proven
to have a width of more than 1500 feet.
It is certain that simple stream erosion could not account for
such a difference of cross section. There is no doubt but that en-
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 93
larging by ice so far as widening is concerned is practically proven.
It may also be overdeepened, by which is meant that it may have
been gouged out deeper than could have been done by a stream of
water alone.
If ice action then be granted, the profile ought to be and prob-
ably is essentially an ice valley profile, 1. e. of a more or less U-
shape, rather than of typical stream erosion form. It is certain
also in this case, if glacial overdeepening is admitted, that there can
be no stream notch in the bottom of it. The significance of this
lies in the probability that the floor is approximately the same level
on a considerable portion of the bottom, so that when once the
margin of this floor is touched the gorge as a whole is thereby-
determined for depth.
After plotting the borings data and relying upon the factors that
seem to be most firmly established, it appears that the following
statements are as definite as the facts will warrant:
a The average slope of the Storm King side of the valley above
river level is nearly 38°, and this is in several steps or sections of
steeper and flatter slopes. The Breakneck side is about the same.
b The average slope of the Breakneck side of the gorge below
present water level (the side on which alone there are enough data >
to plot a fairly good curve) does not vary much from this same
value [see accompanying profile]. And it is also in steeper and
gentler slopes, apparently a series of U-shaped forms set one inside
the other, each inner one deeper than the next outer one. Each suc-
cessive inner step is approximately 300 feet deeper than the last
and 1000 feet narrower.
It is certain that this sort of profile is not as simple as at first
appears. The surprising feature is the close approximation of the
slopes above and below present river level. In view of the fact
that glacial widening has been practically proven, as shown before,
not much importance can be attached to this uniformity or simi-
larity of slope. Ordinarily such a persistence of slope would be
taken to indicate simple stream origin, but having abandoned that
hypothesis, the value of the angle as a factor in estimating prob-
able total depth is lost. In short, one can not assume that the
deepest point is indicated by the intersection of the slopes of the
two sides.
But there is one feature that is at least suggestive. That is the
uniformity of the succession of steps and slopes. It was noted
above that each successive inner one is about 300 feet deeper and
1000 feet narrower, If this uniformity and proportion is main-
YORK STATE MUSEUM
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 95
tained for the next inner one — inside of holes no. Io and no. 22
—there would be room for only one more and its approximate
depth would lie somewhere between 800 feet and goo feet below
tide.
Recent drilling has shown a marked difference between holes
no. 10 and no. 22. WHole no. 10 located 500 feet southeast of no.
22 is nearly 100 feet deeper. Since no. Io is nearly straight down
stream this discrepancy is disturbing. But if one considers the
distance of each from the east bank it is noted that no. 10 is 900
feet out and no. 22 is 800 feet. Hole no. Io is thus about I00
feet nearer the middle of the stream and allowing for this addi-
tional distance according to the profile as known it ought to be
at least 70 feet deeper than no. 22. This corrected difference then
of 30 feet does not seem to be of much importance.
Summary. Everywhere in its lower course the Hudson ex-
hibits the character of a narrow gorge, sometimes of a gorge within
a gorge, most of which is either submerged or buried several hun-
dred feet. |
Depths of 200 to 300 feet are average and for the last 60 miles
of its course represent widths of 1000 to 3000 feet.
Greater depths are believed to be maintained continuously within
a narrower inner notch, but of this there is no conclusive proof
and very little evidence outside of a few Storm King borings.
The Storm King-Breakneck notch is over 751 feet deep. But
it is abnormal at least in width and probably also in depth, due to
ice erosion.
The conditions indicate (a) rapid stream erosion while the con-
tinent stood much higher than now, (0) glaciation which enlarged
the gorge in at least a few places and filled it with rock debris
and later with mud during submergence, (c) finally an emergence
with minor oscillations and erosion to the present time.
ae
4 Origin of the present course of the Hudson
The course of the Hudson is in most respects no more abnormal
than that of the Susquehanna. Both flow across mountain ridges
in such manner as to indicate their superimposed character. Both
date back to the Cretaceous peneplain. But the striking feature
of the Hudson is its straight course. As Hobbs and others have
pointed out, the river is abnormally straight for more than 200
miles — and this in spite of the fact that it crosses the bedding and
other structures of the country rock at nearly all points at an
96 NEW YORK STATE MUSEUM
oblique angle. Such conditions are especially notable south of the
Highlands where the Hudson cuts at a low angle across the ends
of a succession of complex folds of the crystalline metamorphics
for 30 miles to New York city. But this is true Only of the east
side of the river. The west bank is an almost unbroken uniform
escarpment of the Palisade diabase intruded sheet underlain by
Newark sandstones, which if laid down upon a pretty well planed
Pretriassic surface might easily control the Hudson, and which
would not differ from its present course.
The most evident exception to this is the course of the river
from Hoboken to Staten Island. Instead of following the line of
contact between the crystallines and Triassic formations, the river
cuts through the crystallines leaving large masses of serpentine
and associated schist on the west side. This together with the
behavior of the river in cutting across the strike farther north
near the Highlands is believed to strongly favor the fault theory
of location especially south of the Highlands. The same condi-
tions would be favorable to the development of a narrow gorge
and perhaps a very deep one rapidly eroded along the crush zone
of the fault.
From the northern entrance to the Highlands to Haverstraw bay,
where the Palisades are reached, the stream course is not by any
means straight, but shifts from longitudinal structure to cross
structure alternately in a zigzag manner. North of the Highlands
the course is more direct again. On the whole the present explora-
tions have added little to the facts bearing upon this question.
Faults crossing the river are common and easily recognized. Oc-
casionally one appears to pass into the river gorge at a very small
angle and not reappear. In a few places, especially in the High-
lands, the course does not seem to be consistent with the hypothe-
sis of a large fault line. It is to be expected that further work at
the Hudson river crossing will add materially to the facts relating to
the structures within the gorge.
CHAPTER JI
GEOLOGICAL CONDITIONS AFFECTING THE HUDSON
RIVER CROSSING
General statement
This is essentially a study of the geologic features and cond1-
tions shown by exploration to have an important influence upon
the choice of river crossing for the aqueduct. In the beginning it
was possible to consider that any point between Poughkeepsie and
New York might furnish a crossing. The early preliminary in-
vestigations showed that it would be desirable to cross either above
or within the Highlands and subsequent exploratory work throws
light on different possible locations in these regions. Fourteen dif-
erent lines were tested by wash borings. Later some of these were
tested by diamond drill. As data accumulated it was possible to
eliminate many of the trial lines and the more detailed and critical
studies became confined to a few important possible crossings.
In making a comparison of them as to geological environment it
is evident that they fall into two distinct groups’ [see fig. 15].
One, that may be designated the *‘ New Hamburg” group is rep-
resented by the “ Peggs point,” “New Hamburg,’ and “ Dan-
skammer’”’ lines and is characterized by a series of much folded,
faulted and crushed sedimentary rocks, chiefly slates, limestones
and quartzites. The other, that may be called the Highlands group,
is represented by the “Storm King,” “ Little Stony point,” and the
“Arden point” lines and is characterized by crystalline metamor-
phic and igneous rock of a much older series.
A judgment as to the most desirable crossing involves the selec-
tion of one of these groups chiefly upon general geologic features,
and finally a selection of a particular line upon minor differences of
materials or structure.
In the first place it seems necessary to consider, for each group,
1 There have been other suggestions for crossing the Hudson river,
farther upstream and farther down than these—one being at New York
city — but none have had sufficient claim to attention to encourage much
detailed work or so careful consideration as thosé here discussed.
A shift of position of the Hudson river crossing involved a correspond-
ing shift of a large section of the northern aqueduct line. The first choice
of location occasioned a shift southward of all that portion between
Ashokan reservoir and the Hudson.
[97]
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 99
the whole length of pressure tunnels whose position would be modi-
fied by a shifting of river crossing. This is because the aqueduct
will approach the Hudson with nearly 400 feet head —i. e. 400
feet above river level or with an equivalent pressure. For this
reason it is considered necessary to plan a rock pressure tunnel
beneath the river which can deliver the water at nearly the same
elevation again on the east side. |
Thus any one of the “ New Hamburg group ”’ involves a contin-
uous pressure tunnnel reaching from the margin of Marlboro moun-
tain to Fishkill range, a distance of approximately seven miles,
while any of the ‘“ Highlands group” permits the substitution of
two more or less separate siphon tunnels (Moodna creek and Hud-
son river) of considerably less combined total length.
A reliable conclusion as to the choice of crossing is probably best
reached through a comprehensive understanding of the geologic de-
velopment of the region together with a consideration of specific
local conditions. With this end in view a condensed outline of
geologic history, so far as it bears upon the questions at issue, 1s
inserted. But for a more comprehensive discussion of these matters
the reader is referred to the explanatory chapter of part 1.
’
Geology ;
This particular locality, including as it does the Highlands of
the Hudson and the district lying along its northern border, is one
of the most complicated stratigraphically and structurally to be
found in the entire region. The strata represented include more
than half the total geologic scale reaching from the oldest sedi-
ments following the Archean up to and including a part of the
Devonic series [see pt 1]. The rock types include granites, diorites,
gneisses, schists, marbles, serpentines, slates, quartzites, sandstones,
limestones, shales, and, less extensively, other varieties. And the
region bears the evidence of no less than three periods of mountain-
making disturbances, each in its turn adding to the succession of.
foldings, faultings and unconformities.
The oldest formation is a crystalline gneiss —a characteristic
rock of the Highlands. It represents an ancient sediment that has
been completely recrystallized during some of the earlier mountain-
making period. It is older than the Cambric. Interbedded with
it to a limited extent are quartzite beds, ancient limestones (now
usually serpentinous in character) and schistose beds; and in it are
many igneous injections, mostly granites of various types. All
4
100 NEW YORK STATE MUSEUM
these igneous injections are therefore younger than the gneiss and
are very large and abundant in certain cases. The granite of Storm
King, Crows Nest and Breakneck ridge belongs to this type.
Following the sedimentary cycle represented by the above series,
and perhaps others not now preserved, the region was folded into
a mountain range, the series was extensively metamorphosed and
passed through a long period of erosion during which it was again
reduced to sea level position and began to accumulate a new series
of sediments.
The lowest beds occurring upon this foundation are sandstones,
now changed into quartzite. In places they are conglomeritic, and
may now be seen projecting into the valleys along the Highland
border. This formation is of Cambric age, and is from 200 to 600
feet thick in favored places. It forms an almost continuous belt
along the north side of the Highlands except where cut out by
faulting, and extends with similar breaks beneath the later sedi-
ments northward. This quartzite is known as the “ Poughquag.”
Upon the quartzite of this series there was developed a succes-
sion of limestone beds at least 900 to 1000 feet in thickness. This
formation is known as the “ Wappinger”’ and includes some beds
that are of Cambric but for the most part of Ordovicic age.
The final member of this series is a shale and shaly sandstone
in places changed to slate. It is quite variable in actual character
and has a great thickness, never yet successfully estimated, but
probably several thousand feet. This is the so called “ Hudson
River slate” series. In this region they are of Ordovicic age.
This is the succession which the proposed Hudson river lines
has to penetrate in a pressure tunnel. Later Siluric and Devonic
strata lie in the immediate vicinity of this alternative line, but add
no complication to the problem as it now stands. Therefore no
other formations need be considered except the glacial drift. This
covers almost every rock surface and is deeply accumulated in
some places, notably in the narrow gorges and valleys, obscuring
the finer original topographic lines.
A summary of the history of the formations chiefly involved in
this problem with a suggestion of later erosion activities may be
tabulated as follows:
Glaciation
Reelevation
Erosion (interrupted)
Elevation (rejuvenation)
Cenozoic
GEOLOGY OF THE NEW YORK CITY AQUEDUCT IOI
Erosion to peneplain
OP CA ie ponte ar ae Unconformity
A long interval including two mountain-making epochs
and at least one period of general sedimentation
Mesozoic
{Hudson River slates
Ordovicic }
aloo ee limestone
| Poughquag quartzite
reeterd he ae Unconformity
A long interval including mountain folding, igneous
injection, erosion, and perhaps other sedimentations
: Cambric
Paleozoic
The metamorphosed schists, limestones, quartzites
tc., together with accompanying intruded igneous
masses — forming the basal gneisses of the High-
lands 3
Proterozoic
The evidence of such succession and history gathered from the
scattered outcrops of rock in the immediate area, is nowhere better
shown than in the field covered by this investigation.
Structure
When such outcrops as are known are plotted and organized, sev-
eral important facts become clear.
1 The folds run with remarkable persistence northeast and south-
west.
2 The succession in many places is not normal. Often a whole
formation or even two of them are missing and formations that
should be separated are brought side by side. Faulting therefore is
prevalent and the occurrences show that these ee fault lines
usually run northeast and southwest.
3 A consideration of the dips of the strata shows that most of the
folds are overturned as if pushed by some general movement from
the southeast.
4 This same movement causes the faulting to be largely of the
overthrust type, and in some cases the lateral displacement attained
in this way may possibly be several thousand feet.
5 Isolated “islands” of the older rock formation appear out in
the later sedimentary area. They all seem to belong to prolonga-
tion of the ranges of the Highlands and their abundance undoubt-
102 NEW YORK STATE MUSEUM
edly complicates the underground structure throughout a consider-
able belt. ; ;
6 The Highlands area terminates in a serrate margin which, in
the latest thrust movements from the southeast, must have created
very unequal distribution of stresses within the slate-limestone
region to the north causing additional cross folding and faulting.
For the most part these can be traced only a short distance before
losing their identity.
In a mountain folding movement, the uppermost rocks are most
broken and displaced or crushed while those of greater depth may
be bent or uniformly folded or even recrystallized. It would ap-
pear that this latter was the condition of the Highlands rock series
during its earlier history. And even in the latest movements its
lines appear to be less radically disturbed than the slates and lime-
stones to the north. Most of the disturbances that invite serious
consideration belong to the latest period of these mountain-making
upheavals.
Comparison of routes
1 New Hamburg group. This group of crossings is in the
later sedimentary series. Hudson River slates and Wappinger
limestone are the chief formations. But within the southern third
of the tunnel, at least, the underlying Cambric quartzite and the
older Highland gneiss would be cut —the quartzite possibly three
times. The succession therefore will be of considerable complexity
as a whole.
All of the formations involved are thrown into very steep dips
at most places and are consequently liable to rapid and unexpected
changes — some of which probably do not show at the surface.
There are several fault lines belonging to the major northeast
and southwest series to be crossed by such a tunnel — one of them
in each case being met at considerable depth and beneath or adja-
cent to the river. These faults besides being the weakest zones of
rock as a rule, are in addition the most unstable in any possible
future earth movements. Although there is no evidence of recent
displacement along these lines, still such a thing is always possible
and recent serious effects of this kind on the Pacific coast suggest
caution. It is manifestly advisable, if possible, from every stand-
point to avoid crossing several of them.
In the field there are numerous springs of very large flow along
many of the limestone borders. The concentration of them to
these situations in addition to the occurrence of an occasional sink-
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 103
hole, leads to the conclusion that they are more intimately depend-
ent upon the limestone structure for their existence than upon the
glacial drift or any superficial factor. Their abundant flow, some-
times on high ground, indicates rather extensive structural con-
nections and this is believed to be the limestone bed itself and that
such flows would be encountered also in depth. The occurrence of
sinkholes suggest also possible solution channels and cavities and
distant outlets. The types of rock to be encountered on the lines
represented by this group are easily workable. Among them all
the Hudson River slates is probably the most satisfactory from any
standpoint. It is generally easy to penetrate and has a capacity for
healing its own fractures. For this reason it can be considered good
ground, tight and safe. But a considerable distance of the tunnel
can not be kept in slate—perhaps even more of it than can be
proven from surface observations. The other formations are con-
siderably less satisfactory. The limestones are in places shattered
and are liable to abundant flow of water. The quartzite is ex-
tremely hard, as difficult to penetrate as granite, and where crossed
by the faults is probably not healed at all, while the gneiss is doubt-
less of similar character to that of the Highlands crossings to be
discussed later.
Only minor modifications result from a choice of the individual
crossing, whether “ Peggs point,’ ““ New Hamburg,” or “ Danskam-
mer.” In one of them, New Hamburg, it would appear possible to
cross the actual river section wholly in slates. This seems to be the
reasonable conclusion from the diamond drill boring at Cedar Cliff.
But even that line necessitates crossing at least two fault contact
lines immediately at the east bank and beneath Wappinger creek at
depths not immensely less than that below the river itself and both
wholly within the range of influence of the river waters. It would
appear therefore that the situation is not materially altered in the
present discussion, no matter which particular crossing of this
group is considered.
2 The Highlands group [see cross section]. In this group of
crossings there are two separate features to consider. (a) the
Moodna creek valley which these lines all cross, and (D) the Hud-
son river itself. Their characteristics are as follows:
a Moodna creek |see separate Moodna creek discussion]. So
far as known Moodna creek can be crossed almost wholly in slate.
It is possible that the underlying limestone may come near enough
* to the rock floor of the valley to be penetrated but there is little
I04 NEW YORK STATE MUSEUM
direct evidence of it. The ancient valley is deep and probably
marks a line of displacements which can not be avoided, no matter
what route is chosen. The fault contact at the border of the High-
lands is not expected to prove troublesome as it seems very tight
at the exposures seen. The buried granite ridge (a continuation of
Snake hill) which underlies the western end is now known to come
within the limits of the tunnel and adds one more complication.
Except for the fact that the ancient Moodna valley is deep and
filed with heavy drift that is unusually difficult to prospect, there
would seem to be no source of special trouble. It has no lines of
weakness that are not also present in the more northerly districts
and the tunnel has chances of crossing them under more advan-
tageous conditions without so much complication with the lime-
stone series as. characterizes the New Hamburg group.
b Hudson river. Among the Highlands group of crossings there
is considerable difference of structure dependent upon the exact
location of the crossing. The conditions that prevail may be sum-
marized as follows:
(1) Storm King location. This is wholly in massive and gneissoid
granite. The rock is the most massive and substantial body of
uniform type found in the Highlands. The course of the river
indicates some weakness in that direction. This weakness may be
some minor crushed zone or even the jointing alone that prevails
throughout the exposed cliffs. But there is no direct evidence of
faulting, cutting the line and such crushing as may be encountered is
believed to have originated at such depth and under such conditions
as to cause no large disturbance. The freedom of this formation
from all bedding structures and natural courses of underground
water circulation on a large scale is an additional factor. There is
absolutely no other place, within the region, where the Hudson river
can be crossed from gradé to grade in good ground of a single type
with so great probability of avoiding all large lines of displacement.
(2) Little Stony point location. The conditions that prevail at
this point are similar to those that characterize the Storm King line.
The only known difference is in the considerably more shattered
condition of the granite, especially on the west shore at Crows Nest.
It is estimated that this crossing is less favorable by reason of just
this poorer condition of the rock and the somewhat greater yielding
to regional disturbances that it seems to indicate.
(3) Arden point or West Point location. Qn this line the river
would be crossed in the gneiss series proper instead of in granite,
(Ajddng to}v\\ Jo psvog Aq ydeisojoyd) ‘apis jsvo oy} UO JUuLOd MOT {][vUIS
dy} ‘JUIOg AUOJS o][}IT] OF JSOM oY} UO JSON SMOID WOA} SOSSOID YOIYM OUT] JULIO AUOJS 9H] OY} PUL “opis sv
DY} UO UIVJUNOLU possns oy} ‘YOoUYVoIg, O} BSuIYOvod OUI] SUIS{ W40}G 9Y4}— dvs sty} Ul ot] Ssurtsso1s pasodoid oy} Jo
OMT, ‘JUIOG JSOM\ WO YJIOU SUIYOOT Udos sv spuLTySITT oY} OF AVMo}LS UILJUNOW YIUVOSE — SULY WA40IS OY,
CAS
Se tas
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 105
It is largely an ancient stratified series much metamorphosed con-
taining belts of interbedded limestones, quartzites, and schists, in
addition to the more substantial feldspathic gneiss. The eastern
bank of the river bears also abundant evidence of extensive crush-
ing and shearing and is believed to indicate a displacement in this
zone. For these reasons the West Point crossing is considered an
unfavorable route compared to either of the others of the High-
lands group. )
Summary. In a comparison of the geologic features that are
of most importance in contrasting the possible routes for the Hud-
son river crossing the following points are considered of most
importance.
1 The New Hamburg group of crossings involves (a) the long-
est tunnel, (b) the more complicated structures, (c) the greatest
number of known faults, crush zones, and related disturbances,
(d) the more variable series of rock types to be penetrated, (ec)
the greater tendency to encounter heavy underground water circula-
tion, (f) the greater probable susceptibility to disturbance from
future earth movements, and (g) the greater number of uncer-
tainties of rock relations.
2 In contrast the Highlands group admits of (a) shorter totai
tunnel length, (b) the most profound fault lines of the district are
crossed either in high ground or are avoided or, because of the
rocks involved, promise the least possible trouble, (c) the Hudson
river itself can be crossed in a single formation with probability of
avoiding lines of largest structural weakness confining the greatest
pressures and deepest tunnel work within the most uniform and
substantial rock of the whole region.
There are, of course, many unknown or only partially known
features obscured beneath the covering of drift or lying beneath
the river itself; but, however many there may be, it is not believed
that they can materially change the general situation. The major
characteristics are so well marked that any addition to those already
known would in all probability increase the difficulties of the New
Hamburg group of routes at least as much as and perhaps more
than those of the Highlands group.
In view of the above facts and inferences the judgment has been
in favor of the Highlands group of crossings as the more defensible
on geologic grounds as a route for the aqueduct line. Furthermore,
in accord with the preferences already noted, the Storm King loca-
ticn is regarded as the most likely to give satisfactory results.
106 NEW YORK STATE MUSEUM
Quality and condition of rock
The rock of Storm King mountain and of Breakneck ridge at the
Hudson river crossing is a very hard granite with a gneissoid
structure of variable prominence. The color varies from grayish
to light reddish and the structure is always coarse passing into
pegmatite facies that occur as stringers or irregular veinlets. The
grayish facies is of slightly finer grain and more gneissoid. Those
portions that have been sheared are still darker. There are many
joints at the surface running at various angles and an occasional
slickensided surface. The mass is cut by several dikes of more
basic rock (diorite) of widths varying irom a few inches to 8 feet.
These dikes are somewhat more closely jointed than the granite
and consequently a little more readily attacked by the weather. But
where protected they are equally substantial for underground work.
The chief variation from this condition is where crushing or
shearing has induced metamorphic changes. Wherever bed rock
has been reached at this point and to such depths as workings have
penetrated the rock is of this type.
The work includes (@) four inclined drill holes from the river
margin — two starting from the surface and two from chambers
set off from shafts at a starting depth of about 200 feet, (b) several
vertical holes in the river itself, and (c) two large working shafts
20 x 20, one on either side of the river.
_ These give all the data? known as to the condition at depth. From
them it is apparent that crushing and shearing have been prominent.
Many splendid specimens of crush breccia are thrown on the shaft
dump. But its present condition at the depth involved is sound and
durable. The fractures are rehealed. There has been a recombina-
tion of constituents giving a new matrix of complex silicates among
which epidote is the most characteristic, while simple decay is of
little consequence. For strength and permanence the conditions
could not well be improved. There is no reason to apprehend any
change for the worse for the reason that the same tendencies must
prevail at that depth throughout. It would appear therefore that
faulting movements, or the existence of a fault zone of importance
can not become a serious obstruction, because of the tendency to
1Since this paragraph was written four inclined diamond drill borings
have been made from chambers at depths of about 200 feet am the shafts.
These have now penetrated the whole distance beneath the Hudson with
very satisfactory results.
(Ajddng 10}e\\ JO plvog
‘JoOr ‘oz AJOqUIDAON Uoyr, Yydvisojoyd) ‘sulsoq Asojze1O[dxd Ul posvsud Ssit [[IIp Jo our, oy} Aq JouUN) oAnsso.sd
posodoid oy} JO UOT}VIO] OY} Surmoys ULrejuUNnOUL YOoUyeowigG Wot} JIATI uOSphy_y 9Yy} Purv UleyUNOW SUIS, UWIOVS
SS —"
pesse,
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 107
heal up the fractures and so make the rock about as substantial as
before.
It is noted elsewhere that faulting is common in this region, and
that in a considerable portion of its lower course the Hudson prob-
ably follows such structures. It is, however, wholly unnecessary
to assume that its whole course is a fault line. Whether or not
there is a longitudinal fault zone of any prominence in the river at
Storm King is unknown. There are several cross faults, both above
and below this point, that give much clearer surface evidence of
their presence. Fault zones have proven to be objectionable ground
in many places along the aqueduct line, but elsewhere the. data
refer chiefly to situations favoring more ready underground cir-
culation, i. e. at higher levels. In this particular case the rock in
question lies below former ground water level within the belt of
cementation rather than up in the belt of decay, and there is prob-
ably no disintegrated rock from any cause.
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CHAPTER IV
GEOLOGICAL FEATURES INVOLVED IN SELECTION OF
SITE FOR THE ASHOKAN DAM
Topographic features of the southeastern margin of the Cats-
kills, where the chief water supply is available, fixes the approxi-
mate location and bounds of the principal reservoir. The accom-
panying map, a portion of the western part of Rosendale quad-
rangle, shows the situation. The part of the work involving the
chief geological problem was the choice of the principal dam sites
on the Esopus. This is known as the Ashokan dam. This part of
the Catskill system belongs to the Reservoir Department under
Mr Carlton E. Davis as department engineer.
There were originally considered three sites: (1) at “ Broadhead
bridge,” (2) at “ Olive Bridge,” (3) at.“ Cathedral gorge” or the
“Tongore” site. Any one of these seemed possible from a topo-
graphic standpoint. Later developments in regard to storage ca-
pacity and engineering considerations finally reduced the practicable
sites to two—the “Olive Bridge” and the “ Tongore.” These
were then explored thoroughly as an aid to determining whether
or not there were favorable or unfavorable conditions at either
location. Trenches were dug, shafts were sunk, wash holes were
put down, and drill borings were made. The amount of such work
done was sufficient to show the actual conditions both of the drift
and bed rock and incidentally to throw some light on minor matters
in geologic history.
This discussion is essentially a summary of these data and a com-
parison of the geologic conditions indicated by the explorations’ of
these two sites and a statement of some of the geologic character-
istics of the area.
1 General geologic conditions as shown by the explorations
Bed rock is dark colored Devonic sandstone and shale, the
Sherburne formation, lying almost horizontal, strongly jointed,
plainly bedded, and of good quality for the foundation of the dam.
At both locations the present Esopus flows in a postglacial gorge
1Jn this work of exploration a very efficient staff of engineers was engaged.
Among those having very much to do with the features here discussed are
Thaddeus Merriman, division engineer, J. S. Langthorn, division engineer
and Sidney Clapp, assistant engineer.
[ 109 ]
iio NEW YORK STATE MUSEUM
and there is a somewhat deeper buried channel a short distance to
zesS fresh and substantial, due to ionmer weathering, than the pres-
ent exposed surfaces.
In each case glacial deposits reach a thickness of more than 200
feet within the narrow valley or gorge, especially along the north
valley wall within the lumits oi the proposed dam.
Special geological conditions. The factors in which there is
most variation and which are of most significamce in a comparative
study are those belongmg to the glacial drift deposits. In order to
properly estimate the influence oi some of these features it will
be mecessary to briefly consider the types of material represented
at different places and the conditions under which they were
fonmed.
Types of material Till. Heavy bouldery wll, mixed clay,
sand, gravel, and boulders, is the most abundant type of materml.
It forms especially the chief surface material throughout the region,
and is the surface type at both sites. It becomes at places quite
sandy, but 1s almost everywhere good, impervious material because
of its mixed character.
Laminated till. At a few places, notably m the Beaver kill near
its mouth, and m a trench above Olive Bridge, and im the “big
drgway~ above West Shokan, strong lammation appears in heavy
stony till as 1f laid down rapidly in comparatively quict water such
as the margin of a lake. This material is especially mmpervious.
Gravel hillocks. A few small illocks with moraimic contour,
indicating a dumpmg ground for some glacier of a small scale,
occupy the flat mmediately wesi of Browns Station. They were,
at a very late stage, piled mito the course of a former glacial stream
whose delta deposits occupy the sandy bench above the 500 foot
contour just north of Olive Bndge.
Assorted gravel and samd. This material is abundantly developed
just north of Olive Bndge. It seems to have formed a delta de
pcesit at the mouth of a glacial stream that emptied mito the mam
valley at this point The running water washed almost all of the
clay and extremely fme material farther out, where they settled m
the bottom of a small glacial lake that was at that tome held m this
upper portion of the Esopus valley. The dam that held im this body
of water which reached above the 520 foot lime stood near the
proposed “Olive Bridge” dam site. The materials fonming the
dam were im part the glacial till that is now found on that site and
(A[ddng 19}e AA JO prvog
Aq ydersojoyg) ‘(ueyoysy) Wep osplig SAITO 7@ SerVys pue SouoyspueEs oUINGIoYS 94} UL YOUII} JoO-jnyD
gI 93eId
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MARCH 17,/908. 200 ae ES SEs ==> >> SSS = ot Seas Vessel TS SS ee SS
© Bornes. Section of Site on Centerline
Corrected fo Jon. 1909.
Fig 16 Location of the Ashokan dam at Olive Bridge site and a geologic
’ cross section. The small dots in the plan indicate exploratory borings. The
section shows the rock profile indicating a preglacial channel of the Esopus.
The present Esopus flows in a new postglacial:channel at a higher elevation.
The lowest materials in contact with bed rock are heavy stony
till, laminated till and stony laminated clays —all good impervious
material wherever exposed and tight upon bed rock. Sands and
laminated clays are extensively developed immediately northward
of the site and streaks of these deposits interlock to a limited extent
with the till materials of the site itself, but they do not extend far
and die out in wedges among the heavy deposits that characterize
the southern slopes of the hill forming the northern terminus of
the dam. These pervious streaks do not extend at any point con-
tinuously through this hill and consequently as a whole the present
barrier as it stands is practically impervious. The poorer materials
(assorted gravels and sands) characterize the upstream side, and
the better, more impervious materials (till and laminated boulder
clays) characterize the downstream side of the proposed Olive
II4 NEW YORK STATE MUSEUM
Bridge site. It is therefore advisable to locate any such structure
as adam ata point as far down stream on this site as other engi-
neering factors permit.
b “ Tongore” site. At Tongore, bed rock is at least a hun-
dred feet deeper than at Olive Bridge. In the deeper parts, below
the 400 foot line the deposits as indicated by the wash borings [see
sections] are interpreted as a fairly continuous succession of till,
stratified sands and gravels, and laminated sands and clays belong-
ing to two or three different stages of accumulation. Upon this
the heavy upper till was laid down. It is believed that the records
fully support this view and that the stratified or laminated materials
were accumulated at a time when a temporary dam existed at some
point still farther down the Esopus valley. It is apparent further-
more that the most porous zone is at the junction of the upper
till and the lower stratified deposits and in part is represented by
the assorted pebbles of stream wash —1in general not far from the
400 foot line. These middle zone deposits are believed to extend
continuously through the drift ridge that forms the northern half
of this site. As before noted, though rather impervious vertically,
aay
M
yp
Fig. 17 Plan and geologic section at the Tongore site. The dots on the map indicate
exploratory borings and the course of the buried channel of the preglacial Esopus creek
is shown making a right angle bend to the north. The section shows the buried chan-
nel, the new postglacial channel and the great accumulation of porous modified drift-
which is regarded as one important objection to this site for the dam.
some of these deposits allow ready lateral movement of water. This
is held to account for the rather persistent occurrence of springs
or seepage along the creek bank at about this level both above and
(Ajddng 103¥\\ Jo pivog Aq ydeisojoyd) “}flip JO Joo} One YJVousd poLind Aod[[vA 9Y} JO apis yOu
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT
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116 NEW YORK STATE MUSEUM
below the site. The great thickness of these laminated beds, in
places a hundred feet or more, together with the abundance of sand
in them, and the caving tendencies exhibited by them in one of the
large shafts, indicates poor conditions for such a piece of work.
