(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Geologic guide to the Merced Canyon and Yosemite Valley, California : with road logs from Hayward through Yosemite Valley, via Tracy, Patterson, Turlock and Merced Falls"

iiliiiiiiii^ 

■'ill 







1 



LIBRARY 

UNIVERSITY OF CALIFOKSIA. 



QeoJjoc^ Qiddoe 



MenceJt CoA^dfXUt 







1962 




Caiifornk Division of i^ines and Geoiogy 

Bulletin 182 



GEOLOGIC GUIDE TO THE MERCED CANYON 
AND YOSEMITE VALLEY, CALIFORNIA 

With Road Logs From Hayward Through Yosemite Valley, Via Tracy, 
Patterson, Turlock, and Merced Falls 



Bullefin 182 

CALIFORNIA DIVISION OF MINES AND GEOLOGY 

FERRY BUILDING, SAN FRANCISCO, 1962 



LIBRARY 
OWIVERSITY OF CALIFORltlA 

DAVIS ■ 



STATE OF CALIFORNIA 
EDMUND G. BROWN, Governor 

THE RESOURCES AGENCY 
WILLIAM E. WARNE, Adminisfraior 

DEPARTMENT OF CONSERVATION 
DeWITT NELSON, Direcfor 

DIVISION OF MINES AND GEOLOGY 

IAN CAMPBELL, Sfate Geologisf 

BULLETIN 182 

Price $1.50 



Prepared for the annual mealing of the 
AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS 

and the 

SOCIETY OF ECONOMIC PALEONTOLOGISTS AND MINERALOGISTS 

in conjunction with the 

PACIFIC SECTIONS OF AAPG-SEPM-SEG 

at 

SAN FRANCISCO, CALIFORNIA 

March 26-29, 1962 

GORDON B. OAKESHOTT, General Chairman 

NOEL H. STEARN, Generai Vice-Chairmon 

WILLIAM F. BARBAT, Vice-Chairmon for SEPM 

CONTRIBUTING AGENCIES 

California Division of Mines and Geology 

United States Geological Survey 

University of California 

CONTRIBUTING AUTHORS 

Rodney J. Arkley Frederic R. Kelley 

Frank C. Calkins Dallas L. Peck 

Lorin D. Clark Holly C. Wagner 

Clyde Wahrhaftig 

FIELD TRIP COMMITTEE 

PARKE D. SNAVELY, JR., Chairman JACK E. SCHOELLHAMER, ViccChoirmon 

YOSEMITE VALLEY TRIP 

Frank C. Colkins Max B. Payne 

Lorin D. Clark Dollos L. Peck 

Frederic R. Kelley Holly C. Wagner 

Clyde Wahrhaftig 

TRANSPORTATION 

HOMER J. STEINY, Chairman 

KARL ARLETH, Vice-Choirmon 

Harold A. Allsup Ashley Holston 

C. M. Carson Fredi ric R. Kelley 

Marie Clark Harvey Lee 

Irvin Frozier Henry Neal 

Pretz Hertel Harold Rader 
K. A. Wright 



CONTENTS 

Page 

Letter of transmittal -. _ _ 6 

Preface _ ■. _ 7 

Part I— Geologic guide to the Merced Canyon and Yosemite \'alie>', Cali- 
fornia 11 

Suinmarv of the pre-Tertiary geology- of the w estern Sierra Nevada meta- 
morphic belt, California, by Lorin D. Clark 15 

Granitic rocks of the Yosemite Valley area, California, by Frank C. Calkins 
and Dallas L. Peck 17 

The geology, geomorphology, and soils of the San Joaquin \'alley in the 
vicinity of the Merced River, California, b\' Rodney J. Arkley 25 

Geomorphology of the Yosemite Valley region, California, by Clyde Wahr- 
haftig ■ _.._.l.....' 33 



Part II— Road logs from Hayward through Yosemite Valley via Tracy, Pat- 
terson, Turlock, and Alerced Falls 47 

Road log 1, U.S. Highway 50 from Hayward to Tracy, California, by 
Holly C. Wagner and Frederic R. Kelley 51 

Road log 2, Tracy to El Portal via Patterson and Turlock, California, bv 
Clyde Wahrhaftig and L. D. Clark .'.. 55 

Road log 3, El Portal to W'awona Tunnel and a circuit of Yosemite \^allev, 
California, by Dallas L. Peck, Clyde Wahrhaftig, and Frank C. Calkins 61 