The behavior of one of the test shafts throws some light on con-
ditions within the drift deposits. At this place after sinking into
the underlying gravel beds there was “no water” at first, but after
going a few feet deeper there was an abundant flow, that did not
rise much in the shaft. This case seems to support the following
interpretation.
The gravels encountered do not form an isolated pocket or lens,
else it would have carried water full from the first. It must be
a fairly continuous porous zone with large feeding connections else
it would run dry, and it must have an easy discharge else it would
have risen above the level of the first gravels. Therefore it must
be a rather well marked subterranean water passage or porous zone
of considerable extent. Such conditions would make an impervious
core wall to bed rock at this site a necessity and its construction a
matter of considerable difficulty. At this site also because of the
small cross section of the ridge, there is little chance for the inter-
locking of layers or the blocking of the porous ones by a till barrier
to check the lateral seepage, and there is no chance to move farther
down stream to secure such conditions.
4 Summary
Because of the (a) higher bed rock throughout, and (6) the
more uniform and impervious quality of drift deposits, and (c)
the more massive cross section of drift barrier for foundation, and
(d) the perfectly tight contacts of till and bed rock, and (e) the
limitation of the more porous materials to higher levels and (f) the
glacial history connected with the development of all these parts,
“ Olive Bridge” is the preferable location for the proposed Asho-
kan dam on Esopus creek.
CHAPTER V
CHARACTER AND QUALITY OF THE BLUESTONE FOR
STRUCTURAL PURPOSES
Probably no stone marketed in New York State is more exten-
sively known than the “bluestone” of the Catskill region. But it
is noted particularly for a special purpose, 1. ec. as flagstone, because
of its capacity to part or cleave into thin slabs. These slabs are
proven by experience to have remarkable weather resistance and
durability.
Little attention has been given to the question of dimension stone
—whether or not such blocks of as high quality as the flags could
be obtained and where such quarries could be opened.
There are several reasons for this situation. In the first place
(1) the stone is of a dark color and has a dull appearance so that
it is not fancied for the usual expensive structures where large sizes
are used, also (2) the quarries are small, shallow, and are worked
on a small scale by single individuals or groups of neighbors with
few quarrying tools and no transportation facilities for large mate-
rial, and in addition (3) considering the work and equipment neces-
sary and the demand the flag industry was more profitable.
Because of the large demands of the Ashokan dam where nearly
a million cubic yards of heavy masonry construction are to be used
an entirely new situation has developed. It is especially desirable
that a rock capable of furnishing heavy dimension blocks should
be discovered. The usual slab or flag type is unsuited to a consider-
able part of this work. A study of the adjacent region therefore
has been made and explorations along certain promising lines have
been conducted to sufficient completeness to prove that a suitable
stone can be furnished in large quantity. The characteristics of
structure and occurrence as shown by this special study are given,
together with some of the later exploratory data.
Physiographic features?!
All of the rock formations are sedimentary, chiefly sandstones
and shales. They lie in alternating beds of variable thickness and
are almost horizontal. The total thickness is many hundred feet so
1 The principal argument of this discussion has been used in a previous
article by the writer under the title ‘‘ Quality of Bluestone in the vicinity
of the Ashokan Dam” in the School of Mines Quarterly, v. 29, no. 2.
117
118 NEW YORK STATE MUSEUM
that neither the bottom nor the top beds of the series are to be seen
in this locality.
The region is one of considerable relief representing preglacial
erosion. The glacial drift mantle has modified it chiefly by obscur-
ing some of the smaller irregularities of rock contour, and espe-
cially by partially filling many of the stream gorges. Postglacial
erosion has not completely reexcavated the old channels. But the
contour of the uplands reflects the character of the bed rock with
considerable success. The tendency of the more massive and coarse
grained varieties of rock to resist weathering and erosion more suc-
cessfully than the finer grained and more argillaceous or shaly
facies is a general characteristic. Since these varieties form suc-
cessive or alternating beds throughout the whole area, the result is
an almost universal cliff-and-slope surface form. This bed rock
topography is somewhat obscured but not wholly obliterated by
glacial erosion and deposition. Therefore it may be used with con-
fidence in locating or tracing the more durable beds since they
almost invariably appear as a shelf or terrace with a steep margin
toward the lower side and a gentle slope on the rising side.
Structural features
The rock types include bluish gray or greenish gray sandstones
with almost horizontal bedding, and sometimes exhibiting cross-
bedding structure, and compact very dark argillaceous shales. These
two are of about equal prominence, but only the sandstone is of
importance in the present discussion. Its minute structure will be
given in greater detail in the petrographic discussion.
Jointing is common and persists in two sets nearly at right angles
to each other — one striking northeastward and the other toward
the northwest. In some of the best exposures, these joints are
clear-cut and run Io to 18 feet apart, dipping almost vertically. In
the more massive beds there is very little small jointing, so that the
character is especially favorable to large dimension work.
But still more prominent structures are the partings which follow
the bedding planes. These give the rock a decided tendency to
cleave naturally into slabs, the uppermost exposed portion of almost
every outcrop exhibiting this slab structure in more or less perfec-
tion. So general is this structure at all horizons in the sandstones
of the series that there can be no doubt of its connection with
some original sedimentation character. Besides it is a potential
factor in nearly all the beds even when not very apparent. The
(Ajddng Joye \A JO preog Aq yders0j0yq)
‘sjulof JO SjoS JuoUTUOId pue Ie[Nse1 AIZA OM} PUL SUIPpaq [ejUOZIIOY IY} SuIMOYS ‘ssey sUINGIoYyS IY,
' GEOLOGY OF THE NEW YORK CiTY AQUEDUCT 119
exposed places exhibit the character so prominently only because
of the weathering effect, which develops the natural tendency. This
general conclusion is borne out by the well known practice of quar-
rymen of the district of splitting the larger blocks into slabs of the
required thickness by wedges driven along certain streaks that are
known as “reeds.” A reeding quarry is one that has this capacity
well developed, and it is this character in part that has made the
“bluestone” or “ flagstone”’ of New York an important factor in
the production of the United States for a great many years.
For large size dimension stone where great stress is involved it
is evident that this structure would not be desirable. These definite
planes of weakness reduce the general efficiency. A little observa-
tion however shows that there are some outcrops and an occasional
quarry where the more massive blocks do not split well. From the
necessities of the industry these have been avoided or but meagerly
developed. In some cases of this kind the sedimentation is of the
cross-bedded type with somewhat interlocked laminae. If the grain
is coarse such varieties resist splitting with great success. The
thickness of such beds varies from a few feet to 25 feet or even
more without prominent interbedding of shale layers. |
Stratigraphy. These are the sandstones, flags and shales
known as the Hamilton, Sherburne and Oneonta formations belong-
ing to the Devonic period. The strata of the immediate vicinity of
this examination belong to the Sherburne subdivision, but no at-
tempt to differentiate the formations was made. Structurally and
petrographically the different formations are not distinguishable in
this area. On the market the stone from either is known generally
as “ Hamilton flag” or “ bluestone.”
Economic features
There are hundreds of quarries in this general region. Nearly all
are small, and are worked on a small scale without machinery. The
product is almost wholly thin slabs of the flagstone type. This is
supplemented by a small amount of somewhat more massive char-
acter, dressed for window sills; and a very limited output is of
dimension stone of larger size. The general lack of suitable me-
chanical devices and transportation facilities are the chief reasons
for the limited output of the last named grades.
Petrography
The basis of this discussion is a microscopic examination of sev-
eral thin sections made of the different types of rock from the
I20 NEW YORK STATE MUSEUM
quarries whose field geologic features give promise of encouraging
results. The most characteristic variations are illustrated in the
accompanying photomicrographs, plates 22, 22.
Texture. The rock is granular, the individual grains varying
from minute particles in the finer shale layers to three or four
tenths of a millimeter in diameter in the coarser sandstone [pl. 23,
lower figure]. The grains are seldom rounded. Jagged or frayed
or elongate forms are the rule [pl. 23, upper figure]. There is no
marked porosity. When the rock was first deposited as a sediment
it probably had the usual large interstitial spaces of such rock type,
but in this case some subsequent modification — an incipient meta-
morphism — has largely obliterated the voids by the introduction or
development of mineral matter of secondary origin.
In general it is quite apparent that the average grain was orig-
inally more rounded than its present representative.
Mineralogy. The original minerals in order of abundance
were the feldspars, quartz, and probably hornblende, biotite, and in
much smaller amounts others of little apparent consequence in the
present discussion.
All of these have been more or less affected by subsequent
changes. Quartz has suffered least of all, the chief modification
being a greater angularity of form and an occasional interlocking
tendency caused by secondary growth [pl. 22, lower figure].
Both orthoclase and plagioclase feldspars occur. The orthoclase -
grains, which originally made up more than half of the bulk of the
coarser types of rock, have been in places profoundly altered [pl. 22,
upper figure]. In many cases the identification of this mineral de-
pends upon its association and the abundant remnants of character-
istic structure and its normal secondary. products. In the least
affected grains satisfactory identification is not difficult. “Even in
the most modified representatives there is some preservation of
structure indicating size of grain and proving the essentially gran-
ular character of the rock. The plagioclase, although not abundant,
is more readily detected than the orthoclase because it has been
much less affected by the secondary changes.
All original ferromagnesian constituents are wholly altered. There
were some such constituents in the rock, as is plainly shown by the
secondary products. Hornblende and biotite were probably both
present. . |
The secondary products, derived from the original feldspars and
ferromagnesian constituents, include sericite, chlorite, calcite and
Plate 22
Photomicrograph of bluestone, x 25 diameters. The clearer grains are
quartz and indicate the approximate size of other original constituents.
In this case the alteration of the feldspars and ferromagnesian originals
is so complete that their products form an indeterminable complex agegre-
gate of closely interlocked granules, flakes, and fibers of extremely fine
texture.
Photomicrograph of first grade medium grain bluestone, x 25 diameters.
Taken to show angular and interlocking grains indicating secondary
growth and a complete lack of reeding structure. The clear grains are
quartz; the rest of the field is made up chiefly of secondary derivatives
from the original feldspars and ferromagnesian minerals.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT I2!I
quartz as the most important and abundant. Others probably occur
that are less readily differentiated, and among them is kaolin. Occa-
sionally a small amount of massive or granular pyrite occurs. There
are traces of organic remains, especially plant stems, and the pyrite
is most plentiful in association with those beds.
It seems to be the secondary products largely that give the char-
acteristic bluish or greenish color to this stone. Practically all of
the iron freed by secondary changes from the ferromagnesian con-
stituents has entered into new silicate compounds, especially with
the chlorite, which are minutely distributed throughout the whole
mass, giving it all a tinge of the characteristic color of these well
known products. The same amount of iron in the oxid form
would no doubt give as highly colored stone as any of the reds or
browns of other familiar types of sandstone. But the tendency to
form the sericite-chlorite-quartz aggregate in the rock has also an
important bearing on its durability and strength. This is further
discussed in a separate paragraph.
Classification. It is clear that this type of bluestone is a sedi-
mentary rock of medium grain, a sand rock or “renyte.” Since
the silicates are so predominant in the original composition it may
be further identified as a sandstone or a “ silicarenyte.” But in
view of the predominance of the feldspars it should be further
designated as an arkose sandstone. And considering the extent to
which it has been modified by the development of interstitial sili-
cious products and the effect that this has had in perfecting the
bond between the grains, the rock may be classified as an indurated
arkose sandstone.
Special structure. A study of the cause of reeding, or the
tendency to split into slabs, led to the preparation of thin sections
of this structure [pl. 23, upper figure]. It is apparent from them
that the reed is strictly a rock structure and that the perfection of
the capacity to split along these planes depends wholly upon the
abundance and arrangement and size of the elongate and semifibrous
grains and the presence of a more than usual amount of original
fine or flaky material. Almost universally the reed streaks are
darker in color and finer in grain than the average of the rest of the
rock.
In part therefore it is an original character due to the assorting
action of water during deposition, finer streaks alternating with
coarser ones in accord with ordinary sedimentation processes. But,
in addition to that, the subsequent changes that have affected the
whole rock have occasionally accentuated the structure by a ten-
. i
122 NEW YORK STATE MUSEUM
dency of the whole rock to develop elongate or fibrous aggregates. |
It is probable therefore that the parting capacity is in places con-
siderably increased by the very process that has produced just the
reverse results in the more heterogeneous portions of the beds.
Under a sufficient stress the rock will part most easily along the —
planes where this foliate or fibrous character is most persistent.
Even in these cases, however, it may not indicate that the rock is
essentially weak. It simply locates the most vulnerable point in
the stone. In many quarries these streaks are so abundant that
only thin slabs can be obtained —the disturbances of ordinary
quarrying being sufficient to cause parting. The deeper portions of
quarries are, however, much less subject to such behavior. In all
cases the greater slab development of the exposed portion of the
ledge is an ordinary weathering effect, by which the same results
are obtained slowly and naturally and more perfectly than can be
secured artificially on the fresh material of the same beds. The
expansions and contractions of changes of temperature, together
with the rupturing effects of freezing water caught in the pores,
serve finally to weaken every part of the rock. In this process the
prominent reed lines give way so much in advance of the rest of
the rock that they develop into true rifts and separate slabs appear.
It must be appreciated that these ledges have been exposed an 1m-
mensely long time compared with the probable requirements of any
engineering structure, and that this weathering tendency does not
mean a speedy disintegration of the freshly quarried blocks. Still
it is advisable to avoid as many sources of weakness as possibic
and one of the ways is to select ledges where the stone does not
have a reeding tendency, or in which the reed lines are interlocked,
or wavy, or interrupted. These requirements are most fully met in
the coarser beds and especially those exhibiting some cross-bedding.
Two local quarries meet these demands to a marked degree.
Strength. The better qualities of, bluestone have great
strength. Even the reed lines are in many instances stronger and
more durable than the regular quality of some other sandstones
that are usually considered suitable building material. The secret
of this exceptional strength lies in the modifications of texture that
have resulted from the alteration and reconstruction of the mineral
constituents. The breaking up of the orthoclase feldspar, and the
accompanying changes in the ferromagnesian minerals, have fur-
nished considerable secéndary quartz, which has in part attached
to the original quartz grains making them more angular and de-
Plate 23
Photomicrograph showing structure of the reeding quality of “blue-
stone.” Magnification 30 diameters. Taken to show tendency to paral-
lelism of elongate grains.
Photomicrcgraph of best grade coarse-grained bluestone. Taken to
show a quality in which the granular character is still well preserved.
The clear grains are quartz, the others are chiefly feldspars somewhat
modified. The close interlocking and the development of fibrous or
frayed structure and the bending or wrapping of some constituents are
secondary effects.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 123
veloping an interlocking tendency [pl. 22, lower figure]. At the
same time the fibrous sericitic and chloritic aggregates have developed
to such extent as to fill most of the remaining pores, and in many
cases the fibrous extensions have actually grown partly around the
adjacent quartz grains [pl. 23, lower figure]. The effect has been
to develop a silicious binding of unusual toughness. This combina-
tion of changes has made a rock that is now remarkably well bound
or interlocked for a sedimentary type.
Durability. First-class stone of the grades indicated above
would have as great durability as any stone in the market, except
perhaps a true quartzite. With the exception of the almost neg-
lectable quantities of pyrite, occasionally found, there is no con-
stituent prominently susceptible to decay. The rock as a whole
mineralogically is stable and its texture indicates unusual resist-
ance to ordinary disintegrating agencies.
General conclusions
From the microscopic study it is clear that the variety of rock
most fully meeting the demands of heavy exposed construction are
the coarser beds and those freest from reed and shale.
From the field study it is apparent that ledges of suitable char-
acter occur occasionally and that at least three such are not far
from the Olive Bridge site.
From additional explorations it is certain that ledges of high
grade rock occur, and that the grade varies rapidly in the same
bed and that suitable material can be obtained in the immediate
vicinity of the Ashokan dam. No doubt rock of equally high
quality may be obtained at many other localities.
men eats
CHARTER Vi
THE RONDOUT VALLEY SECTION
Because of the fact that the hydraulic grade of the Catskill aque-
duct as it approaches the Rondout valley is nearly 500 feet A. T.,
an elevation more than 300 feet above-the lowest portions of the
valley and more than 200 feet above very large areas of it, a total
width of more than 4 miles being too low for unsupported con-
struction of some kind, and because of the general policy of using
the pressure tunnel system so as to deliver the water at a corre-
sponding elevation on the east side of the valley, and further
because of the very complicated geological features of the district
this section has been the seat of very extensive and interesting
explorations.
Undoubtedly a greater number of obscure features occur here
than on any other single section of the whole aqueduct line. Most
of these features are readable from surface phenomena in general
terms. In all cases the indications are plain enough to serve as a
guide to well directed tests, but many points of critical importance
can not be determined with sufficient detail and accuracy of posi-
tion for such an engineering enterprise without systematic explora-
tion.t The basis and results of this line of investigation which has
occupied the greater part of two years are summarized and plotted
in the following discussion and.charts. The portion receiving
special study is in the vicinity of High Falls.
General geology
Almost everywhere the surface is glacial drift. Where outcrops
of bed rock occur they habitually present the unsymmetrical ridge
appearance usually with a more or less sharply marked escarpment
on one side and a gentle slope on the other. The strike of these
1 These explorations belong to the Esopus division of the Northern
Aqueduct Department. The earliest reconnaissance was done under the
direction of James F. Sanborn, division engineer, who was subsequently
assigned to geologic work over a con iderable portion of the Aqueduct line.
The development of exhaustive explorations and final construction on this
division has been carried on under Lazarus White, division engineer,
assisted by Thomas H. Hogan. The division has been recognized from
the beginning as an important one and in many ways one of the most com-
plex. Thomas C. Brown, now professor of geology in Middlebury College,
was employed for a year on this division during the later exploratory work.
125
126 NEW YORK STATE MUSEUM
features is in general northeasterly and on the gentle slope is the
westerly one.
It is apparent at once that the valley bottom is a complex one
and that its history has been somewhat obscured by the glacial
deposits.
Formations. The following distinct stratigraphic units are deter-
minable in this valley every one of which will be cut by the tunnel
beginning at the west side with the youngest formations:
Feet
Hamilios and Marcellas flags and.suales.- 4 re o-e etek eee 700-+
Onondaga littestone .. 026.4: : 102 ae ee ee ee ee oc eee ee 200
Esopus gritty’ shales. {0.9% bo. 3o5 se tee Se ee ole Saeco &00-+
Port Ewen shaley limestone including the Oriskany transition........ 250-+
Bectraft. crystalline Inmestome £)4 0... hte co eee ese oc ose meee 75
New Scotkind shaley. mestone: “-. 3-5-4 3 cee eee Cae aoe eee 100
Coeymans lamestone. 224.55. on os Se eee eee te eee 75
Manlius limestone including Rosendale, Cobleskill, and the cement
bé@ds . 0.0.2. jc see ate Se es © ee +h Regedieetee 100-+
Binnewater sandstovéve go. <22,05 adh «ei oe eee ae oe dk Wes 50
High Falls shale including small Gace PRICES Bi ce ok ss Seemed 75
Shawangunk cong lomeiates: .s. aca ee oe eee ele kee 250 to 350
Hudson River slates —thickness unknown; probably. more than...... 2000
Approximately 4775
These occur in belts in succession more or less regularly from
west to east. Most of the formations are quite uniform in the
Rondout valley. The Shawangunk conglomerate is probably more
variable than any other as shown by borings. Because of this
general persistence of formation it is possible to estimate approxi-
mately the depth at which any particular lower member lies if some
starting point can be identified. [For detailed description of the
formation, see pt 1]
Structure. The principal irregularities are structural, rather
than stratigraphic. The region on the west side of the valley, the
margin of the Catskills, is but slightly disturbed and lies very flat,
but the region on the east side, the Shawangunk mountain range
and the cement district, has an extremely complicated structure.
The Rondout valley, lying between them, is a transitional zone and
passes from gentle dip slopes and folds in the westerly side to
more frequent folds and thrust faults on the easterly side. In at
least two thirds of the valley it would appear from surface evi-
dence alone that the formations would dip uniformly westward, the
only suspicion of additional complication being given by an occa-
(Zo61 ‘“ABojlOs5 Ur [OOYIS JowIWINS AjistoAruy, eIquinjod Aq ydvis
-0J0Yg) ‘Yoo1d JnopuoY Uo sey YSipY ye opeys sey YStpT puv ouojspurs so}eMouulg IY} UI Pjofy 1oUTM VY
~
v2 93e[d
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 127
sional minor fold seen in the river gorge or an escarpment where
the sedimentary character alone would hardly account for it [see
pl. 24, High Falls]. Explorations have shown that the evidence of
the minor structures is reliable and that disturbances occur at some
places even to the extreme western margin.
Physiography. In spite of the drift cover which obscures
many original inequalities it 1s readily seen that the prevalence of
the gentle westerly dip over most of the area, together with the
succession of so many different beds of varying resistance to ero-
sion, have allowed the development of a succession of long dip
slopes and steep escarpments on a more pronounced scale than the
present topography shows. It is clear that the Rondout is really a
series of these unsymmetrical valleys. The principal large dip
slopes are formed by the Shawangunk conglomerate and the Onon-
daga limestone. In each case an original stream had adjusted its
course fully to the structure and was shifting slowly by the sapping
process to the west against the opposing edges of the overlying
strata which form the bordering escarpment. One of these unsym-
metrical valleys lies along the easterly base of the Hamilton escarp-
ment and is continuous with the lower course of Esopus creek
farther to the north. In the area under special study it is not
occupied by a stream now but is filled with glacial drift so com-
pletely that the original stream has been evicted. It is evident,
however, from computations based upon the average dip of the
slope carried to the base of the escarpment that the bed rock floor
ought to be from 200 to 300 feet below the present surface in the
deepest portion. Borings have proven this to be the case both
along the present line near Kripplebush and also on the first trial
line across the Esopus at Hurley.
The same thing is true near High Falls in the center of the valley
where Shawangunk conglomerate forms the dip slope and the
escarpment is formed by the Helderberg limestones. In this case
the drift filling is very deep also, and Rondout creek flows upon it
quite independent of rock structure except where it has cut across
the margin as at High Falls.
In the eastern half of the valley the hard Shawangunk conglom-
erate forms the chief rock floor and largely controls the contour by
its own foldings and other displacements. Thus the Coxing kill
tributary valley lies in a syncline of the conglomerate with occa-
sional remnants of overlying beds as outliers adding some variety
to the form. The Shawangunk mountains, as a physiographic
128 NEW YORK STATE MUSEUM
feature, owe their present elevation chiefly to the resistance of this
conglomerate which serves as a protective member among the
formations.
On the west side, the foothills of the Catskills form a part of
the cuesta developed by the erosion of Paleozoic sediments, the
inface coinciding with the escarpment along the lower Esopus and
Rondout valleys at this point.
It is certain therefore that the drainage of the Rondout valley
before the Ice age differed materially from the present lines. A
stream, probably the original Rondout, followed near the western
margin of the valley and joined the Esopus as it emerged from the
Hamilton escarpment to turn northeast. Another which had cut
somewhat deeper occupied the central portion of the valley and
probably joined the Esopus at some point farther north — its lower
course is not explored.
Practical questions
The chief practical questions to be given as full answers as pos-
sible are:
1 At what depth must the aqueduct tunnel be placed in order
to be everywhere in substantial bed rock with sufficient cover to be
safe?
2 Where are the most critical places—those whose geologic
characters are such as to demand exploration? And at the same
time which sections may be safely left without testing?
3 What is the rock structure and condition? And are there rea-
sons for believing that the tunnel plan is not feasible at this point.
If so, where can a better one be found?
4 What is the character of underground circulation of water?
5 What formations will be cut at the different points and which
should be favored or avoided wherever possible?
From the fact that the present Rondout flows across solid ledges
at High Falls and at Rosendale from 100 to 200 feet above the
known rock floor of the preglacial gorge where explored it is clear
that the present course is entirely different from the original. The
Coxing kill, the third and most easterly of these streams is not so
much disturbed although it also is shifted.
It is worth noting that the streams of this valley together with
the lower Esopus and the Wallkill river have become so completely
adjusted to the rock structure that they all flow up the larger
Hudson valley, of which all form a part, and join the master stream
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 129
at an obtuse instead of the usual acute angle. They are essentially
retrograde streams.
Explorations. Systematic explorations and tests are repre-
sented chiefly by drill borings through drift into the rock floor.
These were supplemented by two test tunnels for working character
of material and a series of tests on the behavior of certain of the
drill holes, together with other tests on material. The results are
embodied in the accompanying cross sections and the additional
discussion of special features.
Detail of local sections
Kripplebush section. This from the first was reearded as one
of the critical sections because of the buried gorge along the base
of the Hamilton escarpment and because of the doubt as to the
behavior of the Onondaga limestone. On the accompanying section
the borings are plotted and the structure as now interpreted is
indicated. The dip slope formed by the Onondaga limestone is
covered by 200 to 250 feet of drift, mostly modified drift. The
strong valley character of the rock floor is almost wholly obscured
by the glacial deposits and the present brook, an insignificant stream
compared to the preglacial one, occupies a position above the escarp-
ment instead of above the old channel.
After a couple of the central holes were finished, it became appar-
ent that the structure is not nearly so simple at this point as the
general surface features would lead one to expect. It was clear
that a simple dip such as was proven to prevail on the dip slope
would not account for the much greater depth attained by it in.
the vicinity of station 500. The discovery of this additional feature
raised two questions: (1) Is the structure a flexure or is it a fault,
and if a fault whether normal or thrust, and (2) what is the prob-
able effect of this structure on the position and depth of the pre- ~
glacial gorge?
The habit of the district immediately east of the valley would
support the theory of a thrust fault. The nature of the immediate
area would suggest a simple flexure while it is manifestly possible
that a normal fault could easily occur. Later explorations! have
1Since the above was written the tunnel has been completed through the
Kripplebush section. Although faulting is indicated by the borings and
actual occurrence of the beds it is very difficult to find the fault. A part
of the displacement is accomplished by the steepening of the dip but this
will not account for more than half of it.
130 NEW YORK STATE MUSEUM
F Tans
ith: i
TAY
Fig. 19 Geologic cross section of Kripplebush section of the Rondout tunnel line as interpreted from drill borings
GEOLOGY OF THE NEW YORK CITY AQUEDUCT ESE
tested this zone so well that it is practically certain that the feature
must be regarded as a fault of some type with a displacement
of nearly 200 feet. The striking physiographic feature is the
development and preservation of the escarpment on the downthrow
side. This occurrence is certainly a very unusual case in that
regard [see fig. 19].
Because of the intention to construct the tunnel deep enough in
bed rock to reach safe rock conditions the question of depth of
buried gorge becomes an important one. As soon as it was dis-
covered that a fault existed there the problem became of sufficient
prominence to demand more detailed exploration. If the faulting
is accompanied by a broken zone in condition favorable to more
ready erosion, it would be possible that the original stream in work-
ing down this dip slope might become entrenched in the fault zone
aad at that point begin to cut a narrow gorge instead of continuing
the sapping process. In fact, it would undoubtedly do this very
thing if there is such a crushed zone of any consequence and if the
erosion process were allowed to continue long after reaching this
critical point.
As a matter of fact explorations have shown that there is a thin
layer of Hamilton shales still remaining on the Onondaga and the
deepest point found is on the Hamilton shales side. These facts
in connection with the failure to find any deep notch indicate that
there is probably no zone of much greater weakness than the shale
member itself. It is reasonable to conclude that the rock floor can
bé safely regarded as not much lower than 88 feet A. T. and that the
rock condition is not especially bad for tunnel construction? even in
the fault zone. |
Rondout creek section. This is the central portion of the
valley including the depression occupied by the present Rondout
and the exposed edges of the series of shales and Helderberg lime-
stone. The repetition of the dip slope and escarpment, together
with the heavy drift filling and the occurrence of so many forma-
tions together make this an important section. All formations from
the Shawangunk conglomerate to the Port Ewen shaly limestone
occur at this point, and although there is little outward evidence of
disturbance it is certain that whatever difficulty is tc be found in
this variable series is likely to be met here. It is therefore a sec-
tion that requires exploration both for depth of preglacial channel
and for quality of rock.
1 In construction this ground has proven to be good and sound throughout.
5
132
NEW YORK STATE MUSEUM
WH), 4
Wi) °
7
TENE It
Esopus Shales
any
atest
jail eee
TH
\\ CN NS
6/0
AN
NINA
~
SURFACE PROFILE
250'X
Fig. 20 Geologic detail of the central Rondout section constructed from exploratory borings data
WEST
EAST
_ GEOLOGY OF THE NEW YORK CITY AQUEDUCT 133
All of the formations dip westward wherever exposed, but the
dips vary somewhat, nearly all being of low angle. Occasional
minor inequalities of the nature of small rolls may be seen, as, for
example, the small fold in the gorge at High Falls [see pl. 24].
Explorations have shown, as indicated on the accompanying cross
section [fig. 20], that there is a deeper buried gorge here than at
Kripplebush. The deepest point discovered is a few feet below tide
level. The escarpment is steep and is formed by the Coeymans and
New Scotland formations. The dip slope is Shawangunk conglom-
erate, High Falls shale and Binnewater sandstone, with the Manlius
limestone forming the floor.
Identification of the drill cores which penetrate the limestone
indicate that the dip slope is reversed on the west side of the gorge
and that the stream had really reached about the axis of the trough.
A discrepancy in thicknesses and depths in hole no. 34 by which it
appeared that the Coeymans formation was almost twice as thick
as usual and that it contained a broken or crushed zone leads to
the interpretation that there is a small thrust fault here which re-
peats the formation as shown on the accompanying cross section.
Instead of a uniform westerly dip of all formations from the
Rondout westward it is proven that minor anticlinal rolls and even
thrust faults, as in this case, or such faults as in the Kripplebush
case are not to be excluded.
This structural relation has a direct bearing upon the question of
the thickness of the Esopus shales. The Esopus is certainly not so
thick as would otherwise be supposed, by 200 or 300 feet at the
least. The true thickness is still an unknown quantity (estimated
at 800 feet).
It is clear that the aqueduct tunnel will have to be constructed a
considerable depth below sea level at this section, probably not less
than minus 150 feet, even if the character of the formations be
neglected.
But the character or quality of these formations in view of their
structural relation constitutes the chief problem. Because of the
fact that every structure reaches the surface and eventually dips
gently to the west in such manner as to encourage water circulation,
their water-carrying capacity or general porosity becomes of great
importance. A great capacity is all the more serious because of
the heavy drift cover within the abandoned gorge, on top of which
1 This portion of the tunnel and its continuation south to the Shawangunk
range has been constructed at 250 feet below sea level.
134 NEW YORK STATE MUSEUM
the stream flows and which constitutes essentially an unlimited
storage reservoir to feed underground circulation. This is all the
more true if crush zones are extensively developed as accompani-
ments of the faulting.
In general as to perviousness the indications are somewhat ob-
scure. But the data now obtained seem to prove that all the for-
mations except the Binnewater sandstone and the High Falls shale
are compact and fairly impervious along the bedding lines. - Only
where crevices have formed or where crushing occurs is there
likely to be heavy circulation. This is all the more important since
so many of the beds are limestones known to be readily soluble in
circulating water. One of these limestones, the Manlius, exhibits
occasional large open solution joints at the surface — so large that
a surface stream disappears entirely at the so called “ Pompey’s
cave’ and joins the subterranean circulation. But such caves are
probably limited to the surface.
It is near this point, however, that one of the earlier borings at
one side of the present line discovered very soft ground at a depth
of about sea level, i. e. over 200 feet below the present surface,
which shows that similar conditions prevail at certain points to
great depth.
Pumping tests made on hole no. 32 in an attempt to establish
some data on the inflow of water gave very interesting results.
These tests were very thorough. It was proven that the water was
supplied in apparently inexhaustible quantity at maximum pumping
capacity, which was ninety gallons per minute. Futhermore, the
chief inflow seemed to be from the Binnewater and High Falls
formations as was to be expected. Whether a crush zone allowing
free circulation is furnishing a portion of this supply or whether
the whole inflow represents the normal porosity condition of these
formations is not yet proven.*
Other porosity tests have been made in such way as to locate
and measure this factor [see later discussion]. Hole no. 10 shows
an artesian overflow that comes from the Binnewater sandstone.
A working shaft has been put down also in the vicinity of hole
no. 32 and at the same depth found an enormous inflow of water
which drowned out operations for a time. The lateral supply in
this case has been reduced by introducing a thin cement grouting
through holes bored in the surrounding rock irom the surface.
Holes no. 12 and no. 14 also show an artesian flow, but both are
1 In construction the Binnewater sandstone has been found very wet.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 135
shallow holes and the supply comes from near the contact between
High Falls shale and Shawangunk conglomerate.
It 1s certain from these observations and tests therefore that the
Binnewater sandstone and High Falls shale are more porous than
the other formations, and because of the serious difficulties arising
from so heavy inflow of water from them the tunnel grade should
be shifted so as to avoid these formations as much as possible. A
comparison of the accompanying cross section, which is drawn to
scale [fig. 20], will show that a tunnel on one level would neces-
sarily run for a long distance in these beds because of the gentle
syncline. Furthermore, they lie at about the depth that would
otherwise be a safe depth below the buried gorge. But a tunnel
with a step-down, i. e. one run at two different levels could avoid
most of this poor ground. By approaching at a level of about — 50
feet or — 100 feet in the limestone beds to station 600 (hole no. 34),
then stepping down to — 250 feet, the line in a very short distance
crosses these two porous formations and enters the Shawangunk
conglomerate which is more substantial, and, all things considered,
one that seems most advantageous for successful construction. It
will have to maintain a head of more than 7oo feet as the difference
between hydraulic grade and the tunnel level in this section. Under
these conditions rock quality and condition are of greatest impor-
tance and there is no doubt about the advisability of avoiding the
poorest formations in some such manner. |
Coxing kill section. On the line of exploration the Coxing
kill flows over Shawangunk conglomerate and. High Falls shale.
Both dip plainly eastward, and a hole no. 11 located on the east
side of the brook penetrates about 70 feet of drift and shale. But
only a hundred feet to the east Shawangunk conglomerate outcrops
at the surface dipping the same way. It is certain therefore that
a fault occurs here. The dip of the fault plane is indeterminate
from the surface, but the relations and surroundings indicate a
fault of the thrust type.
Later explorations indicate that the fault plane is rather flat
[see cross section fig. 21] so that the shales are repeated above
and below a tongue of conglomerate. Boring no. 11 has also an
artesian flow of considerable volume coming from near the bottom
of the conglomerate. It is a mineral water. |
The chief importance of this section as a problem in applied
geology lies in the influence of the fault and the maximum de-
pression of the conglomerate. If the tunnel, which enters Hud-
son River slates at the Rondout creek section at — 250 feet can
be kept within that formation throughout the rest of its course,
136 NEW YORK STATE MUSEUM
there is no doubt that an advantage will be gained both in the
greater imperviousness of the rock and the greater case of pene-
tration. Wherever the conglomerate is undisturbed it is perfectly
good, but where broken the crevices are but imperfectly healed
and circulation is unhindered. It would therefore be desirable to
know whether at — 250 feet the whole of the downward wedge of
Shawangunk could be avoided. The borings indicate a thickness
of Shawangunk of 345 feet in hole no. 11 where it is cut at a
small angle, and a thickness of 409 feet in hole no. 36 where it prob-
ably lies pretty flat. This greater thickness together with the
LLEELEZEZETXEZ’ Yj.
a Ze Se LiL = 3
sooolt.
Fig. 2x Structural geologic detail of the Coxing kill section
finding of crushed rock at about the — 100 foot level leads to the
conclusion that the formation is overthickened here by the thrust
fault to the extent probably of about 75 feet. The true thick-
ness of the formation at this point is doubtless more nearly 300
feet than either of the figures obtained directly from the two
holes. If this interpretation is used as the basis of plotting a
cross section [see accompanying cross section] it is apparent that
the conglomerate should not be expected to extend more than a
few hundred feet east of hole no. 36 and it probably does not reach
a much greater depth than the — 236 feet represented as its base
in that boring.
1 Construction of the tunnel has progressed far enough through this sec-
tion to prove that the Shawangunk formation does not reach much lower.
It forms the roof of the tunnel for some considerable distance but does not
come down into the tunnel more than a foot or two.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 137
Shawangunk overthrust. At the extreme eastern side of the
Rondout valley near the point where the surface reaches hydraulic
grade again, the surface outcrops pass from High Falls shale to
Shawangunk conglomerate to Hudson River shale in the normal
order but with entirely too small an area of conglomerate consid-
ering the character of the formations. The higher ground is all
Hudson River in the vicinity, and there is abundant evidence of
crushing and disturbance. It is evident that a thrust fault is again
encountered here, one of sufficient throw to bring the Hudson River
slates above the Shawangunk conglomerate — probably a lateral
displacement of very great extent. Explorations have fully proven
the existence of this fault. The accompanying diagram shows a
cross section as now outlined by complete penetration of two
borings.
Two trial tunnels were run to test working quality of Hudson
River slates compared to Shawangunk conglomerate at this locality.
Both are within the influence of the fault zone. Both are there-
fore more broken than the normal with the result that the Hudson
River slates probably show poorer condition than usual and more
troublesome working, while Shawangunk conglomerate probably
shows easier working than usual. It is believed that normally the
two rocks would present a greater difference than was found in
this test.
Special features
Several questions, some of which have a practical bearing, have
been raised as separate features during the exploration of the
Rondout valley.
Caves. One of these is in regard to the possible existence of
underground caverns. This was given a special prominence early
in the work by the experience of one of the drills. After pene-
trating the limestone series near High Falls to a depth of over
200 feet, the drill seemed to leave the rock and enter a space
allowing the rods to drop 28 feet before being arrested by sclid
material. The further attempt to work in this hole resulted in
the breaking of the rod down at this point and the subsequent
failure to recover the diamond bit which is still in the bottom of
the hole. The question is as to the meaning of this occurrence.
Is it a cavern?
“Pompey’s cave”’ has been referred to in an earlier paragraph.
This is clearly not much of a cave. It is essentially an enlarged
joint or series of joints by solution along the bed of a surface
138 NEW YORK STATE MUSEUM
stream to such extent that the stream normally at present has be-
come subterranean. It is the writer’s opinion that the case en-
countered by the drill boring is similar. The apparent cavern is
probably a slightly enlarged joint along a line of somewhat abun-
dant underground circulation and perhaps associated with some
crush zone developed by the small faulting known to occur in this
immediate vicinity. .It is probably not entirely empty but contains
residuary clay, and in all likelihood is very narrow and not exactly
vertical, so that the drill rods were bent out of their normal course
and wedged into the lower part of the crevice. Smaller spaces
of this sort were encountered at a few other points.2
These occurrences seem to indicate that the limestone beds yield
rather readily to solution by underground water, and that this cir-
culation has been at one time active to at least 50 feet below pres-
ent sea level. With present ground water level nearly 200 feet
above sea level it is extremely unlikely that any such action is
going on at so great depth. The occurrence is therefore strongly
corroborative of former greater continental elevation when the
deep stream gorges, now buried, were being made. These deeper
caverns or solution joints probably date from that epoch.
Imperviousness and insolubility. The question of impervious-
ness and closely associated with it that of solubility, is of great
practical importance in this particular work. The immense pres-
sure under which the tunnel will be placed in crossing this valley
makes it impossible to construct a water-tight lining. Everywhere
much depends upon the rock walls to help hold the water from
serious joss. Wherever the-rock is fairly impervious except
occasional crevices or joints they can be grouted and safeguarded
satisfactorily. But where a formation is of general porosity this
can not be so successfully done. Even more difficult to handle ts
the rock wall which is soluble and which therefore with enforced
seepage may tend to become progressively more porous. That this
consideration is not wholly theoretical is shown very forcibly by the
Thirlmere aqueduct of the Manchester (England) Waterworks.
In that case a 3 mile section was built through limestone country
using the same local limestone for concrete aggregate. Although
1JIn constructing the tunnel several clay-filled spaces have been discovered
in the same vicinity at elevation—1o0. One of these extended vertically with
a width of I to 2 feet and from it a great mass of mud ran into the tunnel.
At one point it was connected with a horizontal space of the same kind
extending 15 feet. It can be seen that the original crevices have been en-
larged by water and that they were originally formed during faulting.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 139
this concrete was mixed as rich as I part cement to 5 parts aggre-
gate and the work was well done, excessive leakage reaching a total
of 1,250,000 imperial gallons per day was developed within a year.
It was found that the limestone fragments of the aggregate were
corroded forming holes through the lining of the aqueduct and that
these holes actually enlarged outward. All this was done under cut
and cover conditions with not more than a 6 or 7 foot head on the
bottom of the aqueduct.
In the Rondout valley, the aqueduct will cut no less than 6 lime-
stone beds in all cases under great pressure. This fact will in all
probability tend to increase the action. But, of course, some of the
beds may not yield so readily to solution. Tests made thus far,
however, indicate that all are attacked in water. Considering these
facts it seems desirable, so far as possible, to avoid the limestone
beds wherever rock of greater resistance to solution can be reached,
and further it is equally desirable to use a more resistant rock for
the lining concrete. So long, however, as the formation is not very
pervious so that a new circulation could not be established by the
escaping water there would be little harmful effect.
An average of five analyses of the Thirlmere limestone, different
varieties of the same formation, gives the following:
See iru SIcIMISKIATLETAR ue ae Lek RE RE IE ee 2.77270
Puiiaiiia sand Irom oxi AlsOstPesOs. 12205 oak oc che ce eee odes 0.276
1a EE Seg OVE TE Ge re RU URE es a en se 53.076
TA SURES TODAY EL ORE Pe nO a Se ea a ee a . 390
RMR BTM uCHOUIV CLE Tells Caer aytice ar wick hi Bic dacs Sicle yea alee 6 6 Ep aA es 42.248
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 14!
Rondout siphon statistics
1 Total borings on the sipkon line. Three different boring
equipments have been used owned by different parties and records
have been kept so that the work of each can be followed or com-
pared with the others.
On this division the Board of Water Supply owned and operated
one machine with their own men, another equipment was owned
and operated by C. H. McCarthy, while a third which finally did
a majority of the work, belonged to Sprague & Henwood, Con-
tractors, of Scranton, Pa.
The totals of different general types of material penetrated by
these machines are as follows:
Feet Feet Total Per cent
ot of feet of of core
drift rock depth saved
Gee Wier on auipimente-cgan. “ni. octat os 1740.5" 2I75. 5 3016 89.4
Gropracic. & Henwood! is. ....2022 5.660. 3047 6831 10478 60.04
mereCarthy machine 2 oF 6) OS ee es 181 1228 1409 78.1
The average saving of core by all machines, cutting all kinds of
bed rock was 75.96%
2 Core recovery from various strata. So nearly as can be
done the strata represented in the drill cores have been identified
and summarized as to total penetration and core saving. The core
saving is a factor of prime importance in judging of the quality of
rock and its freedom from disturbance. The following items are
gathered from a study of the whole series.
a@ Holes 6, 10, 12, 13, 15, 17, 18, 21, 22 and 25 penetrate Helder-
berg limestone, a total combined depth of 1096 feet. Individual
holes vary in core saving from 39.3% (no. 13) to 95.3% (no. I5).
The average core saving is 78.19%.
b Holes 8 and 9 are in Onondaga limestone with a total pene-
tration of 197 feet. The core saving varies from 56.2% to 92.8%
with an average of 74.5%.
c Holes 11, 19, 20, 23, 24, 27 penetrate Hudson River shale and
together represent a total of 696.5 feet. The core saving varies
from 16.6% to 89%, with an average of 42.14.
@ loles,6, 10, 11, 12, 14, 16 and 20 cut High Falls shales to a
combined total of 410 feet. The saving varies in different holes
from 17% to 83.3%, with an average core saving of 44.54.
e Holes 8 and 26 penetrate Esopus shale and penetrate 76 feet.
142 NEW YORK STATE MUSEUM
The core saving varies from 73% to 84.6¢, making an average of
78.8%.
f Holes 10, 11, 12, 14, 16, 19, 20, 23, 24 and 27 penetrate Shawan-
gunk conglomerate a total of 1356.5 feet. Core saving varies in
different holes from 33.3% to 100%. The average recovery is 60.524.
g Holes 6, 10, 12, 15 and 16 cut Binnewater sandstone. The
total penetration is 205 feet. The range of core saving is from
30.6% to 74.7%, with an average of 562.
h Holes 7 and 9 cut Hamilton shales to a total amount of 65 feet.
The range of saving is 70% to 81.8%, with an average of 75.9%.
3 Artesian flows. Several of the borings struck artesian fiow
of water. The fact that the sources of this flow are not the same
has led to a tabulation of these data.
RECORD OF ARTESIAN FLOWS
Flow
; Static Flow encountered
Hole Size in head gallons at elevation
mo. inches infeet Minute Day Feet Strata
IO I 18 30 43 200 —I09 ....Binnewater sandstone
II I 10 10 14 400 — 60 ....Shawangunk conglomerate
12 3g I «Fee eee nase — 24 ....High Falls shales
14 TA BAL se A, eee + go : *
20 3% 7G IO 14 400 +108 ....Shawangunk conglomerate
23 2S ae ee wibnatt Ma Nae eet ee ne ‘
31 DME kt coed RE Sas PAE Fs = 1s a hey Ano v3
39 aaa fcr he EES wha MSA hp +112 ....Helderberg limestone
SNE xK 7 ee 432 +203 ....Hamilton shale (possibly
drift)
Pumping experiments and porosity tests
Systematic tests have been made for flow of water, behavior of
ground water and porosity of rock on certain of the Rondout ex-
ploratory holes under the direction of Mr L. White, division engi-
neer. A summary of these tests has been furnished by him from
which is quoted the following:
In addition to determining the location and thickness of the beds
and the general character and condition of the rock from inspection
of the cores, serious attempts were made to determine the relative
porosity and water-bearing quality of the rocks encountered for
the following reasons. (1) To determine the probable leakage from
the siphon when in operation. (2) To determine the probable
amount of water to be handled in construction. These experiments
were divided into three classes: (1) Observation of flow from cer-
CEOLOGY OF aE NEW: YORK CITy AQUEDUCT I43
tain drill holes which showed sustained flow of water. (2) Pres-
sure tests in which water was pumped into holes which had been
sealed off and pressure and leakage noted. (3) Pumping tests in
which water was pumped from 4 inch drill holes by means of deep
well pump of the type used in oil wells, and fall of ground water
during pumping and subsequent rise after cessation of pumping
noted. A description of the first two and the results obtained from
them follows:
A substantial flow of water was observed from the following
holes:
11/17: 50 gallons per minute through 2% inch pipe, static head
10 feet
10/17: 30 gallons per minute through 1% inch pipe, static head
di FCCE
20/17: Io gallons per minute through 3 inch pipe, static head
ge, fcc
The static head was observed by adding on lengths of pipe until
the water ceased to flow over. It will be noticed in the case
of hole no. 10 that the flow from the 1% inch pipe is not that due
to static head of 18 feet, but that due to a head of only % foot. In
other words the friction head is about 17.5 feet, and the velocity
head only % foot. This same condition holds true of the other
holes from which a flow was obtained. This would seem to indicate
that the amount of water is not very great but that it is under con-
siderable pressure. It is believed that this pressure is caused by
gas.
A slight flow was observed from the following holes: 12/17,
14/17, 23/17, 31/44, 39/22, and 5/NE.
The flow from most of these holes has ceased since the pipe used
in boring was withdrawn. There is still some flow from the follow-
mae holes: 11/17, 20/17, 25/17 and 5/NE.
The flow from hole 11/17 is constant at about 10 gallons per
minute. The others are too small to be measured. It will be noted
that the only substantial flows encountered were from the High
Falls shale, Binnewater sandstone and Shawangunk grit, and that
it was possible to force water into these rocks in greater quantities
and at a less pressure than in the other shales and limestones.
Porosity tests. The method of making these tests was as
follows:
Wash pipe equipped with a device for sealing the hole was
lowered to the desired elevation. The seal consisted of alternate
layers of rubber and wood around the pipe preventing the water
from escaping between the walls of the hole and the pipe. Water
was then pumped in and pressure and leakage noted.
The result of the pressure tests was to show in a general way:
(1) That the pressure increased with the depth of seal. (2) That
the leakage decreased with the depth of seal. (3) The maximum
pressure in the grit was 140 pounds to the square inch and minimum
144 NEW YORK STATE MUSEUM
leakage was 5 gallons per minute. (4) In the Hamilton shales a
pressure of 300 pounds to the square inch with very little leakage
was obtained.
The unknown factors are too many and too great to make any
reliable deductions from these experiments. |
DATE APR./IO 1S
°
} oer
ORIGINA eed WATER _LE eae :
NOTE— FEB.Z29-MAR27] PUMPED 490.000 GAL
PROBABLE LIEVEL APR. O.
aad SS
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Fig.723 Curve showing fall of ground water level while pumping;from_boring 34
Pumping experiments were carried on in holes 32/22 as follows:
The apparatus used was a deep well pump of the type used in oil
wells. The holes were of an inside diameter of 414 inches and
were cased to the bottom. A 3% inch working barrel was then
lowered to the bottom of a line of wooden sucker rods. The stroke
was 44 inches and the nominal capacity of pump at. 38 strokes per
minute was 60 gallons per minute or 86,400 gallons per day. The
power was obtained from a 40 horsepower boiler and 35 horse-
power engine belted to a 10 foot band wheel which was connected
to a 26 foot walking beam. In hole 32/22 at station 607 + 50 the
average discharge at 38 strokes per minute was go gallons per
minute or 129,600 per day. The experiment was continued for 15
days and the total amount of water pumped was 1,071,000 gallons.
The ground water level was not lowered. It will be noticed that
the discharge at this point was 50% in excess of the theoretical
capacity of the pump. This was caused by the presence of gas, the
effect of which seemed to be increased by the churning action of
the pump. This may also explain the failure to lower the ground
water.
The experiment at hole 34/22 was similar in character. The
upper 230 feet of this hole had an interior diameter of 474 inches
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 145
and the bottom 274 feet a diameter of only 3% inches. At first a
2% inch working barrel was used to pump from the bottom and a
discharge at 32 strokes per minute averaged 24 gallons per minute
or 34,500 gallons per day. This was continued for about 15 days
and the total quantity pumped was 490,000 gailons. The ground
water level was lowered 17 feet at hole 34 and 4 feet at hole 32,
750 feet away.
The 2% inch pump was then let down to a depth of 200 feet
with a 234 inch casing reaching down to the Binnewater sandstone,
depth of 437 feet. The average discharge at about 40 strokes per
minute was 60-65 gallons per minute, or an average of 90,000 gal-
lons per day. It will be noted that the discharge was much smaller
than at hole 32 owing to the absence of gas. Pumping with a
3%4 inch pump was continued 16 days and 1,532,000 gallons of
—— JAERI 27,1908. 4|P.my.
_-_—_—— —_
Fig. 24 Diagram showing successive stages of ground water level between holes 32 and
34 during pumping
water were pumped in addition to the 490,000 gallons from the 2%4
inch pump. The ground water level in hole 34 was lowered 36 feet
in addition to the 17 feet by the 214 inch pump, but rose g feet in
26 minutes, and 30.5 feet in the next five days. In the next 22 days
it rose 9.15 feet, or .42 feet per day.
Reduced water level in hole 32, 750 feet away by pumping in 34,
15 feet, or 1 foot for each 120,000 gallons pumped. In the first
MUSEUM
NEW YORK STATE
146
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3A31 UBLVM ANNOYS WWNISIBO MoO1dG 1334 NI HLidsd
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GEOLOGY OF THE NEW YORK. CITY AQUEDUCT 147
three days after pumping ceased water rose 5.2 feet, and in 22 days
rose 9.8 feet or at the rate of 0.45 feet per day.
During construction! shaft 4 located at same point as hole 32/22,
station 607+ 50, has proved a very wet shaft, the inflow varying
from 400 to 850 gallons per minute. Pumping at this shaft has
lowered the general water level and correspondingly lowered the
water level in hole 34/22 at station 600 + 00.
ee
ren ——4 HOLE 34 SS
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Fig. 26 Curve showing rise of water in holes 32 and 34 after pumping ceased in hole 34
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1 From this shaft after reaching tunnel grade,— 250 feet, and after running
northward into the fault zone and porous shales, the contractors are pumping
1300 gallons per minute.
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Fig. 27 Profile and geologic cross section of the Wallkill valley from the Shawangunk mountain range to the hilly divide near Ireland Corners on the east side of the valley.
dapted from drawings of the Board of Water Supply)
The exploratory borings are represented, there ik an attempt to classify the drift variety, and the approximate position of the presture tunnel to be conirucied for the Catskill aqueduct is indicated
— eh) A ee ae
suck mok
CHAP TP lat
THE WALLKILL VALLEY SECTION
Between the Rondout and Wallkill valieys the aqueduct is to
follow a tunnel at hydraulic grade which so far as can be seen will
cut only Shawangunk conglomerate and Hudson River slates. No
doubt there are many complicated small structures which because
of the nature of the slates can not be reconstructed. The work
of tunneling is not advanced far enough to add anything. But
in the Wallkill valley, where it is necessary again to plan a pressure
tunnel several hundred feet below grade, a considerable amount
of exploration has been carried on.t .
These explorations [see sketch map fig. 8] are distributed along
several lines crossing the valley at intervals between Springtown,
about 3 miles north of New Paltz, and Libertyville, which is about
an equal distance south.
The geology is simple. Only Hudson River slates form the rock
floor, and so far as can be judged no other formation is likely to
be cut by the tunnels. There are no doubt many complicated struc-
tures, both folds and faults, as indicated by the high dips, but again
because of the nature of this rock it is impossible to discriminate
closely enough between different beds to determine exact relations.
The point of greatest practical importance lies in the fact that
the rock is fairly uniform and, although much disturbed is of
such nature that crevices and joints or fault zones are almost as
impervious as the undisturbed rock. This is because of the tend-
ency of a formation of this composition to heal itself with fine,
compact clay gouge. In fact, the mechanical disturbance produces
or develops the cement filling contemporaneously with the move-
ment. It is chiefly a mechanical filling, whereas the healing of a
harder and more brittle rock like a granite or a limestone requires
more chemical assistance.
An additional practical question involves the estimate of depth
required to avoid any possible buried Prepleistocene gorges and
maintain a safe cover to guard against undue leakage or rupture.
1Explorations on the Wallkill division are carried on under the direction
of Lawrence C. Brink, division engineer. The final construction is in charge
of James F. Sanborn, division engineer, with headquarters at New
Paltz, N.Y. .
149 ca es
150 NEW YORK STATE MUSEUM
To this end most of the explorations were made. Two lines less
than a mile apart on which a few exploratory borings were made
near Springtown indicate two buried channels, a master channel
and a tributary from the west which converge northward. A
maximum depth reaching 70 feet below sea level was found on the
more northerly line almost directly beneath the present stream
channel which flows on drift at an elevation of 150 above tide.
The more southerly profile reaches only sea level indicating a
gradient for the preglacial stream at this immediate locality of more
than 79 feet per mile.
In the vicinity of Libertyville, 5 to 6 miles farther south, where
the aqueduct was finally located, the profile was found to be con-
siderably higher. Intermediate profiles are shown in accompany-
ing figures. The deepest point yet found on the Libertyville line
is 65 feet above sea level. It is worth noting that the gradient of
the ancient Wallkill is therefore shown to be decidedly unsymmet-
rical. The rock floor formation remains the same although it may
vary somewhat in character. Under these circumstances, however,
- a gradient of 13 feet per mile from Libertyville to Springtown
forms a sharp contrast with the 79 feet per mile represented at
the Springtown locality. In view of the remarkable inorease of
gradient and the narrower form it seems reasonable to regard this
as a rejuvenation feature developed at the time of extreme con-
tinental elevation.
How much deeper the lower Wallkill may be, including the so
called Rondout river, which is really a continuation of the ancient
Wallkill and geologically belongs to this drainage line instead of
to the Rondout, no one can tell. But it is at least interesting to
observe that the intervening distance from Springtown to the Hud-
son at Kingston is approximately 12 miles and that a gradient for
that distance equal to the average known in the 6 miles explored,
i. e. 24 feet per mile, would depress the outlet 288 feet more.
That would be equivalent to 367 feet below sea level. If, how-
ever, a steep gradient such as that at Springtown prevails in this
lower portion it is necessarily much lower— for example if a 79
foot gradient is maintained it would be possible to reach a final
outlet at —1029 feet. It is likely that an intermediate value 1s
more nearly correct. This has, however, an important bearing
upon the question of maximum Hudson river depth, especially the
existence of an inner deeper gorge above the Highlands. So far
as this Wallkill profile goes, it supports the gorge theory. It is
certain that the Prepleistocene Wallkill flowed north not very dif-
(AJddng J91e
jo pivog Aq ydeisojoyqg) ‘esurt ureyunoU yUNnSsUeMEeYS IY} UL BeIDQ NOdUOIG PieMO} SUTYOO] AD]RA T[PFIeEM AL
SZ 93e1d
Plate 25
NO. AQUE. DEPT. B.WS. NYE. WALLMILL DIV.
PROFILE SHOWING WASH AND CORE BORINGS
LINE A - SPRINGTOWN
i
&
. ~ z
“ J © > > =
—_— 2 ¥ een 8 Ss 8 Sees S =
oy = 3 Q ~ = ~ < Siow i SS ~ a N“
y -y 8 ¥ ee x S S SN Q s N
; SoS SWS 5 3 S >
3 = $ S Sih aS S Seas CVS y S N
; : s 2
= ra SS Ss S =
te n PS = nee aR
Sare Westie eo ee = % és
S Sas = tS N \ aa = ~
SoA ess aes Beers Ree Bee ee eek
x LS > > ast = © ~ S as ee . XS = ee
= ss > = 3 5 2 2 RN BOR ¥ RF : > oe
———__ gs y << 4S BS Py & ~ 2 NOS Aon sn Q S$ ee
“ SiR Sis SR ek SS Nek ee ee RG Roe NS caste
5 ae ee eS DSESGN x = ROD Do SS Be, ie
ea etee SS he 2 AO Res ek Se Ne ;
W tases niwiiscerdh 9p a SS Ss CS SS Ss XS 8S = 8 SS Se Ss LS ee
ae Ay eee . Pe Ss Shee = ~ = tS NR Gyr “: = 2a 2
pry “3 ee Sey g < Q S S x Nae S = 8 = xa a ee
Ser 220 : . ah = = Ye S LS I ae
aah 5 es
: : aa
i a, 3
j . 2 oe
i Sevico fie, 20
{
' ase
H iat
i ? er)
j .
i
i
f= | Z
ee WALLKILL SiPHON Se
i z }
i
i
i
é ou =k ioe = oe s ‘ai ees |
Profiles of the present and preglacial Wallkill channels near Liberty-
ville, and a diagrammatic section showing the different types of drift-filling
together with the borings which furnished the data
GEOLOGY OF THE NEW YORK CITY AQUEDUCT I5i
ferently from the present stream except on a steeper gradient, but
in all probability the headwater supplies between this stream and
the Moodna have been somewhat shifted. It is possible that some
former Moodna drainage area is now tributary to the Wallkill.
But these changes were wholly glacial in origin and the extent of
such shift is indeterminate at present.
It is a notable fact that a large proportion of the work of ex-
ploration in this valley was done successfully by the wash rig.
The extensive lot of data was gathered without much delay or
difficulty. This is because of the nature and origin of the drift
cover. A considerable proportion of the drift mantle especially
in central and deeper portion of the valley is modified assorted
sands, gravels and silts or muds. In part they represent deposits
in standing water laid down at a time when the lower (north) end
of the valley was obstructed by ice and while waste was poured
into the valley from neighboring ice fields. It is impossible to
reconstruct the beds of these materials with any degree of accuracy.
But it is at least certain that lens or wedgelike layers of differ-
ent quality of material were penetrated, indicating oscillation and
overlapping of deposition conditions, boulder beds and till being
interlocked with assorted sands and gravels. But there is appar-
ently no evidence of ice deposits of greatly differing age. The
accompanying profile and cross section is a representation of ma-
terials on the Libertyville line based upon identifications made by
the inspector of the Board of Water Supply of the Wallkill Divi-
sion under Mr L. C. Brink, division engineer.
CHAPTER VIII
ANCIENT MOODNA VALLEY
Moodna creek enters the Hudson from the west between Corn-
wall and Newburgh not more than a mile north of the entrance
to the Highlands. It is a retrograde stream in its backward flow
similar to the Wallkill. But its channel at present is almost
wholly on glacial drift which it has trenched to a depth of more
than 100 feet below the average adjacent surface. How much
of its retrograde course therefore may be postglacial is not so
clear. It seems necessary, however, to account for all drainage
on the north margin of the Highlands by streams flowing to the
Hudson northward. There is no notch low enough for their escape
elsewhere. The ancient Moodna must have carried most of this
run-off from the district occupying the angle between the Wall-
kill and the Highlands. This stream may have drained even more
of the region now forming the divide with the Wallkill than does
the present Moodna. In any case it must have been a stream of
considerable size, capable of excavating a valley or gorge of
greater prominence during the period of early Pleistocene rejuvena-
tion than now appears. Furthermore its position makes it highly
probable that tributaries of fair size entering in its lower course
were also effective enough to require consideration. This conclu-
sion has led to the exploration of the Moodna valley in consider-
able detail in preparation for the aqueduct work.
The Catskill aqueduct is to cross the stream near Firth Cliffe,
which lies almost directly west of Cornwal!-on-Hudson, and be-
cause of the low surface elevation across this valley, as in the
others, a pressure tunnel in rock is judged to be the most suitable
type of structure. The accompanying sketch map shows the
location. |
Explorations were conducted especially for the buried channels
and character of rock floor.
Geologic features
The region is one of chiefly Hudson River slate. But there
are inliers of the older rocks such as Snake hill which belongs to
a long ridge of Precambric gneiss and granite, brought to the sur-
face by folding and faulting and there are more rarely outliers of
younger formations such as Skunnemunk mountain. Farther north
[53
154 NEW YORK STATE MUSEUM
at Newburgh a gneiss ridge is accompanied by limestone, but in its
southerly extension the slates are in direct contact. This relation
is believed to be wholly due to faulting on both limbs of the anti-
clines. This gneiss ridge disappears southward beneath the drift,
but the borings have shown that it continues across the aqueduct
line, although it has lost its influence on the topography. There
are other inliers of similar character such as Cronomer hill 3 miles
northwest of Newburgh. Between these two gneiss ridges lies the
southerly extension of the Wappinger limestone belt. But so far
as is known it disappears beneath the Hudson River series long
before reaching the line of exploration.
Near Idlewild station, filling the space between the two branches
of the Erie Railroad, there is a syncline containing the series of
Siluric and Devonic strata which spreads southwestward to include
Skunnemunk mountain, an outlier of Devonic strata. This is the
only occurrence of these formations in this region south of the
Rondout valley. The structure and stratigraphic features of this
occurrence have been worked out by Hartnagel. Its northward
extension in all probability terminates abruptly by a cross fault not
far north of the Ontario and Western Railroad.
From these occurrences southward to the Highlands proper
nearly everything to be seen through the drift is Hudson River
slates.
The Highland gneisses are bounded on the north side by a fault
or series of faults. This brings various members of the overlying
series into contact along the margin. In the best place where a
direct observation can be made the gneisses are thrust over upon
the Hudson River slates along a plane that dips about 40 degrees
to the northeast. It is probable that a displacement of as much
as 2000 feet or more could reasonably be assumed at this place.