PLATES 

Plate 1. Geomorphic map and section of the southern part of the 

western Sierra Nevada metamorphic belt In pocket 

Plate 2. Guide map to Highway 50, Havward to Tracy, California In pocket 



(5) 



LETTER OF TRANSMITTAL 

To: Edmund G. Brown 

Governor of the State of California 

Dear Sir: 

I have the honor to transmit herewith Bulletin 182, Geologic guide to the 
Merced Canyon and Yosemite Valley, California, a collection of four significant 
papers on the geologv and soils of this great park area, and a series of road logs 
across the Coast Ranges and San Joaquin \'alley into the Yosemite. This bulletin 
is the result of cooperation between the State Division of Mines and Geology 
and the U.S. Geological Survey and was prepared as the second of two guide- 
books \\'hose publication coincides with the .Annual Convention of the American 
Association of Petroleum Geologists and Society of Economic Paleontologists and 
Mineralogists. 

Description of the geologic features and the road logs extends across the Tracy 
and Vernalis gas fields of the northern San Joaquin V'alley and into the Yosemite 
Valley via the Sierra Nevada foothills across the southern end of the famed 
Another Lode. .'\t the western gateway to the Valley lie historically great gold 
mines and the barite deposits of El Portal. Photographs have been selected to do 
full justice to the beauty and grandeur of the Yosemite, our most-visited national 
park. 

Respectfully submitted, 

DeVVitt Nelson, Director 
Department of Conservation 
January 11, 1962 



(6) 



PREFACE 



In performance of its function as the State's public 
information bureau on geoIog\', mineral resources, and 
mineral industries, the Division of Mines and Geology 
assists the petroleum industry, responsible for two-thirds 
of California's annual mineral production, in the general 
area of exploration. This it does by detailed geologic 
mapping of selected areas and by reconnaissance of large 
areas on the 1: 250,000 scale, and by publication of several 
series of geologic maps and reports, many of which are 
useful to petroleum exploration. These are: Mineral hi- 
formation Service, a semi-popular monthly pamphlet; 
Ammal Report of the State Geologist, Chief of the 
Division of Mines and Geology; the Bulletin series, on 
the geology and mineral resources of quadrangles, geo- 
logic guides to significant regions, or on statewide com- 
modity surveys; the Special Report series on shorter or 
more localized subjects; the new County Report series on 
the mines, mineral resources, and geology of counties; 
and the State Geologic Map, issued as colored, litho- 
graphed, 1: 250,000-scale geologic map sheets, each 1 
degree in latitude by 2 degrees in longitude. 

In planning field trips and a guidebook which would 
be most interesting and of greatest benefit to the petro- 
leum profession, as well as to the people of California, 
the committees felt that the greatest effort should be 
directed toward presentation of geologic features of 
northern California of direct economic importance to 
the petroleum industry; and also that our unique scenic 
geological attractions should not be neglected. The first 



condition has been satisfied by Bulletin 181, Geologic 
guide to the gai and oil fields of northern California; the 
second we hope to meet with this publication. Bulletin 
182, Geologic guide to the Merced Canyon and Yosemlte 
Valley, California. The latter book is the result of co- 
operation between the U.S. Geological Survey, the Uni- 
versity of California, and the State Division of Mines 
and Geologw It consists of four papers by Survey and 
University authors on the metamorphic and granitic 
rocks of the western Sierra Nevada, the geomorphology 
of the area, and on soils of the San Joaquin \'alley along 
the Merced River, followed by continuous road logs 
from Ha\'ward across the Coast Ranges and northern 
San Joaquin Valley and through the Yosemite \'alley. 
Route of the trip is by way of Altamont Pass across the 
Diablo Range and close to the Tracy and Vernalis gas 
fields of the northern San Joaquin Valley. Those inter- 
ested in more geology relating to this area and the oc- 
currence of gas are referred to the section Northern San 
Joaquin Valley, in Bulletin 181, which contains Vernalis 
gas field, b\- Charles F. Manlove, Cretaceous geology of 
the Pacheco Pass area, by Frederick O. Schilling, and 
Type Panoche group (Upper Cretaceous) and overly i?!g 
Moreno and Tertiary strata on the west side of the San 
Joaquin Valley, by Max B. Payne. Bulletins 181 and 182, 
together, constitute a unit which should give the geolo- 
gist an adequate geologic background for understanding 
the occurrence of gas and oil in northern California. 

Gordon B. Oakfshott 
General Chairman, AAPG-SEP.VI 
Convention, 1962 
and Deputy Chief, Division of 
Mines and Geology. 
San Francisco 
January 11, 1962 



(7) 






E 

M 
O 

>- 



o 

a. 

E 






??ii45:! 



^i"''.'^ 







Part I— GEOLOGIC GUIDE TO THE MERCED CANYON 
AND YOSEMITE VALLEY, CALIFORNIA 



CONTENTS 

Page 

Summary of the pre-Tertiary geology of the western Sierra Nevada meta- 
morphic belt, California, by Lorin D. Clark 15 

Granitic rocks of the Yosemite Valley area, California, by Frank C. Calkins 
and Dallas L. Peck .....! ' 17 

The geology, geomorphology, and soils of the San Joaquin Valley in the 
vicinity of the Merced River, California, by Rodney J. Arkley. 25 

Geomorphology of the Yosemite Valley region, California, by Clyde Wahr- 
haftig 33 

PLATES 

Plate 1. Geologic map and section of the southern part of the 
western Sierra Nevada metamorphic belt In pocket 



Frontispiece, Part I, opposite. Winter, Yosemite Valley. Half Dome and 
the Merced River. Photo by Ansel Adams. 



(12) 






'«:*»£:* 




yv^vJ-^. 




rgpa^^-#- 



.^i 






...^,,,^^^:r^%•i«t;tH>^■'■^•- 






> ' 









■^ 





^E 


'3^1 


^Hl 


-1H 


^BBp 


^ ^^H 


^^P^' 




K 


u i^^H 


H|i 


p^l^H 


BEk» 




1 



SUMMARY OF THE PRE-TERTIARY GEOLOGY OF THE WESTERN 
SIERRA NEVADA METAMORPHIC BELT, CALIFORNIA* 



By LORIN D. CLARK 

U.S. Geological Survey, Menio Parle, California 



Plate 1, Geologic mop and section of the southern pari of the western Sierra Nevada metamorphic belt, accompanies this paper. 



Aletamorphic rocks of the western Sierra Nevada near 
the Merced River are of Jurassic and Paleozoic age. They 
are on the west limb of a faulted synclinorium, the cen- 
tral part of which is occupied by the Sierra Nevada 
batholith. Parts of the eastern limb are preserved in the 
eastern part of the Sierra Nevada and in the White and 
Inyo Mountains still farther east. The metamorphic rocks 
strike northwest, as reflected in the map pattern of lith- 
ologic units (pi. 1), and nearly everywhere dip steeply 
eastward or are vertical. The metamorphic rocks are 
divided into three structural blocks by the Melones and 
Bear Mountains fault zones (Clark, 1960). The structure 
within each block is generally homoclinal with younger 
beds to the east, but the gross distribution in the meta- 
morphic belt as a whole has been reversed by fault move- 
ment, for the youngest metamorphic rocks are in the 
western fault block and the oldest in the eastern block. 
Dominant strike-slip movement along the Bear Moun- 
tains and Melones fault zones is suggested by steeply- 
plunging minor folds and b-lineations within the zones. 
The stratigraphic separation along the Melones fault zone 
near the Merced River is at least 10 miles, and the strati- 
graphic separation along the Bear Mountains fault zone 
farther north exceeds 3 miles. Because the dihedral angle 
between bedding and faults is small, the net slip along 
the fault zones must be much greater than the strati- 
graphic separations. 

The Paleozoic rocks, w hich are entirely east of the 
.Melones fault zone in the area of plate 1, are all included 
in the Calaveras formation. The lower part of the forma- 
tion, exposed near Bagby, consists largely of phyllite 
derived from siltstone, but contains some interbedded 
tuff and graywacke and sparse lenticular limestone. These 
are overlain by metavolcanic rocks, derived mostly from 
coarse andesitic breccia but in part from pillow lava and 
tuff. From Briceburg northeastward to the large horse- 
shoe bend in the Merced River, the Calaveras formation 
consists largely of black phyllite derived from siltstone 

• Publication authorized by the Director, U.S. Geological Survey. 




- i-'.AA^A metachert, sparse limestone 
•t, intraformational conglom- 

1 eastward the black phyllite 
persists, but metachert constitutes a considerable part of 
the section, lenses of limestone and dolomite are larger 
and more abundant, and lenses of mafic metavolcanic 
rocks are common. The age of the Calaveras formation 
near the Merced River is based upon H. W. Turner's 
identification of Fusulina, which he considered to be of 
Carboniferous age, from a locality a few miles southwest 
of El Portal (1893, p. 309). No fossils other than crinoid 
debris have been found in the low er part of the forma- 
tion. Because of shearing of parts of the Calaveras forma- 
tion east of Briceburg, its thickness cannot be determined 
accurately, but it is almost certainly more than 25,000 
feet, and it may be more than 50,000 feet thick. 