The contact zone also is much crushed and bears every evidence
of having undergone extensive disturbance of this kind. Others
of this same type occur within the gneisses where weaknesses
formed in this way permit the development of such notches as
Pagenstechers gorge. In some cases the rock beneath the surface in
these zones is more decayed and less substantial than that at the
surface.
Exploration
The first borings made with the wash rig were found extremely
unreliable in the Moodna valley. That is because of the very
heavy bouldery drift forming the greater part of the filling on the
ancient topography. Next to the Hudson river gorge itself, no
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 155
place has presented greater difficulties in penetrating this drift man-
tle. Boulders of such immense size occur that they have to be
drilled like bed rock. In one-of the holes a boulder 30 feet
through was penetrated and 100 feet more of drift found below.
Progress in such ground is extremely slow and costly. This is
so much the more so where as in this case there are long stretches
with unusually deep cover.
A glance at the accompanying profile and cross section will show
a very deep and wide valley. Many of the borings are more than
300 feet in drift which almost wholly obscures the ancient topog-
raphy. The present Moodna is about half as deep and occupies
the extreme eastern margin of the older gorge. Tihere is a sec-
ondary gorge on the west separated from the main channel by a
sharp divide. A few other smaller notches in the line represent
smaller tributary or independent stream courses. One of these
of much interest is known as Pagenstechers gorge.
The rock floor at all points except two in the central Moodna
valley including its two nearest tributaries is Hudson River shales,
slates and sandstones of considerable variation, sometimes much
brecciated. The two exceptional borings are no. 8/A44 and no.
16/A44 on the west flank of the westerly tributary gorge, and
they are in pegmatite and granitic gneiss which is in all probability
the narrow southerly extension of the Snake hill ridge. Here
again neither quartzite nor limestone were found on the flank, a
condition that seems to support the view of a double fault along
the Snake hill ridge.
In striking contrast with the broad central Moodna are the two
narrow and very deep notches farther to the east, the first in
slates and the second (Pagenstechers) in Highlands gneiss.
Special features
Course of the Moodna. The chief interest centers around the
Moodna channel. There are several unusual conditions, for
example:
The rock floor along the profile is almost flat for a distance of
nearly half a mile in spite of the fact that there would seem to
be every reason for a different form. The differences in hard-
ness of rock floor alone would encourage differential erosion; and,
since the structure of the formations, the strike, is almost parallel
to the supposed course of the stream, the influence of different
beds would be at a maximum. Furthermore, the deep gorge of
the Hudson, into which the stream flowed is only 2 miles away;
156 NEW YORK STATE MUSEUM
and if that gorge represents stream erosion to such depth (over 750
feet) it would indicate a gradient of nearly 300 feet to the mile
for the last 2 miles of the Moodna—a condition to say the least
decidedly unfavorable to the development of a flat-bottomed valley.
Of course, if the profile as deterinined can be assumed to run
exactly parallel to. the old stream channel for half a mile it would
be less surprising. But even then it is too flat. For so short a dis-
tance from the Hudson gorge the gradient ought to be much
greater than the variation observed in the Moodna channel. There
are certainly reasons in the structural geology favoring a northeast
course instead of one parallel to the profile line. And if the
stream really did flow across this structure, the differences of
hardness of beds ought to have encouraged a much greater differ-
ence in depth of channel than the profile presents. With structures
all running northeast there is every reason to expect the stream
to follow them.
Recent exploratory data strongly supports the theory that the
Hudson gorge at Storm King gap is widened and possibly some-
what overdeepened by glacial ice. Under normal stream relations
one might consider the Moodna a tributary hanging valley, itself
rounded and smoothed to a broad U-shape by ice. This would be
a very easy solution if it were not for the fact that this tributary
Moodna opens into the Hudson as a reversed stream, 1. e. it opens
against the flow of the Hudson and more or less directly against
the known ice movement. It can not be a hanging valley there-
fore of the normal sort. If a hanging valley of ice origin at all
it would necessarily be one therefore gouged out by ice moving
from its mouth toward its head, a case that so far as the writer
knows has never been observed. The chief objection to this theory
is that in no other gorge or channel (with one exception, the Hud-
son at Storm King gap) anywhere in the region so far as known
is there any evidence of serious modification of an original stream
channel by the ice invasion. Of course, the axis of the valley is
favorable and the situation is peculiar in that it parallels the High-
lands front in this vicinity and the action of the ice may be as-
sumed to have been somewhat concentrated along this margin hbe-
cause of the obstruction.
Inner notch or secondary gorge. Those who habitually em-
_ phasize ice action would no doubt choose to regard this whole val-
ley as shown in the profile, as chiefly glacial in character and
origin. If that explanation is the true one, then it must be ad-
mitted that a deeper smaller inner notch or gorge is unnecessary
and indeed unlikely.
GEOLOGY OF THE (NEW “YORK ‘CITY AQUEDUCT 157
The critical point therefore in the whole argument is as to the
origin of the valley, 1. e. is it essentially a stream valley? Or is it
as to present rock floor form wholly a glacial valley?
If it is a stream valley then no doubt full account must be taken
of the proximity to the Hudson, and the possibility of developing
a temporary graded condition and some adequate allowance must
be made for its work during the subsequent continental elevation
and the deepening of that river to several hundred feet below the
known bottom of the Moodna. In short, one would expect a nar-
row deeper notch in the Moodna floor as a result of this rejuvena-
tion. But on the contrary if in preglacial time the stream were
not so powerful and had not been able to keep pace, and if the
ice movement can be assumed to have concentrated along this line
to such efficiency as to gouge out a groove 3000 feet wide almost
flat to a depth of 300 feet only guided in direction by the original
Moodna, then one may readily abandon the idea of a deeper notch.
One or the other of these types of origin must be the chief
factor in reaching a reasonable opinion as to the presence of an
inner notch.
In any attempt to choose between these factors, one is led to
reconstruct the preglacial drainage lines. When this is done it at
once appears as most probable that there was at that time as now
a considerable area tributary to the Hudson with a stream course
very much like the present Moodna. In other words a fair sized
stream is assured. Once such a stream is granted and the effects
of its work reckoned in full knowledge of the adjacent Hudson,
and its probable behavior is studied in the light of the data ob-
tained in exploration of the valleys of other tributaries, it becomes
more and more difficult to wholly eliminate the inner gorge idea.
It seems to the writer probable that the valley owes its erosion
chiefly to the preglacial stream. But the channel has suffered sub-
sequent widening and smoothing by ice especially in its upper and
broader portion, below which there may yet be a notch. One must
admit that the results of boring prove the notch to be very nar-
wom, Ices tnan 150 feet, or else mot tere at all: In reaching an
opinion as to the possibility of one so narrow, it is worth while
to note that the Esopus, which is a larger stream, has cut down
at Cathedral gorge to a depth of from 50 to 80 feet with almost
vertical sides and only about 150 feet wide. This gorge further-
more is cut in almost horizontal strata of such character that
there is no special structural tendency in them to contract the
stream. At the Moodna on the contrary, in addition to the smaller
158 NEW YORK STATE MUSEUM
size of stream, the rocks stand on edge and run parallel to the
supposed course so that this structural influence is toward a nar-
row and reasonably straight gorgelike form. It is not only pos-
sible that the gorge is narrow, but even probable that it is narrower
than the present Moodna, i. e. less than 100 feet wide.
How deep such an inner gorge may be if it does exist is a prac-
tical question in this particular case, because its depth has a direct
influence on choice of depth of pressure tunnel. Because of the
evident narrowness it is likely that it is not of very great depth
— in view of the quality of these shales perhaps not over a hun-
dred feet.
Is there any one point more than another favorable for such a
notch? There are two facts bearing on this question, (1) the vari-
ation in core saving which indicates that hole no. 5/A44 with
7% has a recovery of only 1/5 the average, and (2) the fact that
hole no. 15/A44+-, which is the next hole, shows the lowest bed
rock in this valley. On the ground of profile therefore and on the
ground of structural weakness there is reascn to choose this space
between no. 5/A44 and no. 15/A44 as the most likely position.
Summary. The very abnormal profile of the Moodna valley
based upon the borings may be due either (1) to parallelism with
the stream course, or (2) to a graded condition of the stream in
some preglacial epoch, or (3) to modification of an original less
prominent channel by ice erosion.
It is the opinion of the writer that the ancient stream crossed the
profile line much as the present stream does, that the additional
narrower valley immediately to the west side is that of a pre-
giacial tributary instead of a bend of the Moodna itself, that there
was a development of a moderate sized somewhat flattened valley
corresponding to the benches and shelves noted in other streams,
including the Hudson, that subsequent elevation of the continent
rejuvenated the stream which cut a deeper narrow inner notch, that
glacial ice moving in reverse direction widened and smoothed this
upper portion of the valley, but that there may yet be a remnant
ot the deeper notch in its bottom, and that the space between holes
no. 5/A44 and no. 15/A44 is the most likely location of this inner
gcrge.
Tributary divide. The sharp divide between the two deep
portions of the valley bottom has proven an evasive feature in the
later exploration. Two holes put down a short distance to the
southward (24/A44 and 20/A44) failed to find the rock floor so
high, one reaching rock at a depth of 181 feet and the other failing
Se ne
—
a te
ALAN
We
AULA HAE
es il x =%,
, rn 3 A
Py - 7 ;
t as oa
on. ~. i 4
AY ut
q
we 3
a ¥
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a v a &
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ae
ne
.
a
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is i“ =
SS
? Wh
x ' ‘
: fs As A | .
3 i. : | Nyt
a \ :
— 300.
300_ SS & R “e es RS Ny Chie
| i ic i ij if Wi ES i A) = Hy Wy ih iil Wl) HUN AN i
20°F (L/h ii =f il ili HN AY \\ WN Vo» NN ANN N j LY Vy Hii AN AN
a eGR nae LL ANN ee) ak MMA A Hh 108 taal ANd iS
oo eM Th mi Gh Uh} MN WH), Hy HY) ttt i I Hing
a Ha katabelt ve Hi Hl i il HA Hh HH i) oo Ih WH My} Oe ie Mh Wl Ve MH Tin iy}! yy) \\\ Lap yO) |. J: GEOLOGI c SE CTION
Oo LALMEETS jj / ili , My NANCOTT Wiha BAK icil Tg
ii LLL cl QL conta OAR ak, eee
y A ce Slate Slate PROFILE ALONG LINE | C-D-E-F am VERT es
PROFILE ALONG LIN: nel
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 159
to find rock even at 213 feet. Two others nearly a thousand feet
to the westward, however, found rock again at approximately the
same elevation as the divide. If this is a tributary stream divide
therefore it must have an east-west trend.
Pagenstechers gorge
This is a notch between Storm King ridge and Little Round top
occupied by a very small mountain stream. The rock floor is granite
gueiss of the Storm King type. Its special characters are (1)
extreme shattering or crushed condition, and (2) extensive decay
along this zone which has softened the rock constituents to great
depth.
Considering the nature of the granite gneiss in general this nar-
row gorge is a surprisingly deep one. But this is no doubt due to
the influence of the decayed crush zone. The drill cores taken from
the holes that penetrated the floor at this place are so much altered
that, after several months exposure to the air, they can be readily
crushed in the hand. Hole no. 16/A45 which is centrally located
penetrated to —196 feet. It is in material of this same condition,
to at least —100 feet. Similar conditions are proven to the north
of the line, shown in the accompanying profile and a rapid increase
in depths. From the surface outcrops farther up the gulch it is easy
to see that the crushed zone extends in that direction with the
strongest lines about s. 70 w. This is doubtless on the strike of the
fault lines of the northern border of the range. It is of more than
usual interest in showing the depth to which incipient decay has
penetrated in these crush zones, and the efficiency of stream erosion
along them.
Overthrust fault
The principal fault line follows the margin of the granite gneisses.
At the best exposure of it the Hudson River slates are overridden
by the gneiss. This represents therefore the cutting out entirely of
the Wappinger limestone and the Poughquag quartzite and a part
of the slates by the displacement which must amount to at least
2000 feet and probably more. The same relation is indicated by the
borings and by the outcrop near the village of Cornwall, but a little
limestone is found midway between the two points along the strike
of the fault. The strike of the fault averages about n. 65° to 70° e.,
but locally, at-the best exposure, it is only n. 35° e. The dip is
southeast at an angle of approximately 45 degrees.
160 NEW YORK STATE MUSEUM
Statistics
Moodna valley
I HOLES BORED UNDER AGREEMENT NO. 18
No Surface Rock Rock Per
Ars elevation in | elevation in | penetration | cent core KIND OF ROCK
feet feet in feet saved
I 86 27 | 22 o | Slate and sandstone
(On porosity test with plug at 58 feet deep the loss of water was 6 gallons
per minute with pressure of o-10 pounds per square inch.) Test unsatis-
factory because of large hole.
2 22005 i | fe) fo) |
aren | 130/13 S007 26 o | Slate
(On porosity test with depth to plug 173.5 feet and pressure o—60 pounds
per square inch the loss was 5 gallons per minute.) Test unsatisfactory
because of large hole.
P4 259.6 39.4 TSA a5 ' 975 | Slate and sandstone
5 2055 B75 £2042 71 | Slate and sandstone
BO2 7, P fe) fe)
y| 297.0 +26 20.07 o | Slate
eee eee ee
2 HOLES BORED UNDER AGREEMENT NO. 40
TFT P OOlO“ROROF#ROFNOR?eR>*OF?*ORO‘OOOOOOOooooooo oa (sao
na Surface Rock | Rock. Per
RAG elevation in | elevation in | penetration | cent core KIND OF ROCK
feet eet in feet saved!
ela 276 201 125 o | Slate
pe Pipl @ 228.8 2 ay fe) a
see: 204.7 257-7 45-3 Oo ae
ET 4 272 at 222.1 13.0 47 60 | Slate and sandstone
Ee. 5 BAG iE 220.1 48.7 o | Slate
6 374.6 250.6 38.0 fo) :
| 168.4 40.4 ETOn 5 88 | Slate and sandstone
Fe 8 188.7 168.2 BAS 76 | Slate and sandstone
9 One 164.3 2S o | Slate
te) E7203 46.3 26 fo) a
BIL 169.9 98.9 Oe we fe) os
4 12 2PM 208.2 255 o | Slate and sandstone
Ate 2207, 74 TiO) 5/ 227 O io
bry: 226.6 2126 pe) fe) ps
Firs 230.1 215 ait 32.5 o | Slate and sandstone
i {16 2O8E3 184.3 32.0 o | Slate
fn 169.2 42.2 25.0 fe) ¢
1In cases which show no recovery of core a method of drilling was employed different
from the others and the rock was ground to pieces. Failure to recover core may therefore
be no indication of poor rock quality.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 161
3 HOLES BORED UNDER AGREEMENT NO. 44
————
NG Surface _ ~ Rock Rock Per .
RA elevation in | elevation in | penetration | cent core KIND OF ROCK
feet feet in feet saved
1y SEs ge 5O7 5 15 Slate and sandstone
12 Ee eas ETE 229.6 20 Slate and sandstone
3 282.9 205 3 58.8 14 Slate
4 277.8 S558 166.7 26 Slate and sandstone
5 209.5 ALO 50.9 7 Slate and sandstone
6 | 2790.5 — 4103 43.0 2 Slate
7 299.2 Sent 102.2 og, Slate
8 ae +89.0 109.3 57-6 | Granite gneiss and
quartz
9 282.6 ge 0 go. 13 Slate and sandstone
IO 230.5 —=30.5 BoA. 7 49 Slate and sandstone
Pe 249.5 ens 63.5 2 Slate and sandstone
12 272.0 =k. 58.6 30 Slate and sandstone
r3 288.4 She 79.0 13 Slate
14 185.1 —39.9 91.8 32 Slate and sandstone
15 301.8 —59.2 a he 45 Slate and sandstone
16 ra ae +25.9 104.6 43 Pegmatitic granite
17 300. eee 7s 2 22 Slate and sandstone
!
1 Porosity test made on hole no. rt shows a loss of .03 gallons of water under roo pounds
pressure with packer at depth of 387 feet. Depth to ground water 217 feet.
Porosity test on hole 2/A44.
Ground water level at a depth of 90 feet = el. + 184.5’.
SUMMARY
Depth to packing fo) 20 40 60 80 | 100 == Gage pressure
in feet 40 60 80 | Ioo | 120 | 140 == Calculated pressure!
TAG) scaler re ey ee eae ae .25 -37 -50 64 79) | Geost—— callonswWost
TOO. ae eee mer eee: 20 BOG] a35 42 252 .07 =
ZA. «oe tep ee eh Meee oie c0eNe 09 Asie. .16 -19 323 28 <
Calculated pressure equals average pressure plus weight of column of water from surface
to ground water level. Gage pressure is given in pounds per square inch. Loss isin gallons
per minute.
162
NEW YORK STATE
MUSEUM
4 HOLES BORED UNDER AGREEMENT NO. 45
eee
No Surface
Nas elevation in
feet
Ia 426.
2 390.
3 442.
4 432.
5 433-
6 180.
7 179.
8 214.
9 179-
Io 260.
TE 214.
12 22
ng LO2s
14 269.
I5 209.
16 28K
17 237%
NHNON HDAHAPNHRH oO Cou Nn
ON
Rock
elevation
feet
DO *
266.
286.
268.
207
T6n :
nos”
142.
alse
2eu):
E55
236.
168.
DNs pR
TZ0 .
iy Oe
387.
in
DN TWHWANAHO NA COMO COM Ff
Rock
penetration
in feet
humnoumonn 1 ~
Per
cent core
saved
on
p
(o.0) (0) [e) (c's) (o} (0) (©) (0) (0) fo) (esy (oe) (oe) (2) (eo) (o
iS
O*
Ke)
KIND OF ROCK
Slate with quartz
Slate
«6
Slate with quartz
Slate
Decayed granite gneiss
Slate
Decayed granite gneiss
Slate
“ec
ec
Decayed granite gneiss
Decayed granite gneiss
and seamy gneiss
Gneiss and dyke rock
CHAPTER IX
ROCK CONDITION AT FOUNDRY BROOK!
Foundry brook is a small stream entering the Hudson at Cold
Spring in the Highlands. It drains a rather abnormally large valley’
bordering Bull mountain, and Breakneck ridge on the east, and its
axis is in the strike of the principal structure of the gneisses which
form the chief rock formation of the floor. This valley is in exact
line with the course of the Hudson from West Point immediately
southward, and its rock formations are similar in character and con-
dition.
There is greater variety of rock composition in this belt, 1. e. the
Foundry Brook—Hudson river belt, than in any other in the High-
lands of similar area. The eastern half of the belt is a typical
development of banded gneisses and schists and quartzites belonging
to the sedimentary representatives of the Highlands gneiss. Small
layers of interbedded limestones are frequent together with serpen-
tine, and mica and graphite and quartz schists. In addition along
the east bank of the Hudson, they are profoundly modified by
crushing and shearing in zones that trend with the formation, 1. e.
in a direction leading toward and through Foundry brook valley.
The west side is much less variable and is bounded at the margin
by one of the most massive types of the region — the Bull mountain
and Breakneck mountain gneissoid granites, which are essentially
the same as that of Storm King mountain. ;
But additional structures enter Foundry brook valley from the
western side at an acute angle with its axis and formational trend.
These additional structures are two well marked faults, which cross
the Hudson — one along the precipitous southeast face of Crows
Nest and the other along the southeast face of Storm King moun-
tain. These are the most pronounced escarpments of the whole
region. The first one crosses the Hudson between Cold Spring and
Bull mountain and in passing northeastward loses much of its in-
fluence upon topography and its movement is probably dissipated in
that direction. A line from the southeastern face of Crows Nest to
the point to be described runs n. 50° e.
1 Explorations at Foundry brook were done under the direction of Mr
Wilham E. Swift, division engineer, now in charge of the Hudson River
division of the Northern aqueduct.
6 163
164 NEW YORK STATE MUSEUM
Explorations
Foundry brook therefore contains structures that could produce
considerable effect upon the quality and condition of rock floor.
The rock floor is covered with heavy bouldery drift—thicker on the
Bull mountain flank than in the valley bottom proper. Where the
aqueduct line ‘crosses the floor is at an elevation of 200 feet to 350
feet A. T. Hydraulic grade of the aqueduct is about 400 feet.
The lowest bed rock found along the line is 182.2 feet and the
channel of the present stream coincides with the preglacial one in
that portion of its course. There are two secondary channels —
probably tributary stream channels on the west side. One of these
lies under 70-80 feet of drift.
Borings were made for the purpose of determining the rock floor
profile and the condition of bed rock. In most of them the ordinary
gneisses and granites were penetrated in normal condition.
But in a few a very unusual condition was found. Hole no. 2 at
el. 347 feet near the west or Bull mountain margin penetrated 49
feet of drift to el. 298. Then the drill passed into gneiss which was
at the top, the first 30 feet, of a fair quality. This is shown by the
core recovered —the first 12 feet recovering over 50%. But the
percentage of recovery rapidly fell off — amounting to only 36¢ in
the first 50 feet. Only 1 foot of core was recovered in the next 30
feet, or only 3%. While from that point el. 220 feet to the bottom
of the hole el. 51.8, at a depth of 295.7 feet from the surface,
nothing but fine decomposed matter was washed up. There was no
core at all. This was at first reported as sand by the drillmen, and,
coming at a time when exploration of deep buried gorges was the
rule at other points of the aqueduct, there were many questions
about the interpretation of this new hole, the first assumption of
the drillers being that an overhanging ledge of a very deep gorge
had been penetrated passing through it into river sands below. A
little study of the material proved that this view is untenable. The
sandy wash from the drill is true disintegrated gneiss much decayed
and dislodged by the drill.
But the meaning of it and the extent of it are after all important
additional questions.
Interpretation and further explorations
It is certain that the soft material and the “‘ sand ” reported from
this boring represent rock decay induced by underground water
circulation. Water circulation is rather free as is shown by the
165
GEOLOGY OF THE NEW YORK CITY AQUEDUCT
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fact that there was an artesian flow from this hole of 10 gallons per
minute after reaching a depth of 80 feet, which increased to 1 5
gallons per minute after reaching a depth of 253 feet. This under-
ground supply is maintained since completion and the pressure is
sufficient to raise the water about 15 feet above the surface.
This is a behavior that is consistent with the geologic conditions.
The boring has no doubt penetrated a crush zone following one of
the faults which enters this side of the valley. The crush zone dips
steeply and the boring has penetrated the hanging wall of more
solid rock in the first 50 feet and, after reaching the broken and
decayed portion of the zone, has swung off parallel to the dip and
avoiding the more resistant foot wall has followed down on the soft
itiner streak.
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This crush zone extends on northeastward across higher ground
where opportunity for taking in surface water is offered. This is
without doubt the source of supply for the circulation which fur-
nishes the artesian flow and which has been so effective in pro-
ducing decay to great depth. But the circulation and associated
decay are probably limited to comparatively narrow zones. There
is no good reason for assuming large masses of rotten gneiss at
great depth. The worst zones are narrow but may be comparatively
deep, i. e. they may extend much deeper than any of the borings yet
made in this valley. The depth of decay is related to the outlet for
underground circulation which in this case is the gorge of the
Hudson.
GEOLOGY:OF THE NEW YORK CITY AQUEDUCT 167
Several other borings encountered similar conditions, especially
those on the west flank of the valley within range of the belt in
which the fault seems to be located.
Hole no. 9 reached the rock floor at a depth of 80 feet, and then
penetrated rock to a depth of 159.7 feet. All of the material is
badly decayed. Only 1 foot of core was recovered from the whole
boring and that is mostly quartz coming from a veinlet or peg-
matitic streak at 141 feet. Water under slight pressure was en-
countered in this hole also. But because of the somewhat greater
elevation Gt tie surface at this than at hele no. 2 there is not a
constant outflow.
Two other holes immediately to the west show much better rock
condition — no. 1 showing 79% core recovery. Also two on the east
side at greater distance [see accompanying profile] show good rock.
But one other no. 3 at a distance of over a thousand feet to the east
encountered another zone of decayed rock, the record being very
similar to no. 2 in that poorer conditions are shown at depth than
near the surface. Rock was found at a depth of 20.2 feet. .From
20.2 to 116 feet the gneiss was quite hard, 55.3 feet of core being
recovered or 57.7¢. But from 116 feet to the bottom 207.5 feet the
material was as bad as in hole no. 2, and no core was recovered.
Several other tests were made on the borings with a view to de-
termining the character and extent of these features more definitely.
For example, if the interpretation given for the behavior of no. 2
and no. 3 is correct it ought to be possible to survey the holes and
determine a deflection from the vertical as the drill deviated from
its course to follow the softest streak. A survey conducted for this
purpose indicates just such a result. The accompanying sketch
shows the data plotted. The drill was deflected 4° 36’ at a depth
of 50 feet, 7° 36’ at 100 feet, 8° 2’ at 150 feet and 9° 40’ at
198 feet.
Pressure tests were made for porosity. on some of the holes in
sound rock. Some of these data are given on the profile.
Some of the rock of this valley, if very extensive, such as that
in borings no. 2, no. 3 and no. 9, would be very poor ground for
tunneling. The practical question involves especially the width of
these zones, are they a foot wide or are they a hundred? In an _
attempt to help settle that question an inclined hole was proposed
that was to run at an angle low enough to crosscut these belts.
Accordingly hole no. 14 was bored inclined 40° 26’ to the hori-
zontal and started on the solid granite gneiss. The results were not
Se Re ge bees emt bee TTS ES SSE eet
168 NEW YORK STATE MUSEUM ‘
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Figure 31
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 169
very encouraging. The decay is shown not to be confined to mere
seams. The doubt raised by so much bad ground has finally led to
the adoption of a different plan for crossing Foundry brook valley
and no further data are likely to be added by this work. As it now
stands the borings at Foundry brook indicate the deepest decay of
any yet made in granites or gneisses except those of Pagenstechers
gorge on the north side of Storm King mountain. Both cases are
of similar origin and history, but Foundry brook is apparently the
more complex in occurrence. There are several parallel zones
along which there is extensive decay to a depth of more than 300
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CHAPTER X
GEOLOGY OF SPROUT BROOK
Three creeks unite to form an inlet at the sharp bend in the
Hudson immediately above Peekskill. The middle one of these
is known as Sprout brook. It occupies a deep and narrow valley
that is well marked for 10 miles in its lower course and is trace-
able as a physiographic feature of less prominence to the north mar-
gin of the Highlands. Its persistence indicates some important
structural control in erosion. .
Geology
This valley lies in the midst of the most typical gneisses and
granites of the Highlands region. And in addition several of the
“iron mines” of Putnam county lie on its western flank. The
rocks are complex granitic and quartzose gneisses and granites.
Foliation and banding and bedding wherever this appears is parallel
to the axis of the valley. The most notable geologic feature is the
occurrence of a broad belt of crystalline limestone throughout the
lower 4 miles. It is undoubtedly chiefly this limestone, which is less
resistant to weather than the gneisses, that controls the form and
size of the valley. As to geologic relations, this is one of the most
interesting formations of the region. It is coarsely crystalline, full
of silicious impurities at many places and carries small igneous in-
jections and dykes, and so far as the bedding can be followed,
stands almost on edge. Although an actual contact is not seen, at
several places the limestone and gneiss approach within a few feet
of each other and it is certain that no other formation can come
between them. This is more certainly indicated in the northerly
extension of the valley where the limestone gradually disappears
leaving only the gneisses and granites. That there may be a fault
contact must be admitted, but of this there is no good evidence in
the field.
Such relations and character show that this limestone is similar
to the smaller interbedded occurrences noted frequently with the
gneisses in the Highlands and elsewhere. [Ii it is of that type then
it is the largest representative yet found in that series. But it is
also in these characters similar to the Inwood limestone of more
southerly areas. The overlying Manhattan schist which is lacking
171
172 NEW YORK STATE MUSEUM
may have been removed in erosion. One of these types it resembles,
but it can not be the Wappinger (Cambro-Ordovicic) as was
pointed out by the writer in a former report.1 The Wappinger,
wherever known to be such, is never intruded and always lies above
a thick quartzite (Poughquag). It does so even in the next valley
(Peekskill creek) less than a mile distant. With the interpretation
of this Sprout Brook limestone therefore is involved the correlation
ard interpretation of the age of much greater areas. That the
Sprout Brook limestone is not Wappinger is clear enough, but it
cculd be either interbedded (Grenville) or Inwood. If it is Gren-
ville then of course it has no direct bearing on the Wappinger-
Inwood question and these two might be equivalents. But if the
Sprout Brook limestone is not Grenville (interbedded) then it must
be Inwood and in that case the Inwood and Wappinger are not
equivalent — which means that there are two series above the
gneisses instead of one—an Inwood-Manhattan series, and a
Poughquag-Wappinger-Hudson River series. At the present time
it is not possible to give with certainty a final interpretation of the
Sprout Brook limestone.
Explorations *
It was at first believed that a pressure tunnel could be con-
structed advantageously at the point of crossing this valley and
borings were made to test rock conditions. The data gathered in
exploration are indicated on the accompanying geologic cross sec-
tion jfig. 32].
Borings indicate that the rock floor has been eroded to a few
feet below present sea level and that the gorge has a drift filling
of more than 150 feet. The central borings penetrate limestone
and indicate a total width of this type of more than 400 and less
than 600 feet. The best estimate on the basis of these explora-
tions is 500 feet. Whether this width represents one thickness
of the formation as would probably be the case if it is an inter-
bedded Grenville layer, or part of a double thickness due to infold-
ing, as would probably be the case if it is the Inwood, there as
no evidence. The thickness seems to be even greater farther south
in the same valley (it becomes ™% mile wide), but it can not be
eg
1 Structural and Stratigraphic Features of the Basal Gneisses of the
Highlands. N. Y. State Mus. Bul. 107 (1907). p. 361-78.
2 Explorations at Sprout brook are in charge of Mr. A. A. Sproul, division
engineer in charge of the Peekskill division.
173
GEOLOGY OF THE NEW YORK CITY AQUEDUCT
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174 NEW YORK STATE MUSEUM
accurately measured and there is no way of guarding against repe-
tition of folds. The valley floor is decidedly terraced at an ele-
vation of about 130 A.T. One side is limestone and the other is
granitic rock. This is probably a local mark of the Tertiary base
leveling work.
Because of the great depth of this narrow gorge, it would require
a 500 foot shaft at each side to lead from hydraulic grade down to
a safe level for the pressure tunnel. For a crossing not more than
2000 feet long this is excessive and the cost becomes greater than
by other methods of construction. Consequently the tunnel plan
has been abandoned and it is not likely that further data bearing
upon these questions will be added.
CHAPTER XI
STRUCTURE OF PEEKSKILL CREEK VALLEY
Immediately east of Sprout brook, described in the previous sec-
tion, is Peekskill creek, which drains the largest valley emerging
from the southern margin of the Highlands. This valley as a
physiographic feature is continuous with the Hudson river gorge
from the sharp bend at Peekskill to Tompkins Cove. There are
important structural features along the strike of this valley which
extend very far beyond the limits of Peekskill creek itself, among
which are strong folding and block faulting. The chief fault con-
tinues to the southwest with still greater prominence and appears
on the west side of the Hudson in the escarpment forming the
southeastern margin of the Highlands continuously for many miles
into New Jersey.
Near the Hudson, Peekskill creek and Sprout brook unite and
the structures and formations characteristic of each valley converge
until in the last half mile of their united course rock formations
characteristic of Sprout brook lie on one side of the valley, those
characteristic of Peekskill creek on the other, and the contact which
follows the divide to that point then passes beneath the waters of
Peekskill inlet. Because of the block faulting which has carried
down overlying formations and protected them from the total de-
struction characteristic of the central Highlands region this valley
is of unusual interest.
Explorations *
The aqueduct line crosses this valley about 3 miles from the
Hudson, and in determining the possibility of crossing by pressure
tunnel in rock a considerable number of explorations were made.
Enough has been done to outline the rock floor profile very defi-
nitely and to determine the condition of the formations.
An examination of the drill cores and records of eeratians
shows the following facts which are compiled as fully as possible
on the accompanying cross section.
Phyllite. One boring (no. I) is in a phyllite whose character
and relation to other formations leads to the conclusion that it
1These explorations were directed by Mr A. A. Sproul, division engineer
of the Peekskill division with headquarters at Peekskill, N. Y.
175
176 NEW YORK STATE MUSEUM
belongs to the Hudson River slate series. This type of rock forms
the whole western side of the valley for several miles. Beds stand
on edge or dip steeply southeastward and are in good sound physi-
cal condition. The rock is everywhere a fine grained micaceous
slate or phyllite and in some places carries pyrite crystals. It is
impossible to estimate the thickness or minor structural habits.
But it is clear that it forms the upper member of a series that
has a synclinal structure and therefore the belt represented by
the phyllite marks the axis of the syncline although the chief val-
ley development lies wholly to one side.
Limestone. Eleven borings (no. 2, 3D, 4C, 11, 13C, 18,
22, 23, 25, 26 and 29) are in limestone. All show essentially a
very fine grained close textured crystalline gray or white or bluish
rock with strong bedding standing nearly vertical or at very high
angles. This, because of its character and relation to other forma-
tions, is regarded as the Wappinger limestone —a formation well
known north of the Highlands, where it is at least 1000 feet thick.
From present explorations it is now certain that a belt 3250 feet
wide is underlain continuously by this formation standing nearly
or edge. Unless repeated of course this would represent approxi-
mately the thickness for Peekskill valley. But it is not believed
to be so thick. It is more likely that there is a threefold occur-
rence brought about by close isoclinal folding (a closed s-fold)
as seen in the accompanying cross section. This view is supported
by at least one occurrence of the underlying quartzite member near
the center of the valley at a point a couple oi miles farther north
On the line of exploration, however, none of the borings pene-
trate any other formation beneath. Attention is called to additional
structural details and physical conditions in a later paragraph.
Quartzite. One boring (no. 5) is in a quartzite. It is very
pure, fine grained, closely bound and typical quartzite. The beds
stand almost vertical and the whole thickness is known from nearby
outcrops to be approximately 600 feet. From its character and re-
lations to other formations it is regarded as the Poughquag —a
well known formation of the north side of the Highlands.
Gneisses. Five borings (no. 7E, 9 B, 17, 27 and 28) are in
gneisses. These are to a considerable extent simple granite gneisses
of igneous origin. But there is the usual variety characteristic of
the Highlands gneisses and no doubt they are representatives of
the great basal gneiss series that is elsewhere referred to as the
equivalent of the Fordham of New York city.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 177
2 Stratigraphy
This is therefore the rock series of Peekskill creek. It is
the only locality on the south side of the Highlands where all
are represented in complete and simple form. ‘There is no doubt
that it is the Poughquag-Wappinger-Hudson River series, in spite
of the complete absence of organic evidence. A similar though
not so complete and clear occurrence is to be found on the west
side of the Hudson near Stony Point and Tompkins Cove. That
is a part of the same structural syncline. It is probable also that
the phyllite so finely developed in the village of Peekskill in the
next small valley to the east is the same. But outside of these
occurrences there are none that clearly represent this same series
as a whole and in the same condition.
No more striking example of this fact can be found than the
adjacent Sprout brook described in an earlier section. There coarse
crystalline and injected and impure limestone occurs alone — no
phyllite and no quartzite. When one remembers that the two
occurrences so strongly contrasted, Sprout brook and Peekskill
creek, converge until they actually unite, and still preserve their
stratigraphic dissimilarity, without any adequate structural reason
for it, the only conclusion possible is that the two occurrences rep-
resent two entirely different series of formations.
The Peekskill valley series 1s Cambro—Ordovicic in age; what is
the other? It is older, at least that is certain. But is it (the Sprout
Brook limestone) as old as the oldest of the gneisses themselves
and therefore interbedded with them representing locally the Gren-
ville; or is it intermediate — Postgrenville and Precambric — with
which possibly other occurrences of rocks of similar habit and
equally uncertain relations belong?
It is on the general similarity of this occurrence to the Inwood
limestone as known throughout Westchester county and New York
city that a tentative intermediate series has been recognized. This
is the Inwood-Manhattan series. Whether it is in reality a separate
older series is not regarded as proven. But for engineering and
practical purposes the distinction is a good one and eminently ser-
viceable. Further discussion may better be continued in a different
publication.
3 Rock surface
The bed rock surface is pretty well outlined by the borings. A
profile based upon them seems to leave no unexplored space of suf-
ficient extent to admit a gorge of great consequence to a lower level
178 - NEW YORK STATE MUSEUM
than is already shown in holes no. 1 and no. 11 [see profile and
cross section, fig. 33]. The elevation indicated by no. 3 D is be-
lieved to be misleading because of the use of a drill that was
capable of destroying a part of the ledge rock that would usually
core. The points believed to be weakened by structural disturbance
and therefore most likely to be attended by erosion and stream
action are in the vicinity of hole no. 11, near the present creek, and
hole no. 25, near Peekskill Hollow road.
4 Buried channels |
From the accompanying cross section it will be seen that the
drift cover is more than 100 feet thick over large portions of Peeks-
kill valley. The rock floor in the middle of the valley averages
approximately 25 feet A.T., while the drift surface except where
cut out by stream erosion is at about 125 feet. In the rock floor
there are two depressions, the large one wholly within the lime-
stone belt and the smaller following the limestone-phyllite contact.
There is not much difference in their depth so far as explored, but
there is a possibility of a somewhat deeper notch in each one. The
depth to which some of the limestone beds are decayed by under-
ground circulation would lead to the belief that a deeper notch may
exist.
The drift cover is chiefly partially assorted sands and gravels in
the central portion of the valley, and more of a till on the eastern
valley side. It is noteworthy that the present Peekskill creek lies
far to one side following closely the phyllite wall. :
5 Underground water
Present elevation above sea level is so slight that there is appar-
ently little encouragement of deep underground circulation. But
at certain points the rock has been found to be very badly decayed
‘to a great depth—to at least 200 feet below sea level. This is
‘believed to have been accomplished chiefly at a time when the re-
~ gion stood at a higher level. Hole no. 22 is especially notable in
this connection. A comparison of the figures of core saving is one
“of the best lines of evidence on this question. Wherever data are
at hand the percentages of saving have been put on the cross sec-
tion. Hole no. 29, for example, shows a saving of only 11% in the
\lower 250 feet, reaching a depth of 207 feet below sea level.
The-present water table profile is shown on the cross section. A
‘great body. of water stands in the assorted sands directly upon bed
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 179
rock forming essentially a great reservoir of supply that has ready
access to the almost vertical limestone beds. This will give a maxi-
mum water supply to holes that penetrate porous or broken por-
tions of bed rock. The attitude of all strata is especially favorable
for admitting an almost inexhaustible supply from a considerable
drift-covered area within which circulation is probably very rapid.
6 Condition of rock
All strata of this valley stand so nearly on edge that drills actually
explore a very limited portion of the whole series of beds. No very
great advantage is gained by excessively deep boring because the
drill follows necessarily almost the same bed from top to bottom.
At best only the immediately adjacent beds are penetrated. This
means that much of the total thickness of beds is untouched by
present explorations, and must be interpreted on the basis of their
general likeness to those more fully determined. The usual suc-
cession of beds is known to be quite uniform in quality and loca-
tions where they can be studied and there is no reason to expect
greater variation here.
Deviations from such normal or uniform conditions are mostly
due (a) to local development of mica from recrystallization of 1m-
purities in the limestone, and (v) to crush zones developed in the
process of folding and faulting which has broken the rock or weak-
ened it enough to permit a more ready circulation of underground
water. Wherever either of these structural conditions prevail, the
rock has been excessively decayed, or disintegrated, or sufficiently
weakened in its binding matter or its sutures to crumble in the
hand ‘or break down to a sand under ordinary boring manipulation.
This’ condition; is known to reach to —297 feet. How much deeper
is not known. ' Probably the decay dates back in large part to pre-
glacial continental elevation at which time probably there was more
ready. deep circulation with possible outlet in the Hudson gorge.
This action has been all the more effective by reason of the attitude
of the beds. They stand so nearly on edge that they present all
their weaknesses of bedding and sedimentation structures to the
destructive surface agents. They admit surface water readily and
favor abundant underground circulation.
Corisiderable faulting occurs. The contact between the eranite-
gneiss and quartzite is a fault contact. Wherever seen this is sound.
But.a crush zone in limestone lies nearly central in the valley, cut
by holes no. 23 and no. 25, where the rock shows a finely brecciated
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1&1
condition some portions of the drill cores being literally crushed to
bits.
In one hole, no, 11, near the. phyilite-limestone contact, a soft,
sandy condition was encountered at a depth of 133 feet, permitting
the drill rods to be pushed down without boring at all, 60 feet
ahead of the casing. This, however, is not believed to indicate any
very extensive weakness. It is probably connected with the bedding
planes or joints rather than with general decay or faulting. Four
or five inches of solution and disintegration along bedding planes
would account for all that has been proven. The fact that the rods
could be shoved down 60 feet while the corresponding outer casing
could be shoved down only half as far seems to support this view.
Summary
If a tunnel were made across this valley there would be approxi-
mately 1100 feet of it in Hudson River slate (phyllite), 3250 feet
tn Wappinger limestone. 600 feet in Poughquag quartzite, and the
rest in the gneisses.
Some weak rock-is certain to be found, especially in the vicinity
of station 367+50 and 345+00 to 350-00. At both places increased
water inflow would be encountered with almost exhaustless supply
from the sands that lie on the rock floor above.
At about this stage in the exploration the Board of Water Supply
decided to abandon the rock tunnel plan. The conditions found
were considered by them too questionable. Steel pipe construction
is to be substituted. As a result it is not likely that much more
detail will be added to the structure of this very complex valley.
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CHAPTER X11
CROTON LAKE CROSSING
It is proposed to finish Ashokan reservoir and the Northern aque-
duct first. This so called Northern aqueduct reaches from the Cats-
kills to Croton lake. Croton lake is the present supply of New
York city and is already connected by two aqueducts with the city
distribution. As a first step, therefore, and as an emergency meas-
ure the Catskill water will be delivered to the Croton system by
finishing the Northern aqueduct first. As rapidly, however, as the
whole project can be carried out the so called Southern aqueduct
will be constructed to continue the Catskill water independently of
. the Croton supply to the city.
The Southern aqueduct department has charge of the line from
Hunters brook on the north side of Croton lake to Hill View reser-
voir near the New York city boundary. During exploratory work
it has been under the direction of Major Merritt H. Smith, depart-
ment engineer, with headquarters at White Plains. Construction
now going on is in charge of Mr F. E. Winsor, department engineer.
The first link in this southerly extension is to be a tunnel be-
neath Croton lake through which the Catskill water may pass in the
same manner as it crosses other valleys. This crossing has been
located a short distance below the old dam on the Croton, about 5
miles up stream from the Hudson.
The problems involved at this point include (1) a determination
of the kinds and quality of rock to be penetrated, (2) their water-
carrying capacity, and (3) opinion as to the proper depth for a
successful tunnel.
Geological features
The Croton valley is one of the very few in southeastern New
York that actually crosses the geological formations and major
structural features instead of following parallel to them. In its
lower portion it passes from gneiss to limestone and to schist sev-
eral times. The reason for this somewhat abnormal course is preb-
ably the development of weak zones by fault movements in this
transverse direction.
Only one of the well known formations of rock is exposed in
the immediate vicinity of the tunnel site. This is the Manhattan
schist, the uppermost formation of the region south of the High-
lands. Along the Croton it varies greatly, the chief type being a
183
NEW YORK STATE MUSEUM
184
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 185
garnet-bearing quartz-mica schist varying from rather fine grain
and semigranular appearance to a very coarse and strongly foliated
structure. This part of the formation undoubtedly represents re-
crystallized or metamorphosed sediments. But associated with this
facies there is a more dense black hornblende schist that, not only
here but at many other places, is thought to represent igneous in-
trusions that have been metamorphosed together with sediments of
various types, until both have lost almost all of their original char-
acters. The hornblendic schist type is not so extensive as the other,
the mica schist, but it is more compact and here as usual is in the
better condition.
Pegmatite stringers occur abundantly, especially in the mica schist
varieties. They are of no great consequence, however, as a factor
in this study. They originated in the aqueo-igneous activity in-
volved in the recrystallization of the rock when it was worked over
into a schist.
Beneath this Manhattan schist formation lies the Inwood lime-
stone, a bed probably at least 7oc feet thick. But at this point how
deep it lies and at what depth it would be penetrated nobody can
tell. None of the drills have touched it. Beneath the limestone in
turn lies the granitic and banded gneisses belonging to the Fordham
gneiss serics, the lowest and oldest of the region.
Along the Croton river nothing but Manhattan schist is to be seen
at the surface for more than a mile above and below the proposed
crossing. The same thing is true for an equal distance on opposite
sides from the river at this locality.
But the structure is folded and the normal northeast-southwest
trend of the folds crosses the river, every arch or anticline tending
to bring the limestone and gneiss nearer to the surface. One of
these folds does expose the limestone and gneiss in a strip extend-
ing from the Hudson river northeastward for two thirds of the
distance to the old Croton dam. But before reaching the Croton
valley this fold pitches down toward the northeast beneath the Man-
hattan schist and passes under the present lake (or reservoir) in
that relation, not reaching the surface again for a distance of about
6 miles. At least one more fold is known to behave in a similar
manner as it reaches the Croton.
These facts make it certain that there is limestone beneath the
schist in the vicinity of the crossing, and that it comes nearer to
the surface in that vicinity than at some other places.
South of the Croton there are several small cross faults run-
186 NEW YORK STATE MUSEUM
ning nearly east and-west. It is believed that similar movements
have affected the rock in the Croton valley itself, modifying its con-
dition so much as to control the course of the stream. The only
immediate bearing upon the problem of the Croton crossing is the
question that it raises about the quality of rock and the necessity
that is introduced of trying to determine whether or not there is
shattering enough to be very objectionable.
Explorations and data
Six drill holes have been made on this proposed Croton lake
crossing — one on either side just at the margin and four others
within the intermediate space of 1400 feet. These inner four have
been made from rafts floated on the lake and have penetrated water,
drift cover, and rock [see accompanying profile and cross section,
pe 27}:
Rock floor. The depth of the preglacial Croton valley is
pretty accurately determined at o feet or sea level. There is no
reason to expect a gorge or inner channel of any consequence.
The drills have penetrated only one formation, i. e. Manhattan
schist. These test holes are believed to be near enough together to
eliminate the possibility of any other formation appearing at tunnel
grade.
Rock condition. The two varieties of schist (1) the coarse
garnetiferous quartz-mica rock, which is a metamorphosed former
sediment, and (2) the darker, close grained hornblendic rock that
is believed to represent an igneous intrusion, both occur in the cores
brought up by the drill. Either under normal conditions is a
good rock. But there are considerable differences in the physical
condition of the rock. Holes no. 3 and no. 4 at the two extremes,
on the lake borders, show sound rock that comes up in large cores
with very high percentage recovery. This is confidently believed to
represent the average condition of the rock in this vicinity at the
sides of the valley.
The central ‘holes, however, nos. 1, 2, 5 and 15, all show more
broken ground. Of these holes no. 2 is much the most broken, the
core recovery being only 14.8¢. The pieces are small and many
are smoothed (slickensided) by movement. ‘The hole penetrates a
typical crush zone resulting from slight faulting movements, and
the low saving is due to the fact that the incipient fractures are not
well bound together. (rehealed) by later mineral change. They are
probably connected with the latest movements of this kind.
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GEOLOGY OF THE NEW YORK CITY AQUEDUCT 187
_..The commonest secondary mineral now filling these crevices is
chlorite, and, although it may completely fill the crevices it has little
binding strength. Any new disturbance or strain readily causes
separation along the same original lines. But in spite of the fact
that the core is broken into small pieces and shows so low percent-
age of recovery it is quite certain that the rock itself is not badly
decayed. An examination of one of the most doubtful looking
cores from the lower part of hole no. 1 showed under the. micro-
scope little evidence of serious decay. ‘This is believed to mean
that underground water circulation is not as abundant as the
fractured condition of the rock would lead one to expect. Further-
more, an examination of the cores in greater detail shows beyond
question that much of the fracturing is entirely fresh and must
have been done by the drill itself. It is certain that the low per-
centage of recovery is in part due to this cause. The small diam-
eter of the intermediate holes is contributory to the same results.
Some allowance must also be made for the difficulty of working
a machine from a raft on the lake.
Comparison of the cores shows a decidedly higher percentage
of core recovery, and presumably therefore of rock solidity in all
of the other three holes — no. 1, no. 5 and no. 15.
Hole no. 2—core recovered 14.8%
“no. I— _ 34.6%
“no. 15 — i 26.3%
“no. 5— = 38.9%
It therefore appears that the last three penetrate rock that is
more than twice as good in its capacity to stand drilling disturbance.
A comparison of quality at different depths is believed to be still
more encouraging. The upper portions of all holes have poor
recovery and comparatively poor looking rock. But in depth there
is a marked improvement.
In view of the fact that the tunnel will undoubtedly be located
somewhere below the —75-foot level, it is really only this lower sec-
tion that is of vital importance to the project. A tabulation and
comparison of core recovery from these lower portions is given
below.
I From total depth of hole 2 From depth —75’ to bottom
Hole no. 2— = 14.8% core recovery 25% core recovery
“ 10. I— == 34.6% ~ 45% ,
Hoehne) TS." == 36,39 . 66% *
ify no. 5 enw — 38.9% se 42% &é
iss NEW YORK STATE MUSEUM
Under the conditions of work, this is a fair saving and indicates
much more substantial rock below the —75’ level. There are many
pieces 10-12 inches in length and ior a 1 imch core this may be
considered very good.
It is clear, however, from a detailed inspection of the cores, that
there is considerable variation somewhat independent oi depth.
There are occasional stretches of poorer ground in the midst of
comparatively sound rock. This is believed to imdicate that the
crushed condition is confined chiefly to certain zones, and that these
zones dip across the formation and across the holes at an angle.
They are probably distributed promiscucusly throughout the central
portion of the valley, but are certainly more abundant and more
strongly marked im the vicinity oi hole no. 2 than at any other poimt
tested. The rock profile shows that hole no. 2 has also the lowest
bed rock. This is a further support to the general explanation of
the valley as given above.
The chief elements of uncertainty remaining after the borings
have been completed are:
1 The exact extent or widths of the chiei crush zones
2 Their dip and strike
2. The possibility of others not yet touched
4 The permeability of the rock for underground water
5 The supporting strength oi such rock m a tunnel of large
dimensions
In spite of the uncertainties enumerated, the conditions are
ertirely understandable. There is little probability of findmg a
worse condition than that shown in hole no. 2. The permeability or
porosity of these zones is of course unknown. The chief reason for
believing that underground circulation is not abnormally heavy ts
the observation that the joints are well filled with chlorite and that
other decay 1s not at all promiment at the lower levels. Further-
more, the rock is a crystalline type of rather successful resistance
to ordinary solution agencies and therefore may be depended upon
to hold its own im its present condition indefinitely. But because
ei the poor binding effect of the chlorite it is to be expected that
blocks will fall irom the rooi of any tunnel where it passes through
a crush zone. Timbering will be required for protection in places,
but the ground will not cave or run. These zones may be expected
throughout a total distance of about 7oo feet—1. e. the space
between no. I and no. 15. The chief belt of such ground probably
lies between holes no. 2 and no. 5.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 189
Summary
The lowest bed rock is about sea level.
This pressure tunnel will cut only Manhattan schist.
All rock is good ground for such work, except in certain narrow
zones where it is crushed.
The extent of such broken ground is not closely delimited, but
occurs at intervals for a distance of 700 feet.
The amount of underground circulation is judged to be moderate
at —100 feet.
The tunnel should be located deep enough to take advantage of
the improved rock conditions shown at about —1o0o feet. There
seems to be no marked improvement below —100 feet as deep as
the drills have gone.
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CHAPTER XIII
GEOLOGY OF THE KENSICO DAM SITE
Kensico reservoir at Valhalla, 2 miles north of White Plains, 1s
one of the links in the Bronx river aqueduct. -It is to be greatly
enlarged and made a very important storage reservoir for the new
Catskill system. In line with this plan a new dam is to be built
near the old site that will rise 100 feet higher than the present
structure. |
Extensive investigations’ have been made to determine the charac-
ter of rock floor for this massive dam. Sites both above and below
the present one have been studied with the question of safety and
efficiency and permanence as well as that of economy of construc-
tion in view. Involved with this is also the source of suitable stone
for its construction. 3
Geological surroundings
Glacial drift covers the rock floor of this and neighboring valleys
to a depth of 10 to 20 feet. No rock is exposed in the valley bottom
at the Kensico site, but at the extremities of the proposed dam the
rock floor comes to the surface in small outcrops. The material
constituting the drift cover is essentially a loose, somewhat porous
till passing into modified types, especially gravels and sands imme-
diately south of the ground tested.
The character of bed rock at the two extremities and beyond the
limits of the dam is easily seen from the outcrops to be Fordham
greiss on the east and Manhattan schist on the west. Between,
although nothing can be seen, Inwood limestone is found by the
borings as was to be expected. No other formations occur, although
the Yonkers gneiss, an intrusive in the Fordham at a little greater
distance figures prominently in studies of material.
The formations are in normal order and are of the usual petro-
graphic character. All dip westward at angles that vary from 45
to 65 degrees and have a general strike a little east of north. It is
evident that the whole series represents an eroded limb of a simple
fold. .
1These explorations have been in direct charge of Mr Wilson Fitch
Smith, division engineer, whose headquarters for the Kensico division is at
Valhalla, N. Y- Preparations for construction have already been begun.
IOI
192 NEW YORK STATE MUSEUM
The Inwood limestone occupies about 800 feet of the bottom and
eastern margin of the valley, lapping well up on the Fordham
gneiss. The drill cores from this formation are unusually sound.
The Manhattan schist shows much broken material. There are
many crush zones. This condition increases still farther west along
the railway near Valhalla station.
The Fordham gneiss appears to be sound where it can be seen
at the surface.
Results of exploration. Many borings have been made. They
prove the general structure and succession of formations, making
the boundaries definite. They increase the evidences of a rather
wide prevalence of weak zones— some of them in the gneisses.
And they also indicate a more extensive surface decay than was
formerly believed to prevail.
The chief problems from the geologic standpoint are connected
with the following features:
I Extent of surface disintegration
2 Extent and distribution of weak zones
3 Depth of decay and perviousness of rock
Surface disintegration. Several borings on ground underlain
by Fordham gneiss penetrated material beneath the drift and above
bed rock that was interpreted as residuary matter from rock decay.
All of this material is of local origin. Later exploration in the
form of a deep trench to bed rock has proven that there is an
extensive residuary mantle of this sort at the eastern side of the
valley below the present dam. In places as much as 30 feet exists.
Undoubtedly this material is a remnant of preglacial soil mantle
that was in some way protected from removal by the ice. Few
places are to be seen in all southeastern New York where there is
so much left in place. In most of it the gneissic structure is still
preserved, but the decay is so complete that it can be cut and
handled like an impure clay.
Weak zones. It has been proven that there are weak zones
in the gneisses as well as in the other rock formations. In some
places the rock is so poor that no core is recovered for distances
of 5 to to feet, and in one hole a seam of this kind 20 feet wide
appears. In every case, however, the drill passes through the rot-
ten material into the opposite wall — indicating a zone of consider-
able dip instead of vertical position. This favors the theory that
the weaknesses follow the bedding largely and are perhaps due to
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 193
difference in the mineral make-up of
the beds fully as much as to dynamic
disturbances. The walls are generally
good. The fragments of core are not
much slickensided. In the schist this
is probably not as generally true.
There are much plainer evidences of
crushing movements in the schist. It
is a locality where one of the folds,
one well developed farther south, is
pinched out and there is rather gen-
eral crushing of the weaker strata.
Depth of decay and perviousness.
As deep as borings have gone there
is occasional decay and broken ma-
terial and streaks that are.pervious.
Final location. The condition of
bed rock, together with other consid-
erations led finally to the selection of
a site above the present dam. In
general the same features character-
ize this site. But the rock condition
is somewhat improved. On the whole
the new situation is a safer one.
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CHAPTER XIV
STONE OF THE KENSICO QUARRIES
The following quarries in the immediate vicinity of Kensico res-
ervoir have been studied in the field:
(1) “ Smith quarry,” which is less than a thousand feet east of
the southern end of the present reservoir; (2) “ City quarry,” which
is on the immediate eastern margin of the reservoir on the east side;
(3) “Garden quarry,” which is a new location about 500 feet from
the eastern margin about midway; (4) “ Outlet quarry,’ 1500 feet
east of the northern extremity of the present reservoir; (5) “ Ferris
quarries’ 1000 feet and (6) “Dinnan quarry” 3000 feet farther
north.
In addition to the field observations a detailed microscopic study
was made on specimens of the rock taken from the Garden, Ferris
and Dinnan quarries.
The question at issue 1s the choice of a rock for the facing and
finish of the new Kensico dam. In view of the use to be made of
the rock, extreme strength is of only secondary importance. But the
questions of abundance, distribution, durability, purity, agreeable
appearance and working quality are vital.
Types of rocks
All of the quarries occur in the broad belt of Precambric gneisses
that forms the eastern margin of the reservoir extending northward
and southward for many miles. The formation as a whole is very
complex. But the basis of it is a black and white banded rock
chiefly a metamorphosed sediment, known as the Fordham gneiss
in southeastern New York. In it are intrusions of igneous rocks
of many varieties and most complicated structure — dykes, bosses,
veinlets, stringers etc., sometimes 1n such abundance as to wholly
obscure the original type. The most abundant of these are, (a) a
rather light colored quite acid rock that is essentially a granite in
composition, but has a sufficiently foliate structure to be classed as
a gneiss and is the same as the “ Yonkers gneiss” occurring farther
south, and (6) a dark rock containing much hornblende and biotite
which is in some cases essentially a diorite in composition, but has a
marked tendency to schistose structure. The former (a) may be
called a granite gneiss and the more massive representatives of the
latter (b) may be classed as a dioritic gneiss. In both cases at
"4 [Q5
196 NEW YORK STATE MUSEUM
times the blending with the original metamorphosed Fordham gneiss
is so intimate that absolutely sharp limits can not be drawn. And
this last condition may well be designated as a third case (c).
The quarries visited represent all three of these cases. Dinnan,
Ferris and Outlet quarries represent essentially the “ Yonkers
gneiss” type (@) of granite gneiss. Garden quarry represents
chiefly (b) the dioritic type of gneiss. City and Smith quarries
represent the last case (c), or the mixed and variable type.
Field character
City quarry. In accord with the above differences in type it
is found that large quantities of uniform material for such purpose
as is proposed can not be obtained from City quarry. The rock
there is badly jointed and is variable to a marked degree. It was
not thought promising enough to test in detail.
Smith quarry. The conditions of Smith quarry are better but
there are similar objections. The amount of uniform material is
greater. It would no doubt furnish an abundance of material suit-
able for use in the construction of the dam interior, but is not at
this point as good a source of facing stone as some of the others
to be considered.
Outlet quarry. Although this rock is characteristic Yonkers
gneiss, it has at this place suffered by weathering a peculiar dis-
coloration to such extent as to make it objectionable, both from the
standpoint of appearance and perhaps of durability.
Garden quarry. There is an abundance of stone at the Garden
quarry. It is fairly uniform. It is no doubt good enough from
every standpoint of durability. It is well located. It can be quar-
ried readily. But it has a very dark color and is undoubtedly less
attractive than a light stone for this purpose. There are no objec-
tionable structures, except where the strong schistose character 1s
developed, and these could be avoided so that with a little selection
a fairly uniform stone could be secured.
Dinnan quarry. This rock is typical “ Yonkers gneiss.”
There is sufficiently large quantity. It is of good quality. It is
situated a little over 2 miles from the proposed dam, but is of easy
access. The jointing and other structures do not seem to be objec-
tionable. It will work somewhat more easily than a true granite
because of the gneissic structure and it has a good medium light
color. The discolorations do not seem to penetrate deep and the
rock shows only slight decay.
Plate 28
Photomicrograph of Yonkers gneiss from “Outlet quarry” taken in
plain light to show prominence of sutures between the grains indicating
the beginning stage of disintegration. Magnified about 30 diameters
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 197
Ferris quarries. The “Old Ferris quarry—is “ Yonkers
gneiss ” considerably more weathered than the Dinnan. It is con-
sidered less promising than the “ New Ferries” quarry which has
been explored by the engineers of the Kensico division. The rock
of this quarry site is not all of one quality. There are essentially
three varietal facies of the Yonkers gneiss type and relationship.
One (@) is essentially a granite. It has a coarse grain and shows
almost no foliate structure. it has a decidedly massive appearance;
but it is not of very great extent. This rock is evidently very
closely related to the true Yonkers gneiss into which it passes on
all sides through an intermediate variety.
This intermediate variety (0) has medium size of grain, is only
slightly foliated and passes without sharp limitations on the one
side into the granite facies and on the other to true normal Yonkers
gneiss. It is not so strikingly massive as the granite, but is more
so than the gneiss proper. This rock may be called a gneissoid
granite to distinguish it from the other.
The true Yonkers (c) gneiss surrounds these two special varie-
ties. It is of finer grain than either of the others and is more
strongly foliate and is strictly a granite gneiss. Varieties (a) and
(6) occur as sort of a lens within the Yonkers gneiss.
The extent of the granite as now uncovered at the site is be-
lieved to represent its limits. The prospect of enlarging the area
will not meet with much success. It is essentially a local develop-
ment connected with the differentiation of the parent magma from
which all three varieties were derived. It seems to have been the
last of the three to solidify, and it has some of the characteristics
of certain pegmatite lenses. |
Although this is certainly an attractive rock and one against
which there is little ground for objection, it is-reasonably certain
that a sufficient quantity of this variety can not be obtained here
for the whole proposed use. And the prospects are not good for
locating another quarry of the same quality. |
The gneissoid granite (b) is of greater extent, in fact it will be
found to encroach on the present area of the granite. It is as good
rock and almost as attractive as the granite.
The regular type of Yonkers gneiss such as that represented in
the Dinnan quarry can be obtained in almost unlimited quantity,
and, with the splendid showing that it makes in further examina-
tion, it has come to be considered the best suited to the purposes
of dam construction at Kensico.
198 NEW YORK STATE MUSEUM
Petrographic character of the rocks
This line of investigation is confined to four sets of samples.
No. 1 The granite of the New Ferris quarry
‘2 The gneissoid granite of the same quarry
3 The Yonkers gneiss of Dinnan quarry
4 The dioritic gneiss of Garden quarry
1 Granite. The rock is coarse grained and well interlocked.
The chief constituents are orthoclase, quartz and microcline.
There are but small amounts of dark minerals, and there is not
much decay. |
Both surface material and the drill core were examined. The
deeper material shows a little calcite, that may be original, occur-
ring in irregular grains. They do not seem to indicate decay.
There is a little kaolin alteration of the feldspars, but not to a
serious degree. There are no injurious impurities in the rock such
as might cause rapid disintegration or discoloration.
The rock is undoubtedly of good grade as to strength, composi-
tion and durability.
2 Gneissoid granite (Ferris quarry). The rock is of medium
grain, containing quartz, the feldspars and a little mica.
There is very little alteration, and no serious decay or injurious
constituents. A small amount of sericite and calcite present are
not considered of consequence, and as in the case of the granite,
the calcite is believed to be original.
The grains are intimately interlocked and the rock is certainly
of good quality and very similar to the granite proper.
3 Yonkers gneiss (Dinnan quarry). This rock is fine grained,
and is composed of quartz, mica and the feldspars among which
microcline is very abundant.
The condition is good,— very little alteration, close structure, but
with a. little more granular appearance than any of the other types.
It is a good rock and gives good durability tests.
On badly weathered surfaces the Yonkers gneiss breaks up into
a granular product like sand long before it decays to earthy matter.
This seems to be caused by expansion and contraction of the dif-
ferent constituents under changing weather conditions inducing a
weakening of the sutures. Sometimes there is very little decay
even along these sutures, but as they open slightly they become the
channels for moistiire and staining solutions. This makes the
boundaries of the grains very well marked in weathered specimens.
Plate 29
Photomicrograph of Yonkers gneiss of the type to be used in the new
Kensico dam. Dinnan quarries. Magnified 30 diameters
_ Photomicrograph of diorite gneiss from “ Garden quarry.” Magnifica-
tion 30 diameters. The constituents are hornblende, biotite, feldspars
and quartz.
GEOLOGY, OF THE NEW “YORK CITY AQUEDUCT 199
Such incipient disintegration is common in the more even grained or
granular varieties.
The accompanying photomicrograph [pl. 28] is taken in plain
light and shows the outlines of the grains due to this cause.
4 Dioritic gneiss (Garden quarry). Rock is of medium grain
and with a strong tendency to schistose or foliate structure. The
dark grains are hornblende and biotite, the light grains are feldspars
and quartz.
The rock is fresh, durable and has +9 injurious constituents. It
is good enough for the use in ali respects, but has a dark color and
is more strongly foliated than any of the others considered.
It is evident from these observations that the rocks considered
are all of suitable mineralogic character for the purposes of large
dam construction. For very large quantities of material, however,
it is probable that neither the coarse granite nor the gneissoid
granite could be depended upon for uniform supply. The true
regular Yonkers gneiss, however, is very abundant, and can be relied
upon for indefinite amounts. The dioritic gneiss is also abundant.
The immediate region is not capable of furnishing any better rock
than those described above.
Additional tests
Some instructive tests were made by the Board of Water Supply
under the direction of Mr J. L. Davis who has charge of the
testing laboratories. A few of these applying to the rocks at
Kensico are tabulated below.
The tests cover: specific gravity, weight per cubic foot, porosity
in per cent, ratio of absorption, per cent water absorbed, ratio of
drying 24 and 48 hours, retained water pounds per cubic foot 24
and 48 hours.
In the accompanying tabulation the terms used are subject to the
following limitations as to definition:
I Ratio of absorption, sometimes called porosity, “is the ratio of
the weight of water absorbed to the dry weight of the stone.”
2 Porosity gives “the actual percentage of the stone which is
pore space.” “The difference between the dry and saturated
weights of the sample is multiplied by the specific gravity of the
rock and the product added to the dry weight. This gives the
weight the specimen would have provided it contained no pore
spaces. The difference between the dry and saturated weights
200 NEW YORK STATE MUSEUM
multiplied by the specific gravity of the rock is then divided by the
above computed weight of the poreless specimen. This ratio ex-
pressed as a percentage is the actual porosity. Expressed as a
formula, the computation is as follows:
(Saturated wt. — Dry wt.) S. G.
== Porosity.”
(Saturated wt. — Dry wt.) S. G. + Dry weight
3 Ratio of drying. An attempt has been made to determine the
comparative and actual rates at which the saturated rocks give up
the absorbed water under ordinary atmospheric conditions. “ The
ratio of drying was computed by dividing the weight of water
lost during exposure by total weight absorbed. The weight of re-
tained water was computed.” The comparison is most useful in
rocks of like petrographic general character.
The other terms need no explanation.
TABULATION OF TESTS
' o ie}
Q, = , :
5 = ES 2 4 a) Bate oF Reames water
S| no S ee) 2) @ re) drying pout Ss per
| ao] §9 coh De ge Sa cubic feet
Sol eel es 5G ONS BQ
Name & oO | a6 3
4 — » arm
n Oo te © | a5 ¢ 3
3 of | GB G SG S)
3.9 | 0 Opn ip aoe S 24 48 24 |. 48
Chan lS Sea ae x hours | hours} hours | hours
a ee Poe reo ee Be
ae ‘ec ee) ene x2 Sac OO
Granite, Ferris | x] 0.34] 0.77| 2.66| 164.71 |
quarry, core No. 461| 2| 0.31| 0.84| 2.65| 164.04 0.26 aie) 52.8 aos aoe
ak a ee ee ee | gear age | H
Gneissoid granite, 1| 0.3210 0.85H 2.63) 161.03)
Ferris quarry, | ©.28|) 67.48] 69.88 .146 -145
core No. 468 | 2) "0n25| tony E) (265126278 |
se ee ees |S oo eee ee
Yonkers gneiss, I] 0.30] 0.87| 2.64] 163.3 ) |
| t | 0.30| 88.16] 88.16 .057 Ob
Dinnan quarry 2| 0.39] I.o1| 2.64] 161.0) |
ee ee | Fe —— |
Dioritic gneiss, 1] 0.42) 0168) 2.83) D754 |
Garden quarry, Pi on2rl-62e5 |) 62.5 -137 5137
core No. 459 2| 0.24| 0.68] 2.86) 174.8 } |
Gneissoid granite, 1] 0.37| 0.96| 2.63] 162.5 )
Ferris quarry, r | r.08] 86.7 | 88.2 252 -215
surtace 2| ©0.98| 2.50| 2.62| 159.4)
Granite, Ferris t| ©.44|) ©.12| 2.63 re teh
0.40] 70.0 | 74.0 207 -180
quarry, surface 2| 0.19| 0.50] 2.71] 167.3 }
Mr Davis concludes from a careful analysis and interpretation of
these tests that the Yonkers gneiss is of superior durability.
CHAPTER XV
THE BRYN MAWR SIPHON
Geologic conditions as shown by exploration for a proposed pres-
sure tunnel
Bryn Mawr is a railway station 2 miles northeast of Yonkers.
The general features of the vicinity, its topography, succession ot
formations and the boundaries are shown on the accompanying
sketch map which is largely copied from United States Geological
Survey Folio No.83. The Southern aqueduct follows southward
along a Manhattan schist ridge until, at a point about a mile northeast
of Bryn Mawr, a cross depression of so great width and depth is
reached that some special means of crossing has to be devised.
Near Bryn Mawr station a gneiss ridge rises and continues south-
ward. The proposed line follows this ridge.
Explorations have been made as usual by drilling to determine
if possible whether or not a bed rock pressure tunnel is practicable.
The following questions may be made to cover most of the
practical issues of the study:
t What formations would the tunnel cut?
2 Which of these would show most questionable ground?
3 What portion of the line is regarded as most critical — whose
development would show whether or not a tunnel is practicable?
4 What special conditions are shown by drill borings?
5 What interpretation is to be placed on the peculiar results from
hole no. 4 where there has been unusually great difficulty in drilling?
6 What experiences in similar ground have a direct bearing on
this case?
Formations
The formations that would be encountered in the Bryn Mawr
siphon are:
1 Manhattan schist (top), the usual micaceous type, also called
Hudson schist in United States Geological Survey Folio 83.
2 Inwood limestone (middle), the usual coarsely crystalline dolo-
mitic and micaceous type, also called “ Stockbridge dolomite” in
tie Folio, same as’ Wuckahoe marble,” same as “Sing Sing
marble,’ same as limestone at Kensico dam and also at Croton dam.
201
202 NEW YORK STATE MUSEUM
3 Fordham gneiss (bottom), the usual black and white thinly
banded type, a much folded and strongly metamorphosed rock, the
oldest of all.
4 Yonkers gneiss, the usual type, gneissoid biotite granite very
uniform and granular. This formation is an igneous intrusive that
cuts up through the Fordham gneiss and is therefore younger.
Whether it is also younger than the limestone and schist is not
clear.
5 Quartz veins and lenslike segregations of quartz, also pegma-
titic streaks, are occasional occurrences in all of the formations.
They are most abundant in the schist, but are seen also in the Ford-
ham gneiss. A similar development was encountered in the lime-
stone in hole no. 40.
6 Glacial drift, chiefly modified drift, partially stratified sand
and gravel, reaching more than 125 feet in depth, covers por-
tions of all formations.
This last formation (no. 6) is the only one that may be wholly
avoided in the tunnel proper. The chief interest lies in its hindrance
to exploration and its possible usefulness as a source of sand and
gravel supply. |
Weakest formation. The Inwood limestone is the most ques-
tionable ground. This is believed to be so chiefly because of the
greater solubility of the rock, its granular and micaceous character,
and the probability that a line of displacement accompanied by some
fracturing crosses the siphon line in this formation. If a very
excessive amount of shattering occurs in this zone it may have
induced a condition of disintegration to such depth as to endanger
the tunnel. 3
There are no surface indications of a serious condition at depth
for any of the other formations.
Critical zone
The critical zone is probably not far from the contact between
gneiss and limestone. There are two reasons for this opinion. The
first is related to the nature of the folding. The formations are
squeezed into a close syncline pitching northward. In cross section
the strata at any point around the head of this trough dip inward,
and, because of the more resistant Fordham gneiss forming the floor
of the trough, the drainage and seepage and consequent tendency to
decay might be expected to follow along its upper contact.
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Location map showing by the dotted belts the distribution of Inwood lime-
stone in the Hastings-Yonkers district and the position of the Bryn
Mawr tunnel section as well as shaft 13 on the New Croton aqueduct
with their relations to the limestone belts. Manhattan schist and Ford-
ham gneiss occupy the rest of the area.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 203
The second reason is related to the probable later faulting move-
ments. It is evident from the map [Folio 83] that the formations
in the vicinity of Bryn Mawr are bulged up. One would expect
the trough which contains the schist and limestone of Grassy Sprain
valley to continue uninterruptedly southwestward and join with
Tibbit brook valley. But a cross fold has bulged the formations
up so much that for a distance of a mile erosion has removed all
oi the formations except the gneiss. Bryn Mawr station is about
central on this bulge. Evidence of such a movement is readily
seen on the gneiss along the northerly margin where it slopes down
toward the limestone. The movement had developed a little-shear-
ing and has tilted the minor folds downward toward the north at
angles varying from 30° to 80° from the horizontal. This angle
becomes somewhat more accentuated as the limestone is approached,
and it is believed that it may pass a short distance into the limestone
border. There is, however, no great amount of crushing evident in
the gneiss and this may hold also in the limestone.
The fact that Sprain brook crosses the formations along this
northerly margin and flows for 2 miles in a southeasterly direction
may indicate a still later movement, probably faulting. There is no
surface evidence of it except the abnormal course of the creek.
But, if there is such a fault, it also crosses the siphon line in the
same zone, i. e. in the vicinity of the limestone-gneiss contact, not
far from the location of the present course of the brook.
Therefore it seems reasonable to conclude that the critical zone
is near the contact, probably on the limestone side, and in the
vicinity of the present course of Sprain brook. It is also probably
cut deepest here by erosion. If this zone is in good enough condition
to stand tunneling the rest of the line ought to be.
Conditions indicated by borings
' All rock formations stand very steep. They vary from 80° to
go°. This means that very few beds can be explored by one hole,
and that any weakness or crevice is likely to make a showing in
excess of its true proportions.
The cores show considerable crushing. Some of the fractures
are not healed, although weathering from circulation is not present
on all of them. The micaceous layers are most affected by circula-
tion. Some beds of this variety are considerably weakened even at
depths of over 200 feet. Occasional seams have been encountered
that give no core at all for several (even 20 or 30) feet. But the
204 NEW YORK STATE MUSEUM
greater proportion of the recovered pieces are comparatively solid
even where the total percentage of saving is very low. It is evi-
dent that some of the core, a considerable percentage, has been
ground to pieces in the process of boring. This is especially notice-
able at hole no. 40.
Hole no. 40. Much trouble ee been met in this hole. A
careful analysis of the record and core and the behavior of the drill
is interpreted as follows:
1 Partially assorted drift, chiefly sand and gravel was penetrated
for 125, fect.
2 Limestone bed rock of fairly sound sees was struck at
about 125 feet (about el. —40). .
3 The casing that was put down to shut out the sand failed to
reach solid rock, and this permitted a continual supply of pebbles
and sand to run into the hole and obstruct the work with each pull
up. The presence of these pebbles was also instrumental in grinding
the core to pieces, and this accounts chiefly for the low saving.
4 After this opening was plugged up with cement, the drilling
was continued successfully until a somewhat broken quartz vein
was encountered and this has been followed for about 35 feet. Its
broken condition afforded another opportunity for fragments to fall
into the hole, and on top of the drill, bringing the work for a second
time to a standstill. It is certain also that the drift pebbles still fall
in. As the formation stands vertical here it is not surprising that
any feature should show an apparent extent quite out of proportion
to the real value. The quartz vein is probably of no great breadth.
Small seams containing mud may also be followed 15 or 20 feet
and still be of no great significance in the formation as a whole.
The rock fragments (core) recovered in this hole are fairly sound.
5 In spite of the many delays and difficulties of this hole, it is
apparent that the general rock formation is not responsible for it
all. The failure to reach solid rock contact with the casing has been
the cause of part of it. Later the penetration of a rather rare
quartz vein, a thing that would not often be found in the limestone,
has added to the trouble. Both of these causes are so rare that
they may almost be given the value of accidents.
But the last 100 feet or more of the hole, from depth 225 feet to
335 feet, shows an unusually questionable condition. Only a few
rock fragments are saved and they include limestone and quartz
vein matter. The rest is wholly disintegration sand of rather com-
plex composition but carrying very much mica. This is all wash
GEOLOGY OF THE NEW YORK CITY AQUEDUCT
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GEOLOGIC SECTION
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BRYN MAWR
“NOLLWLE
line from north to south showing the geologic structure interpreted from drill borings
-
Fig. 36! Section along the aqueduct
205
206 NEW YORK STATE MUSEUM
material except one sample, which is a “dry sample” and is still
more strongly micaceous.
Borings nos. 40, 45 and 46 are ail within the zone that was con-
sidered, from surface indications, to be likely to carry the deepest
gorge and to show the weakest rock. Because of the heavy drift
cover (more than a hundred feet) it is manifestly impossible to
locate the weakest zone more closely or judge of its exact condi-
tion except by borings.
Hole no. 42 at station 634 +28, penetrates 82.4 feet of drift and
reaches bed rock at about elevation 21 feet A. T. The rock is good,
substantial, coarsely crystalline limestone. It shows as sound con-
dition as can be expected in this formation even under the most
favorable situations.
Hole no. 46 at station 644+-77.4 is just south of the brook. It
penetrates 72 feet of drift and reaches bed rock at elevation 14
feet A.T. The rock is Fordham gneiss of typical sort and in per-
fectly good condition. There is no question about the soundness of
the rock from this point southward.
Hole no. 45 at station 643+ 52.5, 125 feet north of hole no. 46
penetrates drift for about 150 feet (possibly a few feet less, 145
feet). This drift cover is interpreted as mostly sand (modified
drift) to 115 feet and a boulder bed from 115 to 143 feet. After
the true ledge is reached it is sound and shows no unusual or ques-
tionable conditions. It is Fordham gneiss.
Interpretation
1 Weak zone. There is little doubt that this last 100 feet of
hole no. 40 is in the decayed weak zone that was expected to de-
velop in the vicinity of the contact between the gneiss and the lime-
stone. It would be expected to pitch northward along the floor of
gneiss and extend beneath the southerly extremity of limestone at
this point [see fig. 36].
2 Contact. Hole no. 40 cuts limestone, hole no. 45 cuts only
gneiss, therefore the formational contact lies somewhere in this
177-foot space.
3 Position of old channel. Bed rock surface is lowest at hole
no. 45. But since the rock itself is sound gneiss, it is not believed
to represent the lowest possible point. This is still more certain
because of the fact that the pitch is northward so that this becomes
a dip slope on which the preglacial stream could glide against the
edges of the limestone beds [see diagram], and because the condi-
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 207
tion of the rock a little farther north (at hole no. 40) shows that
these limestone beds are actually much weaker than the gneiss.
Therefore the deepest portion of the buried channel is to be expected
between holes no. 40 and no. 45, and probably nearest to hole no. 4o.
4 Depth of old channel. How deep the buried channel may
be can not be accurately estimated. But if the same dip slope as
is shown by the rock surface from hole no. 46 to no. 45 prevails
northward toward hole no. 40, a depth somewhat below —100 feet
may reasonably be expected. In the absence of data bearing upon
the depth of other portions of this ancient channel or of the lower
Bronx river with which it must have been connected, it is impossible
to estimate more closely.
5 Interpretation of hole no. 40. There is so little rock actually
saved from the more than 200 feet of possible core on this hole
that its real character is very obscure.
There are three possible explanations for the condition found in
the last 100 feet.
a The drill may have followed a large mud seam.
b The material may be only residuary rotten limestone still wholly
above the gneiss.
c The actual contact may have been penetrated and a part of
this rotten material may be decayed gneiss within a crush zone.
The difficulty in drawing absolute conclusions is increased by the
fact that matter falling in from above has been a continued source
of trouble and is more or less mixed with the rock material of
lower points. Therefore, the fact that the sand taken from the
lowest points, 335 feet, is silicious instead of calcareous, may not
prove satisfactorily that the rock at that point is wholly silicious.
It is worth noting, however, that the harder rock in the upper
portion of the hole was in places much crushed and that mud seams
were encountered before reaching this last 100 feet.
It is also worth noting that the same dip slope of rock surface
as prevails between holes no. 46 and no. 45 if continued northward
to hole no. 40, would cut that hole a considerable distance (75 feet)
above its bottom.
In view of all the conditions, therefore, it is judged that there is
a crush zone here, that hole no. 40 penetrates it, that it 1s badly de-
cayed, that the plane of the crush zone dips steeply northward and
cuts both limestone and gneiss, that a tunnel at about —300 feet
would cut this zone south of station 640 and north of station 642,
and that all other portions of the line are in comparatively satisfac-
208 ; NEW YORK STATE MUSEUM
tory condition. This zone for a hundred feet is likely to be wet,
weak, and would require extra precautions and additional expense
in construction.
6 Evidence of faulting. Whichever interpretation of hole no.
40 is taken is in support of some displacement in the nature of
faulting between holes no. 40 and no. 45. If the gneiss rock floor
is not reached in hole no. 40, then the greater northward slope of ©
it from hole no. 45 to no. 40 than is shown from no. 46 to no. 45
indicates a downward movement. If on the other hand, the iden-
tity of the formation in the lower part of hole no. 40 be considered
undetermined, and its condition attributed to decay in a crush zone,
the presence of the crush zone itself indicates. movement of a fault
nature.
Conclusions as to character of the crossing
In considering the geological conditions as a factor in the prob-
lem of practicability of a tunnel, it is necessary to note the follow-
ing points:
1 In view of the fact that the deepest point in the ancient chan-
nel is not yet found, and that it will probably go below —I00 feet,
it would be necessary to figure on a tunnel grade down well toward
—300 feet.
2 It would be necessary to figure on a wet and weak zone of at
least 100 feet along the tunnel and a more expensive construction
at that point.
3 The ground at such depth south of station 642 is unusually
sound. The ground north of station 636 may be counted good.
The ground between 636 and 640 may be considered fair, and the
ground from 640 to 642+, troublesome, containing the chief ele-
ments of uncertainty.
Fig. 36, which is a geologic section along the line at this point,
shows the distribution of these features drawn to scale. .
CHAPTER XVI
A STUDY OF SHAFT 13 AND VICINITY ON THE NEW CROTON
AQUEDUCT
[See outline location map, pl. 30]
There has been reference made occasionally in connection with
the Bryn Mawr explorations, as well as others, to the remarkable
piece of bad ground encountered in 1885 on the New Croton aque-
duct near Woodlawn in the Saw Mill valley. This experience has
been the source of much misgiving. Because of its evident im-
portance and close relationship to conditions that may exist in the
same formation at points on the Catskill line, an examination of
this ground was made for the purpose of comparison. The mean-
ing of that case and its bearing on the Bryn Mawr questions aze
given below: z
Engineer’s records
This ground and its remarkable behavior is describéd by Mr J. P.
Carson in the Transactions of the American Institute of Mining
Engineers, September 1890, pages 705-16 and 732-52.
A description is also given in Wegman’s Water Supply of the
City of New York, 1658-to 1895, on page 152.
From Mr Wegman’s report is taken the following:
The south heading was started from this shaft on June 1, 1885.
It advanced at the rate of about 80 feet per month for 392 feet
through good limestone rock (dolomite), which then became softer.
On December 9, 1885, when the heading had reached a point 407
feet from the shaft a fissure was encountered from which about
100 cubic yards of decomposed limestone clay, sand and dirty water
poured into the tunnel, partly filling it for a distance of 125) feet.
After three days delay, when only clear water was flowing into the
tunnel, the fissure was plugged with straw. The heading was ad-
vanced 20 feet further until on December 22, 1885, an outpour three
times greater than the first occurred, covering everything in the
heading out of sight * * * borings were made on the surface
with a diamond drill to determine the extent of the soft ground in
front of the tunnel. It was found to lie in a pocket in the rock,
209
210 NEW YORK STATE MUSEUM
which had a length of 110 feet on the axis of the tunnel and ex-
tended for a short distance below the invert of the conduit. The
soft material, consisting of sand, gravel, clay and decomposed rock
had a depth of about 160 feet from the surface to the top of the
tunnel. It exerted such a pressure against the timber bulkhead that
the 24-inch oak logs used as “ rakers”’ (braces) became crushed in
24 hours and had to be continually renewed.
The chief points of present interest are that the tunnel, at a depth
of about 160 feet from the surface, and after passing through sev-
eral hundred feet (407 feet) of good dolomite, came into rotten rock
and soft ground 110 feet across on the line. It was so soft that
it ran into the tunnel in great quantities and exerted such pressure
as to make progress in it a very troublesome and costly matter,
taking “ 60 weeks to advance the tunnel 85 feet’ and costing “ $539
per foot.” The material caved in so freely as to form a pit on the
surface.
Statement of geologic conditions
It is not possible to interpret the conditions at this locality as
fully as one would wish because of the vagueness of some of the
statements, but the following facts and explanation are essentially
COELECE :
1 The rock is the Inwood limestone, the same kind and same
general conditions as all of the limestone belts that occur in the
region of the Southern aqueduct.
2 The soft ground penetrated at the point in question — 407 feet
south of shaft 13 — called in the Carson report and others “a fis-
sure’ or “ pocket,” etc., is in reality a fault crush zone. The fault
plane probably dips steeply southeast and strikes n. 50° e. cutting
the tunnel line at an angle of something like 20°.
3 The point is well up on the side of the valley more than a
hundred feet above Saw Mill river, and the strike of the fault zone
in its southwesterly extension cuts into the lower portion of the
valley, so that underground circulation would be encouraged along
the zone in this direction.
4 The limestone outcrops very near by on the west side of the
line and the Manhattan schist occurs near by on the east. The atti-
tude of the beds is such as to indicate a fault of the thrust type,
The accompanying figure illustrates this relationship in a cross sec-
tion at right angles to the axis of the tunnel [see fig. 37].
GEOLOGY OF THE NEW YORK CITY AQUEDUCT ZIi1
Crush Zone
Fig. 37 Sketch of the geologic structure at shaft 13 on the New Croton aqueduct.
Interpreted from field observations
5 It would appear probable that this zone was penetrated at the
worst possible level, i. ec. near enough to its wholly decayed upper
part to furnish no resistance at al! to the overlying sand and gravel,
and not deep enough to reach the more substantial (although prob-
ably crushed) rock that may reasonably be expected to prevail at
no very much greater depth.
The chief point is that the weak spot has a reason and is not an
accidental thing that might be expected just anywhere. But it must
be admitted, in spite of this fact, that a casual examination of the
iocality would not make one suspicious of its existence, and it is
surprising that the spot could have caused so much trouble.
From the above it will be seen that in several respects the Bryn
Mawr case is somewhat similar to this. They both indicate fault-
ing; they are in the same type of rock; they both show or indicate
caving tendencies.
On the other hand, there are certain elements of difference some
of which are capable of very materially modifying any conclusion
that might be based upon the simple facts of likeness. For exam-
ple — it should be expected (1) that the fault movement at shaft 13
would be the greater because of lying in the more prominent lines
of such displacement of the region, (2) being a thrust movement,
the crush effect is probably more prominent at shaft 13 than at
Bryn Mawr, (3) occurring at greater elevation above probable cir-
culation outlet, the opportunity at shaft 13 for extensive and rather
deep decay is the greater, (4) being cut so near the surface (160
feet), its condition there is not necessarily a reliable guide to the
seriousness of decay at a greater depth.
212 NEW YORK STATE MUSEUM
Comparison of Bryn Mawr and shaft 13
The following statements embody an opinion on the points raised
or suggested in connection with a reference to the New Croton
difficulties at shaft 1 3. The items are therefore treated by compari-
son or contrast so far as possible:
1 Type of rock. The rock explored at the Bryn Mawr siphon
is the same formation as that in the Saw Mill valley cut by the
New Croton aqueduct, i. e. the Inwood limestone — sometimes
called “Stockbridge dolomite.” It is the same also as the other
large limestone belts in Westchester county. There are occasional
small strips of limestone of another type, but its behavior could not
be very different.
2 Soft material. “Is any material of this sort” (like that in
the New Croton tunnel near shaft 13) “likely to be encountered
either in the crushed zone at boring 40 or elsewhere in the lime-
stone belt?”
It is sure to be encountered, especially near hole 40, if that zone
is cut shallow. The behavior of the lower portion of this hole is
very similar to the described case near shaft 13. The only prob-
ability of avoiding it lies in placing the tunnel deep enough to cut
more substantial rock. The single hole upon which all this argu-
ment is based can scarcely be considered a thorough enough ex-
ploration to build up a quantitative statement as to depth or width.
There is no evidence, either on surface or in the exploration
holes, of any other such zone on this line.
3 Depth and extent. Under the circumstances, the increased
depth makes it less probable that so much ground of like behavior
would be found. Again, it is not likely that precisely the same
conditions would so effectually halt operations or be considered so
nearly insurmountable at this time. One of the many serious
objections is that the tunnel would have little strength or resist-
ance to a bursting pressure. It must be admitted that if caving
ground were penetrated it would prove very difficult to handle with
the gravel cover at the depths now considered, 1. e. 300 feet or
more below the surface. :
4 Water. “ What are the probabilities in regard to the quan-
tity of water to be met in the crushed zone near boring 40? Can
any limit be set which it would be extremely improbable that the
inflow would exceed, on account of the topography of the country
and the nature of the overlying materials?”
There is likely to be much water. Nearly all of the overlying
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 213
drift is sand and gravel that is probably saturated and in such con-
dition as to permit easy flow to any lower outlet. It may readily
carry 8-10 quarts of water to the cubic foot or about 2 gallons.
The area covered by such deposits is about 2500 feet long on the
southerly base along the creek and at this margin is approximately
150 feet deep. The northerly margin is variable and reduces in
places to o feet in thickness. It may, however, really represent
500,000,000 cubic feet of this gravelly material holding 1,000,-
000,000 gallons of water as a nearly permanent supply.
This overlying material is necessarily a menace of no mean pro-
portions. Every crevice or crush zone remaining unhealed will
have water and plenty of it, the inflow being limited only by the
size of the cracks and their abundance untii the reservoir should
be drained. There is no hardpan bottom to act as a dam.
Outside additions to this permanent supply are confined to that
received from rain and the stream. The rainfall on the area and
immediately available as addition to the underground supply in the
lower sands, together with the stream flow, which would probably
sink into the sands, if an attempt to drain the underground supply
were made, may be expected to furnish additional water at a pos-
sible rate of 2500 gallons per minute. How much of all this is
available at tunnel level depends wholly upon the openness of
structure in the rock. There is nothing else to materially control
the permanent and additional supply.
There is evidence in hole 40 of considerable crushing. That
means capacity for water circulation, but how much no one can
tell. There is also much rotten rock in the same hole. This means
that circulation has been easy and effective, but how much now no
one can tell. The single hole (no. 40) in the absence of any other
corroborative data is not sufficient to base more elaborate or precise
quantitative estimates upon.
5 Solubility. What is “the nature of the limestone with
reference to its resistance to solution?”
This limestone is, as all limestones are, more easily attacked by
circulating water than most other rock types [see Rondout Valley].
The Inwood limestone such as occurs at Bryn Mawr is crystalline,
often contains much mica and then is inclined to be foliated in
structure, and it prevailingly stands steeply inclined. Because of
these features in which it differs from the Rondout Valley lime-
stones, it is likely to be more generally affected by decay along the
zones permitting circulation than any of the Rondout Valley types.
214 NEW YORK STATE MUSEUM
The Rondout Valley limestones are affected along joint planes, but
the effect is almost wholly confined to a simple enlargement of these
crevices. In the Inwood an additional effect is the weakening of
the sutures or bond between the individual granules resulting in a
tendency to weaken the whole mass as far as there is much pene-
tration of seeping water. It would have less tendency to produce
openings or caves, but greater tendency to produce a rock that
would crumble in the hand or that would gradually assume the con-
dition of a lime sand or a micaceous mud.
As to the effect of water from the aqueduct on fresh portions of
this rock, it is certain that the rock would be attacked wherever
exposed to direct action. Its method of attack is by solution, and
the rate of attack may safely be reckoned as not materially different
from that assumed or being established by experiment and expert-
ence on the Rondout Valley types.
In the final consideration of the difficulties at Bryn Mawr the
engineers have decided to abandon the tunnel plan. It is probable
therefore that no additional explorations of direct bearing on the
problems of this ground will be made.
CHAPTER XVII
GEOLOGICAL CONDITIONS AFFECTING THE LOCATION OF
DELIVERY CONDUITS IN NEW YORK CITY
Hill View reservoir is the terminus of the Southern aqueduct.
The Catskill water is to be delivered at this point, just north of the
New York city line on the Yonkers side, at an elevation of 295
feet. From this reservoir the water is to be distributed by an inde-
pendent system of conduits to the principal centers of consumption
in lower Manhattan and Brooklyn.
It is believed that distribution can be most economically made
and the system be most permanently established by constructing
the main trunk distributaries as tunnels in solid bed ‘rock at con-
siderable depth below all surface disturbances.
Preliminary investigations have been carried on by Headquarters
department, Mr Alfred D. Flinn, department engineer, beginning
in 1908. As the active work of exploration was entered upon Mr
William W. Brush, department engineer, was assigned to this special
division of the department’s work and most of the preliminary ex-
ploration borings were planned and finished under his immediate
supervision. With the resignation of Mr Brush to take the post
of deputy chief engineer in the Department of Water Supply, Gas
and Electricity, Mr Walter E. Spear, department engineer, was
secured to continue the difficult work of finishing explorations and
preparing for construction.
Studies of conditions affecting such a system and explorations
designed tc test the ground in line with these studies have been
made. The work thus far done in an exploratory way has been ©
confined to one main distributary.
Section A. Preliminary geological study
As a preliminary step toward the systematic study of local con-
ditions affecting possible conduits, trial lines were laid out on the
1Few engineering enterprises, probably, have been planned with so care-
ful regard for all known geologic conditions. The geologist and the en-
gineer worked alternately on the same problems until, in the opinion of both,
the best possible line was selected. It is the writer’s belief that so sys-
tematic a method has seldom if ever been carried out in engineering work
of this kind. On this account, and in part to illustrate some of the pre-
liminary stages in such work, many of the original facts and arguments
and suggestions are given without change in the following discussion.
215
216 NEW YORK STATE MUSEUM
city map from Hill View reservoir to Brooklyn by three different
routes. So far as the topography and city development and other
engineering considerations could be forseen either route could be
used. Studies of all kinds were expected to indicate which would be
the most favorable and whether or not it might be advisable to shift
even the best one to still more favorable ground. These are shown
on the accompanying map which also covers the local geology of
the immediate vicinity of the lines [see pl. 32].
General questions
When the problem of the practicability of a rock tunnel for
distribution conduits was first studied, several general questions
were raised which indicate the lines of investigation followed.
1 What is the character of the rock along the projected conduit
lines shown at the depths required for such tunnels?
2 Will the rock at moderate depths be such as to permit success-
ful and economical construction of tunnels to be used under the
hydraulic pressure due to Hill View reservoir?
3 Does the character of rock in the vicinity of the lines vary
sufficiently to materially affect the cost of a tunnel if the lines be
shifted approximately 1000 feet either way from those shown on
the original map as trial lines?
4 Are the suggested locations of conduit lines adapted from a
geological viewpoint to the construction of pressure tunnel con-
duits, and, if not, what changes in these lines would be advisable?
5 Is the thickness of rock covering sufficient at all points to
obviate trouble from open seams and disturbed surface rock?
6 What borings and other field investigations should be under-
taken to determine the practicability of construction of pressure
tunnels along the lines suggested?
In line with this series of questions a thorough geological investi-
gation was begun, the chief conclusions of which are given below.
Geological formations
There are six local formations of sufficient permanence and in-
dividuality of character and of sufficient areal importance to be
treated as units in this study. These are described in some detail
in part 1, but for convenience are briefly listed as follows:
1 Glacial and postglacial deposits of boulders, clay and sand, with
silt beneath the rivers.
Plate +31
oe
~~ “roe
iL Parr e,
Reproduced from a model
p of New York city and environs.
A relief ma
by Howell.
”) ? “f ei
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 217
2 Manhattan schist, the most abundant formation, chiefly mica
schist with very subordinate hornblende schists, and usually with
abundant pegmatite lenses and veiis.
2, The Inwood limestone, a white, dolomitic marble when fresh,
which shades into impure, micaceous varieties.
4 The Fordham gneiss, varying from a thinly schistose or
quartzose rock to a strongly banded or a very massive and much
contorted gneiss. The oldest formation of the district.
5 The Yonkers gneiss, an original intrusive granite, now
squeezed into a gneiss. Younger than the original Fordham.
6 The Ravenswood grano-diorite or as it might be called in
engineering practice, granite; an original, intrusive rock now some-
what gneissoid from pressure. Younger than the original Fordham.
The Manhattan schist, the Inwood limestone and the Fordham
gueiss are cut by veins or dikes of coarsely crystalline granite,
technically called pegmatite. They are of irregular distribution and
do not affect the tunneling operations one way or another.
All the formations older than the glacial drift have been com-
pressed into a series of northeast and southwest folds, and all have
as a rule a steep or almost vertical dip. The axes of the folds are
not horizontal, but usually pitch downward to the south at low
angles. Erosion has developed a series of ridges trending north-
east and southwest. The limestone being a softer and more easily
eroded rock, almost always underlies the valleys or flats and the
river channels. It is certain also that there is some faulting.
Rock at depth
The distribution of geological formations along the proposed
lines has been shown on the accompanying map [pl. 32]. In gen-
eral the kind of rock at tunnel depth will be the same as at the
surface as indicated on the map for each point. Such error as there
is, arises from two causes: (a) Uncertainty as to the exact location
of some of the contact lines between two formations (usually due to
drift cover), and (0) dip and pitch of the strata.
- In the first case (a) where the drift is particularly heavy, it is
sometimes impossible to fix a contact line accurately from surface
features alone.
In the second case (>) it must be appreciated that nearly all of
the formations dip eastward at a very steep angle, so that a form-
ation would usually be found to extend a little further east at depth
than at.the surface. And also all formations pitch southward, so
218 NEW YORK STATE MUSEUM
that they would be found to extend considerably farther south at
depth than their surface outcrops. This angle of pitch is from
ne” £0.30".
In nearly all these cases, however, the obscurity of the actual
surface boundaries is as great a source of uncertainty as the effect
of dip and pitch, so that the boundaries as mapped may be con-
sidered sufficiently accurate for this comparative study of the lines.
It is worth noting that the rock at the proposed depths of tunnels
would be, as a rule, more substantial than at the surface. But there
are several places on all of the lines where the exact condition is
unknown at the surface as well as at depth. The chief points of
this character will be noted in a later paragraph.
Comparison of lines*
A comparison of the three lines submitted as the basis of ex-
amination — (a) the westerly one, (b) the central one, (c) the
easterly one [see accompanying map, pl. 32], as to rock formations
likely to be cut by them, furnishes the following figures:
Line A. Going southward from Hill View reservow
Feet
6200 Yonkers gneiss — good rock
1 400 Fordham gneiss
1400 Probably largely Inwood limestone with one weak zone
(at Van Cortlandt lake)
5 600 Fordham gneiss — good rock
2400 Near contact with limestone, probably in gneiss
1600 Crossing Harlem river — Inwood limestone
4000 Inwood limestone — probably fairly good rock
800 Inwood limestone — probably containing bad zone to
Speedway
16 400 Manhattan schist (to 135th st.)
2000 Along contact between schist and limestone
4200 Inwood limestone with one weak zone (to s. end of
Morningside Park)
1The statements of quality and extent of certain formations and zones
are capable of some modification as exploratory work progresses. Some of
these are noted in later sections of this report under special headings, such
as The Lower East Side, and The East River-Brooklyn. section. For the
present purpose, as showing the development of the geologic basis of the
project it seems preferable to leave the accompanying comparisons in their
original form as presented to the board.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 219
12 800 Manhattan schist probably good quality (to s. end of Cen-
tral Park)
21000 From Central Park to East river —no outcrops — mostly
Manhattan schists at tunnel depth. Condition largely
conjectural'— probably mostly good rock with occasional
weak zones
6000 Manhattan island to City Hall, Brooklyn. Containing an
unknown! zone in the East river and unknown quality
of rock in Brooklyn.
Summary of Line A
Feet
6 200 Yonkers gneiss
7000 Fordham gneiss
2400 Contact (probably in gneiss)
12000 Inwood limestone
2000 Contact (probably in ieseone)
29 200 Manhattan schist (good)
21 000 Estimated Manhattan schist (fair)
6000 Almost unknown
85 800 total
Line B. Going southward from Hill View reservoir
Feet
8000 Yonkers gneiss — good quality
13 000 Fordham gneiss — good quality
6 800 Inwood limestone, probably mostly in fair condition, except
at two points (to Cromwell av.)
6 600 Inwood limestone, unknown condition, but probably largely
poor (to Harlem river)
600 Inwood limestone — unknown condition (Harlem river)
4600 Inwood limestone — unknown condition — probably fair
(to Mt Morris Park)
800 Manhattan schist. good
800 Probably Manhattan schist — unknown
2800 Inwood limestone — unknown condition — probably at least
one bad zone (to 106th st.)
12000 Manhattan schist along Central Park — good
1 Explorations since conducted by the Board of Water Supply have proven
the quality and character of the rock floor at these places. For the revised
statement on these sections see the special discussions.
220
Feet
8 600
I4 000
6 000
Feet
8 000
13 000
2I 400
12 800
23 400
6 000
84 600
Feet
6 000
17 400
5 000
9 800
1 800
600
6 400
I 000
I 200
I 800
I OOO
I 000
7 O00
2 800
18 400
NEW YORK STATE MUSEUM
To Broadway — Manhattan schist (little known except
from tunnels already made)
To East river, probobly Manhattan schist (same as line A)
Manhattan island to City Hall, Brooklyn — uncertain con-
dition (same as on line A)
SUMMARY OF LINE B
Yonkers gneiss — good quality
Fordham gneiss — good quality
Inwood limestone — variable quality
Manhattan schist — good quality
Estimated Manhattan schist — fair
Almost unknown
total
Line C. Going south from Hill View reservoir
Yonkers gneiss — good rock
To Webster av.— Fordham gneiss — good rock
Along contact between limestone and gneiss
To 138th st— Inwood limestone with probably two bad
zones
To Bronx kills-—along contact between limestone and
gneiss — uncertain quality
Across Bronx kills— mostly in limestone containing a
fault zone — probably bad ground
Crossing Randall's and Ward's islands and Little Hell Gate.
Nearly all is Manhattan schist of good quality
Crossing Hell Gate —. Inwood limestone
Crossing Hell Gate — Fordham gneiss of good quality
Astoria point — probably Fordham gneiss of good quality
Crossing another limestone belt
To Vernon av.— Fordham gneiss of unknown quality con-
taining one fault zone
To Nott av.— Ravenswood grano-diorite — good rock
To Borden av.— Probably Ravenswood grano-diorite.
To Fort Greene Park Brooklyn — almost wholly unknown
but contains probably 5000 or 6000 feet of poor ground
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 221
SUMMARY OF LINE C
~ Feet
6000 Yonkers gneiss — good quality
17 400 Fordham gneiss — good quality
6 800 Along contact between limestone and gneiss (questionable)
12400 Inwood limestone — with several bad zones
6 400 Manhattan schist — probably good quality
3.000 Fordham gneiss -— probably good quality
1000 Fordham gneiss — unknown quality
9 800 Ravenswood grano-diorite — mostly very good rock
18 400 Almost wholly unknown
81 200 total
Tabulated summary — Types of rock formations
LINE A (WEST) LINE B (CENTRAL) LINE C (EAST)
Per Per Per
Feet cent Feet cent Feet cent
Monlrense oneiss: sone. i vie-c8 6200:5 7(7 <2) 8000, “(9 4) 6000 (7.3)
Bordham (\etieiss 22.25 625.24 7000. (S21), 13000 (45.3))) 21400 (26:3)
Gantachn zones iss. ae ae A 400. (5:1) O (O)i a eGrsco!, (853)
Inwoods limestone i). 3..42: PAOGOs C14.) . 21 400 (25.3) 12400 (15.2)
Manhattan Schist sos ...c55 « FO 200 (56.5) 36200 (42.6) 6400. (7.8)
Ravenswood grano-diorite . ) (0) O (0) 9800 (12.0)
Olay cc coh oe oe Sow sss 6000 (7.0) 6000 (7.0)- 18400 (2.6)
Motawlenetiy we). faves: LS5 O00 2 See oc S41 G00» 255. ie S200" Geae fs:
Summary of quality
LINE A LINE B LINE C
Per Per Per
; Feet cent Feet cent Feet cent
Goad sock, 1st; erade. 552+. 42400 (49.4) 33800 (40.0) 39800 (49.0)
Probably, air, 2d’ erade;.:. 30800 (35:0) 34800 (4r:t) 13600 (16.7)
Probably poor, 4d strade.... 6600. (7.7) too00 (11.8) 9400: (TY.6)
MIMOSH MNKHOWIE ook... --< 6000 8(7.0) 6,000" 3) (751) ae. te 700.” (22:7)
85800 100.0 84600 100.0 81200 100.0
Argument on choice of line
In judging the quality of rock and its suitability for this con-
duit the factors of most weight are the same as those repeatedly
mentioned in connection with other portions of the Catskill aque-
duct line. That is, in brief, that the harder crystalline rocks of the
Fordham gneiss and) Manhattan schist types wherever known to be
222 NEW YORK STATE MUSEUM
free from fault crushing and surficial weathering are the best
variety ; that the more heavily buried areas of these rocks, together
with those limestone areas that are known to be the most substan-
tial of its class, should be regarded as fair or second grade; that
the more obscure areas of limestone and all portions crossing
faults or rivers or crush zones in any rock must be regarded as
poor or third grade. This rating is based wholly on rock char-
acter and without any consideration of cost of construction.
From the above it is clear that line A has more “first grade ”
rock than either B or C and less “ third grade” ground.
Line C has three times as much “unknown” ground as either
B or C and less “ first’ and “ second grade ” rock.
In other words, the three lines are estimated:
Se i ae a pie oh
Birst rade ‘rocky ..u Sie Pee ee 49.4 40.0 49.0
mecond Sradewocks) S22 le mens ae oe 35.9 AI.1 16.7
First and second grades together....... Obes 81.0 O527
Mihind grade cocliis: eC e) ee: 7.7 11.8 LAG
Wrknown’ erounde enn a Beles. aks 720 Het 22.7
In addition to these differences of quality, it appears from a
study of the areal geology along the respective lines that a tunnel
would pass across limestone contacts from one formation to an-
other six times on line A, four times on line B, and seven times
on line C. These may all be considered points of probable
weakness.
All of the lines cross belts of well known weakness believed to
represent fault zones. Line A crosses three such zones, line B
crosses two, and line C crosses at least three.
Furthermore, all of the lines cut limestone for greater distances
than seems desirable or necessary. The weakest ground and the most
uncertain quality of ground that can be mapped falls within the
limestone areas. In this respect line A with 13.9% of limestone
ground is preferable to line B, with 25.3% or line C, with 15.29.
From the above it is apparent that line C is least defensible.
Line A has some advantage over both of the others, especially in
quantity of first grade rock quantity of first and second grade
together, low amount of the known poorest grade and small extent
of the so called “unknown” ground.
The chief advantage of line A over line B lies in its much
smaller limestone area (12,000 feet vs. 21,400 feet or 13.9% VS.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 223
25.3%), and the chief advantage of line A over line C lies in its
much smaller amount of “unknown” ground (6000 feet vs. 18,400
feet or 7.0% vs. 22.6%). On these grounds line A is the least ob-
jectionable of the three lines proposed.
But it is also clear from an examination of the field, as is shown
on the accompanying map [pl. 32], that it is possible to avoid
some of these objectionable features or certain parts of them and
materially improve the figures by shifting the line to a sort of com-
promise position between line A and line B. This compromise
line, or the trial lines from which the final tunnel line may result,
should follow as closely as possible the gneiss and schist ridges
and should avoid the limestone areas and known weak zones wher-
ever possible.
Depth of tunnel
The rock formations in general at the required depths are no
more objectionable on Manhattan island or in The Bronx than at
other localities on the Southern aqueduct. There are weak places
and crush zones to be crossed and some of them can not be avoided
by any possible manipulation of the line, but these most question-
able spots constitute but a small proportion of the whole distance.
The depth most suitable must depend chiefly upon the depth neces-
sary at the worst spots.
Comparative cost of construction if lines are shifted
The question is best answered by reference to the geological map.
It will be noted especially that the belts of the different rock forma-
tions are usually narrow, and that they run nearly parallel to the
average direction of the lines. Therefore a shift of line to no great
distance would at many points place it within an entirely different
formation. It is also notable that all of the lines run along or near
the contacts between formations for long distances. At such points
a very small shift would wholly change the type of rock and rock
quality. Some shifting is desirable.
In general it may be assumed that the limestone belts would be
easiest and cheapest to penetrate wherever they are fairly substan-
tial, but they undoubtedly also contain the greater proportion of
weak and troublesome ground and must be considered least desir-
able from the standpoint of maintenance and durability. The
gneisses are probably most expensive to penetrate and the schists,
medium. Both are more expensive than limestone but both are
more likely to prove acceptable for other reasons.
224 NEW YORK STATE MUSEUM
The question of shifting the lines is a complicated one and hinges
more upon rock conditions, durability, and location of weak zones,
than on any possible cost.
Advisable changes in lines
None of the suggested lines are defensible from a geologic point
of view for the reason that a much better one may be obtained by
no very serious shifting.
In the general consideration of relative advantages of different
_ possible locations of the line, it is. believed that the following large
features are of most immediate importance:
1 The ridges as opposed to the valleys.
2 The hard formations as opposed to the softer ones.
3 The crossing of few contacts as opposed to crossing many.
4 The location well within a formation as opposed to location
aiong a contact zone.
It is distinctly preferable from a geologic standpoint (1) to fol-
low the ridges, (2) to keep in the hard formations, (3) to avoid
many changes from one formation to another, (4) to keep away
from contact zones, and (5) to avoid weak zones, if possible, or
cross known troublesome zones at the most advantageous point.
Recommendations of new lines F, G, H, I
The original lines A, B and C are marked on the map in blue
[pl. 32]. In addition several trial lines are sketched in yellow, any
one of which would give better geological conditions than any of
the three original lines. The newly suggested trial lines differ from
each other chiefly in the points at which they cross the limestone
belts and weak zones. In all of them the central idea has been to
follow the gneiss and schist ridges as persistently as possible. All
unite at Central Park and are intended to follow Fifth avenue,
Broadway, the Bowery and Market street to East river along one of
the original lines. North of Central Park they differ from the orig-
inal lines. The westerly one crosses the Harlem river at 176th
street and may be designated line F. The easterly line may also cross
the Harlem river at 176th street and may be designated line G; or
it may continue southward and cross the Harlem at 1 55th street.
It will then join the first one in the vicinity of 144th street and is
called line H. The alternative easterly one which crosses the Har-
lem at 155th street and follows Seventh avenue to Central Park is
line [.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 225
Details of rock conditions along these lines are as follows:
Line F. (Westerly) beginning at Hill View reservowr
Feet
7600 Yonkers gneiss — good quality
15 000 Fordham gneiss — good quality
2000 Fordham gneiss — probably 2d grade
1200 Harlem river crossing—partly limestone—3d_ grade
14 800 Manhattan schist — good quality
1600 Manhattanville crossing — 3d grade — some limestone
2600 Manhattan schist—good rock—through Morningside
Parle:
800 At south end of Morningside Park — perhaps some lime-
stone — 2d grade
1 400 Manhattan schist — good — to junction
12000 Manhattan schist — along Central Park — good
20600 To East river — Manhattan schist —less known’— (fair)
(2d grade)
6000 To Brooklyn “ unknown”!
85 600 Lime G
Feet
8 400 Yonkers gneiss — good rock
17600 Fordham gneiss — good rock
which brings it to the Harlem river where the other line (F) 1s
joined. Although the line is about 1400 feet longer, it avoids some
low ground (2000 feet) along the east bank of the Harlem river,
some of which may be in poor condition. Total length of line,
87,000 feet.
Line H
Feet
8400 Yonkers gneiss — good quality
23 800 Fordham gneiss — good quality — to Harlem river
I 000 Crossing Harlem river — probably fault zone in gneiss
Soo Fordham gneiss — good quality
1000 Limestone — 2d grade
1 200 Manhattan schist — good quality — to junction with the first
line (F) at 145th street
From this point the line is the same as F and G. Its chief ad-
vantage is the great distance which it has in Fordham gneiss.
Total length of line, 85,600 feet.
* Subsequent explorations made by the Board of Water Supply have elimi-
nated this unknown ground: See later discussion.
226 NEW YORK STATE MUSEUM
Line I
Feet
8 400 Yonkers gneiss — good quality
23 800 Fordham gneiss— good quality —to Harlem river
1000 Crossing Harlem river -— probably fault zone in gneiss
4 400 Fordham gneiss — good rock — to 135th street
4600 Inwood limestone — probably fair — 2d grade
2000 Inwood limestone — probably poor quality — 3d grade
1000 Manhattan schist — good quality |
At this point the line unites with line F. Total length of line,
&2,800 feet.
A tabulation of these figures indicating estimated extent of rock
types is given below:
LINE F LINE G LINE H LINE i
Feet Feet Feet Feet
ioral lenothy or Immest ena cere oe Oe ee 85 600 87000 85600 83800
Benet in Yonkersveneisse eae ee wee ee 7600 8400 8400 8400
Meneth im Hordham someissen+. see sone ee 17000 17600 25600 29200
Length in Inwood limestone and marginal
COMEACES {Sree 6 eRe ee ce ah ee 3600 -3 600, 3400) 8 “6iGon
iencth wn Manhattan senistene. seers 51 400 51400 42200 33600
Comparative summary of types of formation (Comparative dis-
tances are expressed in percentages)
A B Cc F G H I
WEOmkerS /ST1EISS 42 so mete ele 7D UOBAS 73. “Oxare EOSOMNOnO gGNe)
ordliam! ST1eisse si. penoeee ee eek Sup 5.3 926).3)" 10-6" 20e2 5 2OnOr sane
CGontactwones sy: Sea aaa Beaty LOKOM) O43
ihiwood limestone aan... ween AAO). QE) mt Ee) UWS GE EN
Miainhiattans SCHISt see eee ue BOu5 "4220. (72S 6020, 950-0) 49.5 ou
Ravenswood grano-diorite! ...... 0.0 OL Om 1250.4, OL0) 40053 OL O05 Ono
Too little known to classify!..... 7.0 | 2720. 22-06 7.05) Os ps7 Onur
1The Ravenswood granodiorite has been proven by later explorations to
extend into the territory here marked as too little known to classify.
As a group it is especially noticeable that the new lines F, G, H,
I, have a very much lower percentage of contact zones and lime-
stone. The percentages of gneisses have been notably increased,
and the unknown and questionable formations have been reduced
to approximately the lowest terms.
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 227
Estimated summary of quality
LINE F LINE G LINE H LINE I
Feet Feet Feet Feet
ood. sock, first. otadex.. os. hi6% 53 400 56 800 54 600 AQ 600
Fair ep SCCONG MAT Lyle see eae oe 23 400 2I 400 22 400 25 200
Poeria: ~hird:. *“ br Heater 2 800 2 800 2 600 3 000
Unknown! (Brooklyn) .......4...: 6 000 6 000 6 000 6 000
85 600 87 000 85 600 83 800
1 All of this rock is now known to be of good quality.
In other words these new lines show:
LINE F LINE G LINE H LINE I
Percent Percent Percent Percent
Pet EAE: GOCK © os sess ba fa kona elads 62:3 65.3 63.8 59.1
Second Shao ed at UES A RE AO anes oes Tega ig 24.6 26.1 30.0
First and second grades together....::.... 89.6 89.9 89.9 89.1
MP ECmenAGe LOCKE vs OP iekios waves Sateen es 3.2 220 3.0 3.6
Seerikimauwt ) stron) oN ony. a deco a sale 7.0 6.9 7.0 7a
*Results of recent boring explorations show that this ground is first
grade also.
A comparison on this.basis with the original lines A, B, C indi-
cates that these new lines F, G, H, I, make a better showing,
especially on first grade rock and that all show decided reduction in
the third grade ground.
A B Cc F G H I
Rigsieorade LOCK. 2.05. 5666 2 bss « 40.4. 40.0). _AG.0: 6253626553, (02281). 50.1
DecOnMeerade TOCK ..c58 660 - Se BEFO.e ALS 10:7 227 03.6 242m 20uh aoc
EiteSh tated SECON! 152 ce gee eais cer 8523) (SIO) 165...7). 80.67 | S0.01460-0) 480.1
Mhindvemade rock sof esses es Tet Oe TO) ee ORES OnE. 30
Raker mia vac eee te oes faye ZEON Pat QB | WAKO KONO uae Own re I
1 Now known to be first grade.
On geological grounds, therefore, it is confidently believed that
any one of the new lines (F, G, H, I) would give decidedly better
results than any one of the original ones (A, B, C). The poor
and the questionable and the unknown ground can not be wholly
avoided by any possible line, no matter how roundabout. In these
lines, approximately as drawn, the objectionable points are reduced
to a minimum with almost no increase in total length of conduit.
The objectionable portions are also restricted in large part to the
8
228 NEW YORK STATE MUSEUM
Harlem river, where we already have the experience of the last
aqueduct (the New Croton aqueduct) as a guide, and a very few
other spots.
General conclusions
Line I is the shortest possible defensible line. Its chief objec-
tonable feature is a rather long stretch, 6600 feet of limestone,
from 135th street to Central Park, upon the quality of which there
are no data. It crosses the Harlem river fault probably in gneiss.
- But it crosses the extension of the Manhattanville fault in lime-
stone.
Lines F, G and H are almost equally defensible. Line G is
longest, but is in some respects — especially in following the ridge
crests — one of the best possible locations.
It should be appreciated that many other matters, such as
municipal works already completed or projected, or matters of
engineering practice, are likely to make it necessary to modify any
line proposed, and that the final line is more likely to be a com-
promise, considering all interests.
A graphic representation of the comparative merits of the pro-
posed lines is given in plate 33. This is strictly a geologic study.
The lines are properly placed on an outline map of the city corre-
sponding exactly to those drawn on the geologic map, plate 32.
The geologic formations that each would cut are represented on
longitudinal sections which follow each line, and the attitude and
‘structure of each formation are indicated.
Revised lines
Subsequently two revised lines based upon the preceding studies
were examined to determine preference. Later one of these, or a
slight modification of it, was adopted as the one to be explored.
It was soon determined on the same reasoning as was applied to
the first group of lines that the most westerly line — the line keep-
ing as much as possible within the gneiss and schist ridges —
would be the most likely to give satisfactory conditions. By this
method of selection the unknown or untested and doubtful ground
was reduced to its lowest limits. It was found that nearly all of
the very weak spots could be located by inspection in the northern
portion of the line, but south of 59th street the question is de-
cidedly more difficult because of the heavy drift cover. No rock
outcrops occur south of 30th street, and one is reduced to the
evidence of deep borings.
se)
SRE
%
pe ae ae LEM
- POET ST
meg heen
1a
es
~~
ae
“tf
= iby
) 7? ; rat RP, 7 :
SE sp na es rs i Pa
; i. he ‘vad | . on 4 . »
7 2 < _ - oes ae é
4 ws ’ as ' a
N. Y. State Museum Bulletin 146
Plate 33
—
y Grariic Geovocic Stupy
E .
cron SS a
ms “ 3
Arternative Lines Inwood Limestone Fordham Gneiss Yonkers Gneiss Ravenswood i l {
SOR aii! Grano-diorite pipe) SAN
SRE S
New York Ciry Ys SS ;
g Z =
Distaisution Conpuir
The attitude of the different forma-
“ions and their approximate amounts
are indicated by longitudinal sections
along the alternative lines whose 4 ance iP
courses are indicated in detail on the : cs aa SS SS SSS
accompanying geologic map, SSS
New York Bay
pive®
Pian of lines same as on
Geologie Mop
Original lines -A,B.C,D.E. in blu
New lines-F.G,H1. in orange:
This limestone can be avoided
by running farther South with
lines For G before turning to
= Wi
noe
DISTRIBUTION CONDUITS
ercioee stcrient ex provost Usts
2000 6 2000F
puntg relationship of the
7 folded formations
Huoson RIVER
Huoson River
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 229
Points for exploration north of 59th street
It was soon evident that extensive exploratory work would have
to be undertaken and the following points were selected at which to
begin. :
1 The Harlem river crossing, where the distribution conduit line
crosses the river just below High Bridge [see later description].
The only good evidence as to character of rock at this place is from
the pressure tunnel of the New Croton aqueduct which crosses the
river a short distance above.
2 The Manhattanville cross valley (125th street depression).
This is the most important cross depression on the island of Man-
hattan. It is apparent after a little investigation that the bed rock
floor lies deep and that if it were not for the drift filling the tides
would surge through this -valley making a direct connection between
the Hudson and East river. It was the least known as to depth
and character of any point along the proposed line.
3 The depression between Morningside and Central Park. At
that place limestone on the crest of a pitching anticline reaches
farther south than on either side and is more deeply eroded. The
other zones of large importance are in southern Manhattan the
geology of which is a special study.
CHAPTER XVIII
AREAL AND STRUCTURAL GEOLOGY SOUTH OF 59TH
STREET
The necessity for exploration in certain sections of this area can
not be appreciated without a statement of the local geology and
especially of the revision of both areal and structural geology that
the writer has based upon an exhaustive study of all the available
drill cores and other data to be found in southern Manhattan, East
river and Brooklyn. |
Below Central Park there is now little geology to be gathered
from a study of the present surface. But as far south as 3Ist
street the bed rock geology is pretty well known from earlier re-
ports and from recent improvements that have exposed the under-
lying rock. All of this portion is mapped as Manhattan schist ex-
cept one small area of serpentine at 59th street between Ioth and
11th avenues. There is no reason to modify this usage. A careful
study of a great number of rock borings from the Pennsylvania
Railroad tunnel across Manhattan at 32d street proves beyond
question that bed rock is Manhattan schist, including almost all
known variations and accompaniments, for the whole width of the
island along that line.
Still farther southward the points that have yielded exact in-
formation about bed rock are less numerous, and below 14th street
are confined to deep borings or an occasional very deep excavation
for foundations. Even these sources of information are lacking
over large areas. The greater number of borings available are
along the water front. Their distribution is such as to indicate that
the west side and central portion and southerly extremity of the
island are all underlain by Manhattan schist. This is true eastward
to the East river at 27th street, and as far eastward as Tompkins
square at 1oth street and almost to the Manhattan tower of Brook-
lyn bridge in that vicinity.
To the eastward of these limits, i. e. to the eastward of the line
projected from Blackwell’s Island to the Manhattan tower of
Brooklyn bridge, there is a more complicated geology. The borings
of the East river water front are decidedly variable. They are
certainly not all Manhattan schist of the usual types. Those most
unlike the Manhattan are at the same time most like some varieties
231
232 NEW YORK STATE MUSEUM
of the Fordham, and indicate that these formations both occur.
The lack of any data in the beginning of this investigation except
on the water front made it impossible to draw more than very gen-
eral lines. Drawn in this way, the lines of course are too straight,
but it is certain that they indicate more nearly the actual existing
areal distribution of formations than any of the maps now in exist-
ence.1 They indicate a southward extension of the Blackwell’s
Island belt of Fordham gneiss toward the Manhattan tower of
Brooklyn bridge. How much of this anticlinal fold of Fordham
actually brings this formation to the surface it is impossible to say,
but that it may be expected to be encountered along this line is
evident. |
On the east side a parallel belt of Inwood limestone is indi-
cated and this again is succeeded by a Fordham gneiss area
which occupies the rest of the eastern margin. Explorations
made along the line of the gas tunnel across East river at 72d
street? indicates comparatively narrow belts of limestone there
in both the east and west channels. The limited width of limestone
at these points, together with the occurrence of two strongly de-
veloped disintegration zones, seem to indicate rather extensive
squeezing out and faulting of this formation along fault planes
17In the summer of 1908 the writer was assigned the task of studying
in detail the evidences of geologic structure beneath the drift in southern
Manhattan. Before any drilling was attempted in the city by the Board
of Water Supply, a thorough canvass was made of all previous borings in
this district and the cores and records were personally inspected. More
than 300 such borings were found in which some of the core could be
secured for identification and classification as to formation and condition.
Most borings were given no weight at all in the final summary of this
evidence unless the rock core or at least fragments of it could be secured.
After all of these newly assembled data were tabulated and plotted on the
map, it was evident that if the identifications were correct the areal and
structural map of southern Manhattan needed extensive revision. A new
map therefore was made and presented to the chief engineer of the Board,
October 30, 1908. This has been used since as the basis for exploration of
the Lower East Side section. This original tabulation and map only
slightly modified was published under the Areal and Structural Geology of
Southern Manhattan Island [N. Y. Acad. Sci. Annals, April 1910, v. I9, no.
II, pt 2]. The extensive explorations of the board have made further revision
necessary [see accompanying map, pl. 34]. Exploratory boring is still in
progress (October 1910) and some slight modifications of boundary lines
may yet be made.
2 This is taken from Prof. J. F. Kemp’s description of The Geologic Sec-
tion of the East River at Seventieth Street, New York [N. Y. Acad. Sci.
Trans. 1895. 14:273-76].
; eels
5 ey > Leg.
Y FEZ : > MRL Z i © Ree aes eget ee oe
ERE» yf oy s\) y 7 A “:
Gy,
@
Wy, Mi
46
m2 SS
way >:
Base map reproduced from a copyrighted map by B. Belcher Hyde, 5 Beekman street, and here used by permission
N. Y. State Museum Bulletin ;
aeuaee
Lt
No reurrcs,
i
46 2 eo >
= = ? >0
wet bas wt Ss 3s
ZzEs ae ze Sing SE
Q Eos zs os 52 a= Os
} 222 O® Ses jt" Ps
Zz fw 8 os a ose? ae
o 3 we £08 ox
ira) epee ZE == uso8 Vi
wes NE 52 ageae we
© 1°38), | = = = : 2230 Gus
LJ Zs KS as 3 WS fe Yeates
=i ef <° o> 15 fl Y=" s,
xES xr? >. aD Dos 1\-“S: wl oi fa” psu] als 5 his QS -200
| Ay zy y i He n il Col tecg Ne H ® SN
| ed aS ; Re Bes | | | id (ees i PRRs ai -300
iv | 8 RR By BS OMA ow GH HS GSR IMENT RR, SU
—400 Manhattan i iB S$ Inwood Dail See is He wf Ns iol # ie -400
/ | as cy 7 Te + hla de a oi bye OO
Schist | aN Limestone & “ah ld oF v ll. nN Aral Hil i | x
— 400 H aes S HH SiH ‘ Hy & in 9 i 2 ; i ‘ fy - 500
3 : A ae aT i ak i
—600 Inuroed Limestone I! d i i ql Se os 6 ft 2 HI i 3 “fl Fordham Gnevss Serves -600
LH = 2 P ii A mostly granodiorite
700 Fordham Gneiss Serves [' I: i z ou sll i I i -700
fi with oecasiona/ ‘haa = a Ve
limestone be wy i
— 800 BS bed ) Fordham Gneiss Series with many narrow Interbedded Limestones we -GOO
— 900 -900 City of New York
-/000 000 BOARD OF WATER SUPPLY
-//00 400) CATSKILL AQUEDU CT
Shisp= S = GEOLOGIC DETAIL OF LOWER EAST SIDE
: } fF < QYfE=
Deore): Pp oftle line 1's intended to Ma ZS. = N Lippe are prosected parallel lo Sike
mark the fim) -ock decay as interpreted = y W; hole*Lé ald (Ole 3
Vane iemg?! of rock aecay age Kecovery OVEl 25 fe Shown this Goronmate bearirg of strike 1s NLEL 200
from borings Each proms ment depiess/oi Kecovery Under , 2» 9 | Y) 200 0 eet
F OCTOBER 18, 1910
probably tncheates a cresh 2078 belonging No fecovery a7) ry) i
to a fault.
| j
CHAPTER XX
THE GENERAL QUESTION OF POSTGLACIAL FAULTING
WITH ITS BEARING ON THE PERMANENCE OF ENGINEERING STRUC-
TURES.
Evidences of postglacial faulting and other recent movements
have of late attracted a good deal of attention. The experience of
San Francisco in the exceptionally disastrous earthquake and fire,
traceable directly to earth movements of the nature of faulting
which dislocated or injured the water conduits rendering them use-
less, is fresh in the minds of men everywhere who have public
responsibilities of this kind. If displacements are occurring at
the present time, or if any related movements are continuing, or if
there is evidence of recent disturbances of this sort in this region,
they have a decidedly important bearing upon the permanence of
all engineering structures that cross them.
No undertaking is more vitally concerned with this question than
the Catskill aqueduct. Although the principal factors to be taken
into account have been considered in other connections. [see
“Faults”? and “ Folds,’ pt 1] a unified statement may encourage
a more intelligent understanding of the bearing of these structures
in southeastern New York on this specific question.
The region included in this discussion extends from the Catskill
mountains to New York city. It will be convenient, for the pur-
poses of this argument, to divide the whole area into three districts
whose boundaries are determined by decided differences in com-
plexity of geologic history. These lines necessarily follow closely
the boundaries of greater stratigraphic unconformities. The
youngest groups of strata have suffered only such changes as have
accompanied movements of later geologic periods. But before they
were formed the underlying groups of rocks were just as pro-
founaly affected by earlier disturbances. In this region, at least,
three such groups of large importance exist. The oldest or lowest
has been affected by riot only everything that has influenced the
younger strata but by disturbances of a still earlier time which very
much increase their complexity.
On this basis it is convenient to think of the three districts as
follows:
A Catskill district. Including that portion of the region west
and northwest of the Shawangunk mountains and marked by the
271
is)
os)
7 NEW YORK STATE MUSEUM
prevalence of Siluric and Devonic strata, i. e. all strata above the
Kudson River slates. These strata have been affected by only one
great mountain-making movement — that of the Appalachian fold-
ing, and minor disturbances of still later date.
B Hudson river district. This includes that portion of the
region lying between the northern border of the Highlands and the
hawangunk mountains. It is marked by the prevalence of Cam-
ric and Ordovicic strata, i. e. Hudson River slates, associated with
‘appinger limestone and Poughquag quartzite as the chief bed
rock. These strata have been affected not only by the Appalachian
folding but also by a still earlier one — that of the Green mountains
and the Taconic range. They were folded mto mountain ranges
and worn down in part again before the Siluric and Devonic strata
oi district 4 were im existence. Therefore as a structural problem
this district (B) is approximately twice as complex as the other.
C Highlands district. This includes all of the region com-
monly known as the Highlands of the Hudson as well as the rest
of the area south of the Highlands proper to New York city. Its
rocks are the oldest —- much the oldest. They had been folded into
mountain structures and in part worn down before any of the
others were accumulated. They have also suffered extensive
igneous intrusion so that im places these igneous types prevail.
And besides they have been metamorphosed far beyond the pomt
of any other group. No other series of strata has been so pro-
foundly affected. They form the lowest group. All thimgs con-
sidered this district should be structurally three times as compli-
cated at the first one (4), and adding the igneous and metamorphic
complexities, it is probably more near the truth to consider this
Highland district’ four or five times more complex.
All of the formations from-the oldest to the Middle Devonic are
involved. For the specific formations and their succession and rela-
tion the reader is referred to that discussion in part 1 [see p. 29,
et. seq.|-
|p if)
=
Structural features
Except the most westerly part of the region, that occupied by the
Upper Devonic strata, all formations are compressed into iolds.
Many of the smaller folds, especially those in the Catskill district,
are still complete. The easy subdivision of strata possible in this
district also simplifies the problem of detecting small changes of
altitude. But for the most part the larger folds have been beveled
=,0f extensively by surface erosion so that only the truncated limbs
GEOLOGY OF THE NEW YORK CITY AQUEDUCT 273
are now to be seen, and the strata therefore appear as narrow belts
that dip steeply into the ground. This is more marked in the
Hudson river district than in the Catskill, and 1s still more strikingly
true of the Highlands.
There are evidently at least three different epochs of folding inter-
rupting the processes of sedimentation and followed by periods of
erosion before sedimentation was again resumed. These breaks
constitute so called stratigraphic unconformities and occupy the
relative positions indicated in the foregoing tabulated scheme [see
ptr).
In each epoch of folding the compressive forces accomplishing
this work. seem to have acted in a southeast-northwest direction
causing successive series of folds with a northeast-southwest trend.
The total amount of crustal shortening accompanying these move-
ments is not known, but that it must be many miles is indicated by
the fact that the strata of the older series of formations stand pre-
vailingly on edge. All stages between small amount of movement
to very great displacement are represented.
Accompanying the folding in each epoch there has been a ten-
dency to rupture and displacement of the “fault” type. There are
multitudes of them varying from movements of too little amount to
be regarded in a broad way to those of several hundred feet. Most
of the larger and more persistent ones are strike faults and follow.
the main ridges or valleys, sometimes governing the location of
escarpments or gorges. Dip faults crossing the formations also
occur and doubtless have guided the adjustments of many tributary
streams, and occasionally portions of the larger water courses. The
thrust fault is most common. This is especially true of the larger
ones and particularly those parallel to the trend of the other struc-
tural features.
The chief effects of these movements may be summarized as
follows:
1 Formations are cut out of their normal order and nonadjacent
ones are brought in contact.
2 Cliffs (escarpments) and sharp gulches are more common.
3 Crush zones (belts of brecciated material) are developed.
4 The crush zones give an additional control of stream adjust-
ments.
All of these effects are common. Many of those faults dating
back to the earlier epochs are obscure and not readily located. Many
of the older weaknesses of this sort have been healed by recrystalli-
274 NEW YORK STATE MUSEUM
zation so that they are now as sound as any other portion of the
rock. A good deal depends upon the type of rock and the conditions
under which the movement took place. In some of the more open
ones, circulating water has seriously affected the rock and in places
there 1s extensive decay even in the harder crystalline formations.
Age of the faulting. The chief epochs of folding and faulting
are those of the mountain-making movements — one Precambric,
another Postordovicic, and still another Postcarbonic. All of these
date very far back in geologic history, and since the last of these,
othing akin to them in importance has been felt in the region.
In Posttriassic times however there was small faulting south of
the Highlands, that affected the areas of Triassic rocks of New
Jersey and Connecticut.
Whether or not there continued to be slight movement along some
of the older lines it is now impossible to say. It is at least clear that
all of the great movements belong to very ancient time, and that the
last period of geologic time as we know it for this region, has been
one of comparative stability. The chief exception is evidently con-
nected with the continental elevations and depressions of the
glacial epoch.
Recent movements. The effects of glaciation make it possible
to determine whether or not there has been further movement in
postglacial time. Conditions are not everywhere favorable enough
to detect minute changes, but where they do obtain, the evidence is
capable of very definite interpretation. The essential features of
these conditions are
1 A bed rock ledge that has been left well smoothed by glacial
scouring.
2 Protection from postglacial destruction so that the original
unevenness as left by the glacial smoothing can not be mistaken.
If on such a ledge, as now exposed, there are steplike offsets or
minute escarpments that could not have remained had they been
present during the ice action, then there must have been displace-
ment to this extent, since the original smoothing took place.
A few such evidences have been found in New York and New
England, and have been noted in geologic reports. W. W. Mather
in his report on the First District of New York (1843) pages 156-57,
was the first. The data as now known may be found in the last
bulletin of Geologic Papers of the New York State Survey [see
N. Y. State Mus. Bul. 107 (1907) p. 5-28]. The following para-
GEOLOGY OF THE NEW YORK ‘CITY AQUEDUCT 275
graphs are intended as a brief summary and comment on the facts
as there given: |
Localities where some postglacial displacement has been
detected.
1 Copake, N. Y., on the eastern border of the State near the
southwest corner of Massachusetts
2 Rensselaer, N. Y.
Sy oouth, lino, Nai.
4 Defreestville, N. Y. (near Troy)
5 Pumpkin Hollow, N. Y. (near Copake)
6 Kilburn Crag, N. H.
fF Port Kent N: ¥> (incerta)
8 Attleboro, Mass.
In addition to these there is reference to similar occurrences at
St John, N. B. and in the province of Quebec. All of the known
localities lie a considerable distance beyond, north and northeast, of
the Catskill aqueduct line.
Causes of displacement. In southern New York all of the
cases of postglacial faulting yet discovered lie in the area of slates
belonging to the Hudson River series. Whether the belt now occu-
pied by this formation is therefore to be considered the most un-
stable zone, or whether there is some tendency to slight readjust-
ment inherent in the slates themselves causing these movements, is
not clear. It would seem consistent with known recent geologic
history to connect these displacements with the general elevation
and subsidences accompanying and following the glacial occupation.
It is perfectly clear that the whole continental border in this region
suffered considerable subsidence during glacial time. Also the ter-
races and deposits along the Hudson prove beyond question that
during the ice retreat, at the very close of the glacial occupation,
the land surface stood from 80 to 150 feet lower than now. There-
fore an elevation of this amount has occurred in postglacial time,
and probably, judging from the condition of the terraces themselves,
took place soon after the glacial ice withdrew.
The stresses and inevitable warpings accompanying these mass
movements seem to be sufficient to account for all displacements
known to be of this age. There is nothing in them that necessarily
promises a renewal of mountain folding. But it appears that the
movements have almost all been of the thrust character and in this
respect they differ not at all from the commoner type of the region.
276 NEW YORK STATE MUSEUM
Amount of displacement. he greatest throw noted on any
single Postglacial fault in eastern New York is given by Wood-
worth as 6 inches, and he remarks that this 1s imperfectly shown.
Usually the displacement is distributed over a zone in which several
small faults occur instead of a single larger one. This may mean
that the whole disturbance is essentially superficial.
At South Troy it is stated that a total displacement of 12 inches
is thus distributed through a number of small fauits within a dis-
tance of 30 feet.
At Rensselaer a total of 5 inches is given.
At Defreestville a total of 13 inches is indicated in a distance
Of Teeverec:
At Copake, at two different spots, a total of more than 7 ches
was measured within a space of 12 feet. Woodworth thinks that
the total displacement for the locality may exceed 2 feet.
At Pumpkin Hollow a total of 17 inches is estimated.
Conclusion. If such rates prevail over larger areas beneath
the drift, it is clear that rather profound changes would be indi-
cated. But thus far there is no indication of such continuity.
Likewise if it were certain that the movements are now in
progress, it would be a matter of greater concern. But there is
no direct evidence to prove it. |
Estimates of the length of postglacial time differ greatly. The
shortest ones worthy of consideration range from about 5000 to
10,000; the longest run above 100,000 years.
Some intermediate value is probably nearer the truth — say
25,000 years.
Adjusting the postglacial faulting problem then to these time
estimates the summary of it all would be as follows: Somewhere
within postglacial time, i. e. approximately 25,000 years, move-
ments of strata have developed at a few places in eastern New
York that appear as small faults with total throw in each locality
varying from a few inches to perhaps as much as 2 feet. Whether
the movement has been gradual and continuous or concentrated
largely into some small portion of this time is not known. Whether
the effects are extensive or, on the contrary, very local and super-
ficial, is likewise unknown. But in any case there are no known
instances of violent and large displacements, such as would be
likely to cause great damage to sound structures, in this region in
postglacial time.
LED EX
Appalachian mountain-folding,
66, 73.
Aqueduct, see Catskill aqueduct.
Arden point, 97, 104.
Arden point line, 85.
Artesian flows, 142.
Ashokan dam, construction of, 13;
elevation of reservoir, I7; stone
used in construction, 38; geologi-
cal features involved in selection of
site for, 109-16; location map, 113;
Olive Bridge preferable location,
116; to be finished first, 183.
\spidocrinus scutelliformis, 42.
Athyris spiriferoides, 38.
Atrypa reticularis, 40, 43.
spinosa, 40.
Atwood, T. C., acknowledgments to,
7; division engineer, 237.
63,
Beaver Kill, 110.
Becraft limestone, 42, 55, 126.
Bensel, John A., member of Board
of Water Supply, 13.
Berkey, Charles P., consulting geolo-
gist, 6, 19, 75; cited, 48.
Binnewater sandstone, 44, 55,
133, 134, 140; porosity, 135.
Bluestone, character and _ quality,
117-23; economic features, I10;
petrography, I19Q—23.
Borings on the lower east side, tabu-
lations and interpretations, 254-65.
Breakneck ridge, 85, 91-95, 100, 163;
quality and condition of rock, 106-
a
Brink, Lawrence C., acknowledg-
ments to, 6; division engineer, 21,
149, ISI.
Bronx valley, geologic cross section,
193.
Brown, Thomas C.,
Esopus division, 125;
on limestone rocks, 140.
126,
employed on
observaticn
o--
ae
Brush, William W., acknowledg-
ments to, 6; division engineer, 14,
Ba OTS:
Bryn Mawr, geologic section at, 205;
comparison with shaft 13, 212.
Bryn Mawr siphon, 201-8.
Bull mountain, 163.
Calyx drill, 26.
Cambric quartzite, 102.
Cambro-Ordovicic formations, 45-46,
50; 108:
Carson, J. -P*-cited)-200:
Cat Hill gneissoid granite, 52, 57.
Catskill aqueduct, water supply pro-
ject, 90-16; generalities of construc-
tion, 14-15; estimation of cost, 15;
present plans for, 15; time for
completion, 15; problems en-
countered in the project, 17-24;
gathering data, 21-23; relative val-
ues of different sources of infor-
mation and stages of development,
25-28; geologic problems, 75-276;
general position of aqueduct line,
77-80; location map, 8o.
Catskill creek, IT.
Catskill district, general geology, 2
74; of simple structure, 31; post-
glacial faulting, 271-72.
Catskill. formation, 47% 55, 63.
Catskill Monadnock group, 73.
Catskill supply, area in square miles,
11; daily supply in gallons, 11;
estimated daily supply, 11; esti-
mated cost, II; storage in gallons,
II; part of supply available by
TOTS. BS.
Catskill system, parts of, II-14; con-
struction of certain parts in ad-
vance of the rest, 13.
Catskill watersheds and aqueduct,
ATG TA
Caves, 137.
Cedar Cliff, 103.
Cement beds, 44.
Cenozoic time, 64.
Chadwick, Charles N., member of
Board of Water Supply, 13.
Chonetes coronatus, 38.
mucronatus, 38.
Chop drill, 26.
Clapp, Sidney,
109.
Clark, cited, 44.
Clays, III.
Coastal plain, 73.
Cobleskill beds, 44, 55, 126.
Coeymans limestone, 43, 55, 126, 133.
Cold Spring, 85, 163.
Conglomerates, better quality of wall
than limestones, 140.
Continental elevation, 67-60.
Cortlandt series of gabbro-diorites,
assistant engineer,
cre ee
52, 53, 5/-
Coxing kill, 127, 128.
Coxing kill section, 135-36; struc-
tural geologic detail, 136.
Cretaceous deposits, 36-37, 54.
Cretaceous peneplain, 67.
Cronomer hill, 154.
Crosby, W. O., consulting geologist,
6, 19, 75; cited, 36.
Cross sections, Rondout valley, 140.
Croton aqueduct, study oi shait 13
and vicinity, 200-14; comparison
of Bryn Mawr and shait 13, 212-
14; map showing location, 239.
Croton lake crossing, 183-89.
Croton river, average daily fiow, 9.
Croton water, average daily consump-
tion, 9.
Crows Nest, 100, 163.
Crystallines, south of the Highlands,
47, 50; older, 57-
Dalmanella testudinaria, 46.
Dalmanites selenurus, 40.
Dana, J. D., cited, 48; mentioned, 46.
Danskammer crossing, 85, 97, 103.
Darton, mentioned, 46.
Davis, Carlton E., acknowledgments
to, 6; department engineer, 13.
78 NEW YORK STATE MUSEUM
Davis, J. L., tests of Kensico rocks,
199.
Delancey and Clinton street section,
structural geology, 253-66.
Delivery conduits, geological condi-
tions affecting the location of con-
duits, 215-29.
Devonic strata, 37-43, 55-
Diabase, 37, 240.
Diamond drill, 26.
Dikes, 106; pegmatite, 52.
Dinnan quarry, 198.
Division engineers, responsibility oi,
=
Driit, kinds of, 33-36; glacial, 32-36, ©
54, 100, 202.
Dwight, mentioned, 46.
East river crossing, 233, 238.
East river section, 250-53; structure,
233-
Engineers, division, responsibility oi,
=a fh
Esopus creek, 11, 69, 77, 112, 128.
Esopus division of Northern Aque-
duct Department, 125.
Esopus shale, 40, 55, 126, 140; thick-
ness, 133.
| Esopus valley, geologic cross section,
78.
Esopus
P45,
Exploration zones, special, 237-70.
watershed, development of,
Fault zones, 206.
Faults, 60-62, 135, 163; postglacial,
general question of, 271-76.
Favosites helderbergia, 43.
Ferris quarry, 198.
Firth Cliffe, 153.
Flinn, Alfred D., acknowledgments
to, 6; department engineer, 13, 215.
Folds, 59-60, 272.
Fordham gneiss, 47, 52, 57, 62, 185,
IQI, 192, 202, 206, 217, 218, 219,
220, 221, 225, 220,, 232, 233, 234,
237, 238, 255, 257, 258, 260, 261,
262, 264, 265, 266, 268.
INDEX TO GEOLOGY OF THE NEW YORK CITY AQUEDUCT 274
Formations, summary of, 54-57.
Foundry brook, 27; rock condition
at, 163-60.
Foundry brook valley, structural de-
taude TOS.
Garden quarry, 198, 199.
Geographic features, 30-31.
Geographic history, 65-74.
Geologic conditions affecting the
Hudson river crossing, 97-107.
Geologic knowledge, practical appli-
cation to engineering plans, 19.
Geologic problems of the aqueduct,
75-276. .
Geology of region, 29-74; summary
of formations, 54-57; outline of
history, 62-65; local summary, 2605—
66. : |
Glacial drift, 32-36, 54, 100, 202.
Glacial period, 64, 71.
Gneisses, 176; dioritic, 198, 199;
Grenville series, 50-52, 57. See
also Highland gneiss.
Grabau, A. W., cited, 37, 44.
Granites, 99, I00, 100; gneissoid,
198; of the new Ferris quarry, 108.
Grassy Sprain valley, 203.
Gravel, IIo.
Gravel hillocks, 110.
Gravel streaks, I1I-12.
Grenville series, 50-52, 57, 62.
Hamilton shales, 38, 55, 78, 119, 126,
140.
Harbor hill moraine, 35.
Harlem river crossing, 237, 238-44;
map showing plan of exploratory
borings, 239.
Hartnagel, cited, 44; mentioned, 154.
Healey, John R., acknowledgments to,
6; exploratory work by, 237.
Henry street, interpretation of hole
No. 207 on, 261-65.
Hester street, interpretation of hole
No. 202 on, 259-61.
High Falls, 125, 127, 133.
High Falls shale, 44, 55, 126, 133, 134.
135; porosity, 135.
Highlands, 30-31, 73, 81; crystal-
lines south of, 47, 56; postglacial
faulting, 272.
Highlands gneiss, 50, 57, 99, 102, 154,
163. Sce also Fordham gneiss.
Highlands group, crossings, 97, 103-
4, 105; more defensible as a route
for the aqueduct line, 103.
Hill View reservoir, 215; elevation,
V7.
Hipparionyx proximus, 41.
Hobbs, mentioned, 95.
Hogan, > Thomas! Ey
vision engineer, I25.
Hornblendic gneiss, 263.
Hudson river, 60; water to be used
for fire protection, 10; wash bor-
ings, 26; depth of buried channel,
89; submarine channel, 90-01;
Storm King-Breakneck mountain
profile, 91-95; origin of the present
course, 95-96; crossing, geological
conditions affecting, 97-107; out-
line map showing possible cross-
ings, 98; difference of structure
in crossings, 104; postglacial fault-
ing of district, 272.
Hudson river canyon, 81-96; points
of exploration, 83-88; comparative
sections at Peggs point and Storm
King, 92; study of profile, 94.
Hudson River slates, 46, 56, 83, 100,
102; 103, :120,, 135,137, E40, emer
WS Suen 27.
Hudson schist, 201.
Hurley, 127.
assistant di-
Idlewild, 154.
Igneous types, 52-54.
Imperviousness and insolubility, 13&-
39.
Inwood limestone, 47, 49-50, 56, 172,
185, 191, 192, 201, 202; 210; 21a
217; 218, (210, -220; 227) 226. aan
237, 238, 240, 242, 243, 245, 246,
ZAG. (254) SEGe 250. 201, 202. aoe.
268, 269.
280
Ithaca beds, 38, 55.
Jameco gravels, 39.
Jerome Park reservoir, interbedded
development of limestones in vicin-
ity of, 268.
Jura-Trias formations, 37, 54.
Kemp, James F., acknowledgments
to, 6; consulting geologist, 6, 19,
75 3 Cited) ele. 232:
Kensico dam site, geology of, I9I-
94.
Kensico quarries, stone of, 195-200;
additional tests, 190.
Kensico reservoir, to be enlarged,
132
Kripplebush, 127.
Kripplebush section, 129-31.
Laminated sand and clay, III.
Laminated till, IIo.
Langthorn, J. S., acknowledgments
to, 7; division engineer, 2I, 109.
Leperditia alta, 44.
Leptaena rhomboidalis, 40, 42.
Leptocoelia acutiplicata, 40.
Leptostrophia magnifica, 41.
perplana, 40.
Libertyville, 149, 150.
Limestones, 99, 100, 176; resistance
to solution, 139; analysis of, 139;
of Sprout Brook valley, 171; in-
terbedded, older than the Inwood,
266-70.
Liorhynchus mysia, 38.
Little Stony point, 85, 97, 104.
Location map, 80.
Long Island, Cretaceous and Ter-
tiary strata, 32; glacial deposits,
35-36.
Lower East Side zone, 238.
Lowerre quartzite, 47, 50, 56.
McCarthy, C. H., boring equipment
owned and operated by, 141.
Manhattan schist, 47, 48-49, 56, 171,
Teas. TOO) MGOl, MOS) ZOl. eigeeeno:
NEW YORK STATE
———
MUSEUM
219, 220,,, 221, 1225, .220, 233, 235
237, 238, 240, 241, 242, 243, 244,
245, 246, 247, 248, 240, 254, 257,
261, 265, 268, 2609.
Manhattanville cross valley, 237, 244-
45.
Manlius limestone, 43-44, 55, 126,
133, 124.
Marcellus shales, 38, 55, 126.
Matawan beds, 37, 54.
Mather, W. W., cited, 268, 274.
Merrill, cited, 48.
Merriman, Thaddeus, acknowledg-
ments to, 6; assistant chief engi-
MEET, is) TOG,
Mesozoic time, 64.
Miocene deposits, 36.
Miocene fluffy sand, 54.
Moodna creek, 103-4; wash borings,
265 (course Of155—50:
Moodna valley, ancient, 153-02; sta-
tistics, 160.
Morningside to Central Park sec-
tion, 245-50.
Mountain-forming
60.
movements,
59-
New Ferris quarry, granite, 108.
New Hamburg, 81.
New Hamburg group, crossings, 97,
102—3, 105.
New Hamburg line, 83-85.
New Paltz, 140.
ew Scotland shaly limestone, 42,
lope UAL oe
New York-Wesichester district, 30.
New York city, gorge at, 91; sec-
tions of gorge at 32d street, 92;
geological conditions affecting the
location of delivery conduits, 215-
29; areal and structural geology
south of soth street, 231-36; struc-
tural geology of the lower East
side, 253-60.
Newark series, 37, 54.
Newburgh, 154.
Northern aqueduct
first, 183.
2
to be finished
INDEX TO GEOLOGY OF THE NEW YORK CITY AQUEDUCT 281
Oil-well rig, 26.
Olive Bridge, 110; site, 112-14; pref-
erable location for the proposed
Ashokan dam, I16.
Oneonta formation, 38, 55, I19.
Onondaga limestone, 39-40, 55, 78,
T20, 127, 120.
Oriskany beds, 40, 55, 120.
Orthothetes woolworthanus, 42.
Pagenstechers gorge, 154, 155, 159.
Paleozoic time, 63.
Palisade diabase, 37, 54.
Pebble beds, 111-12.
Peekskill creek, 172:
Peekskill creek valley, structure of,
175-82; geologic cross section and
detail of borings, 180.
Peekskill granite, 52, 53, 57.
Peggs point, borings, 89; gorge at,
QI, 92.
Peggs point, crossing, 83, 97, 103.
Pegmatite, 53, 57, 185; dikes, 52.
Peneplain, Cretaceous, 67.
Pennsylvania borings opposite 33d
street, New York city, 80.
Phyllite, 175-76, 181.
Physiography, 30-31, 65-74.
Piedmont belt, 73.
Platyceras dumosum, 40.
nodosum, 4I.
Pleistocene glaciation, 71.
Pliocene deposits, 36, 54.
Pompey’s cave, 137-38.
Porosity tests, 142-47.
Porosity of Kensico rocks, 199.
Port Ewen beds, 40, 42, 55, 120.
Postglacial faulting, general ques-
tion of, 271-70:
Poughquag quartzite, 47, 56, 100, 172,
£70, Tote72)
Pressure tests, 27.
Pumping experiments, 142-47.
Quartz, 202.
Quartzite beds, 90, 100, 176.
Quaternary deposits, 32-36, 54.
Raritan formation, 37, 54.
Ravenswood granodiorite, 52, 53,
Big 2204 220.) 220° 233." B52
260.
Rhipidomella oblata, 42.
Ridgway, Robert, acknowledgments
to, 6; department engineer, 13.
Rondout cement, 44.
Rondout creek, 11, 69.
Rondout creek section, 131-35.
Rondout siphon statistics, 141-42.
Rondout valley section, 125-47; en-
gineering problems, 17-19; geology,
31; special features, 137-40; analy-
sis of limestones, 139; cross sec-
tions, I40.
Ronkonkoma moraine, 35.
Rosendale cement, 44, 45.
Rosendale limestone, 126.
57;
205,
St Nicholas Park, 246.
Sanborn, James F., acknowledg-
ments to, 6; division engineer, 21,
125, TAO;
Sands 110} sii
Sandstones, 100.
Saw Mill valley, 200.
Schistose beds, 99.
Schoharie creek, II.
Schoharie shale, 40, 55.
Sedimentation structures, 58.
Shales, better quality of wall than
limestones, 140.
Shaw, Charles A., member of Board
of Water Supply, 13.
Shawangunk conglomerate, 45, 55,
627-126, SIZ 7) 143) 43h i364 140r
thickness, 136; overthrust, 137.
Shawangunk mountains, 31, 127.
Sherburne beds, 38, 55, 100, 119.
Shot drill, 26.
Sieberella galeata, 43; figures, 43.
pseudogaleata, 42; figures, 42.
Siluric. Stkatay 49-4550 55:
Sing Sing marble, 201.
Siphon line, total borings on, 14t.
Skunnemunk mountain, 153, 154.
to
CO
bo
Smith, J. Waldo, credit due, 5; chief
engineer, 13.
Smith, Merritt H., acknowledgments
to, 6; deputy chief engineer, 13;
department engineer, 14, 183.
Smith, Wilson F., acknowledgments
to, 7; division engineer, 21, I9QI.
Snake hill, 153.
Solubility, question of, 138.
Southern aqueduct, general location
map, 184; terminus, 215.
Southern aqueduct department, 183.
Spear, Walter E., acknowledgments
to, 6; department engineer, 14, 215.
Special exploration zones, 237-70.
Spencer, J. W., cited, 9o.
Spirifer arenosus, 41; figures, 4r.
concinnus, 42.
macropleura, 42; figures, 43.
mucronatus, 38; figures, 39.
murchisoni, 41.
perlamellosus, 42.
Sprague & Henwood, boring equip-
ment owned and operated by, I4I.
Sprain brook, 203.
Springtown, 149, 150.
Sproul, A. A., acknowledgments . to,
6; division engineer, 21, 172, 175.
Sprout brook, 175, 177; geology, 17I-
74; geologic cross section, 173.
Staten Island, Cretaceous and Ter-
tiary strata, 32.
Stockbridge dolomite, 201, 212.
Stony Point, 177.
Storm King crossing, 85, 9I-95, 97,
104, 105; trial profile, 94.
Storm King gorge, 89, 92, 156.
Storm King granite, 52, 57, 100, 104;
quality and condition of rock,
106-7.
Storm King mountain, fault along
the southeast face of, 163.
Stratigraphy, 31-57.
Strophalosia truncata, 38.
Stropheodonta becki, 42.
Strophonella headleyana, 42.
Strophostylus expansus, 4I.
Structural features, 58-62.
Styliolina fissurella, 38.
NEW YORK STATE MUSEUM
Surface configuration, history of, 66.
Swift, William E., acknowledgments
to, 6; division engineer, 21, 83, 163.
Taonurus caudagalli, 4o.
Tertiary deposits, 36-37, 54.
Tertiary incomplete peneplanation,
69-70.
Tertiary reelevation, 70-71.
Thirlmere aqueduct of the Man-
chester, England, Waterworks, 138.
Thirlmere limestone, average of five
analyses, 139.
Tibbit brook valley, 203.
Till, 110.
Tompkins Cove, 177.
Tongore site, 114-16; plan and geo-
logic section, 114; detail of drift
character, II5.
Topographic features, 30-31.
Tuckahoe marble, 2or.
Tuff crossing, 83.
Uncinulus campbellanus, 42.
Unconformities, 58-59.
Valhalla, ror.
Van Ingen, cited, 44.
Veatch, cited, 35, 36.
Wallkill river, 60, 128.
Wallkill valley section, 149-51; drift
conditions, 25.
Wallkill-Newburgh district, 31.
Wappinger limestone, 46, 56, 83, 100,
102, 354. 172, 270,. Tole 2c
Wash rig, 25, 81.
Water, increase in consumption, 9;
reports on available sources of
supply, 10. See also Catskill sup-
ply.
Water Supply Board, staff, acknowl-
edgments to, 7; members, 13; de-
partments, 13-I4.
Wegman, cited, 209.
West Hurley, 77, 78.
INDEX TO GEOLOGY OF THE NEW YORK CITY AQUEDUCT 283
West Point location, 104. Woodlawn, cited, 209.
West Shokan, III. Woodworth, mentioned, 276.
White, Lazarus, acknowledgments
to, 6; division engineer, 21, 125, | Yonkers gneiss, 191, 196, 197, 198,
142. 202.257. BIG 210, 220, 221. 2a
Wilbur limestone, 44. 226; of superior durability, 200.
Winsor, Frank E., acknowledgments
to, 6; department engineer, 14, 183. | Zaphrentis prolifica, 40.
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