Mesozoic rocks, consisting of complexly intertonguing 
metavolcanic and metasedimentary strata, constitute the 
central and western fault blocks and occur in the eastern 
fault block east of Bear \"alley. The Mariposa, Consum- 
nes, and Logtown Ridge are among the named forma- 
tions. The metavolcanic rocks were derived chiefly from 
tuff and volcanic breccia of intermediate composition, 
but in part from basaltic lava, some with pillow struc- 
ture, and in part from felsic lava and pyroclastic rocks. 
Th e meta -se.dim<'r.«->.-"- — 's were derived largely from 

i mixed-pebble metaconglom- 
)me sections. Metamorphosed 
abundant locally. Both the 
J volcanic rocks accumulated in deep 
water ,for graded beds are common in both tuff and 
gravwacke throughout the region. Marine fossils found 
in both metavolcanic and metasedimentary rocks are of 
Late Jurassic (Callovian to Kimmeridgian) age. Every- 
where in the \\ estem Sierra Nevada the Jurassic strata 
are truncated by faults or erosion— the section in the cen- 
tral fault block along the Merced River is nearly 18,000 
feet thick and is among the thickest preserved. 




(!5) 



16 



California Division of Mines and Geology 



[Bull. 182 



Two groups of plutonic rocks intruded these meta- 
morphic rocks during Late Jurassic time and one group 
in middle-Cretaceous time. The first was an ultramafic 
group, now represented mostly by serpentine, and the 
second a group ranging from diorite to granodiorite. In 
middle-Cretaceous time, according to radiometric data 
obtained by Curtis, Evernden, and Lipson (1958, p. 7-9), 
the Sierra Nevada batholith was intruded. Rocks of the 
batholith are described by F. C. Calkins and D. L. Peck 
elsewhere in this guidebook. 

Uplift and perhaps gentle folding occurred in the 
Sierra Nevada region between about middle Permian 
and middle Jurassic time, but most of the deformation 
of both Paleozoic and Mesozoic strata occurred during 
Late Jurassic and possibly Early Cretaceous time. The 
synclinorium was formed in Late Jurassic time, before 
intrusion of the older series of granitic rocks, and re- 
sulted in the northwest strike and steep dip of Mesozoic 
and Paleozoic strata. Axes of folds formed during this 



deformation plunge northwest and southeast at angles 
of less than 30°. The second major deformation also 
began before emplacement of the older granitic rocks, 
but possibly continued into the Early Cretaceous. The 
major faults, pervasive shearing of the eastern block, and 
steeply plunging minor folds and lineations resulted from 
the second major deformation. 

Referejices 

Clark, L. D., 1960, Foothills fault system, western Sierra Nevada, 
California: Geol. Soc. America Bull., v. 71, p. 483-596. 

Cloos, Ernst, 1932, Structural survey of the granodiorite south 
of Mariposa, California: Am. Jour. Sci., 5th ser., v. 23, p. 289-304. 

Curtis, G. H., Evernden, J. F., and Lipson, J., 1958, Age deter- 
mination of some granitic rocks in California by the potassium- 
argon method: California Div. .Mines Special Rept. 54, 16 p. 

Turner, H. W., 1893, Some recent contributions to the geology 
of California: Am. Geologist, v. 11, p. 307-324. 

Turner, H. W., and Ransome, F. L., 1897, Description of the 
Sonora quadrangle I California]: U. S. Geol. Survey Geol. Atlas, 
Folio 41. 




Photo 1. Interbedd«d metochert and block corbonoceous phyllite of the Calaveras formation of 
Paleozoic age near the Geologic Exhibit marker. These beds ore little folded, but nearby are intricately 
folded strata. Photo by U.S. Notional Pork Service. 



GRANITIC ROCKS OF THE YOSEMITE VALLEY AREA, CALIFORNIA* 



By FRANK C. CALKINS 

U.S. Geological Survey, Menio Park, California, ond 

DALLAS L. PECK 

U.S. Geological Survey, MenIo Pork, California 



BRIEF HISTORY OF DISCOVERY AND GEOLOGIC 

INVESTIGATIONS 

The Yosemite \'alley was discovered by William Penn 
Abrams in 1849, but it first became well known when 
it was rediscovered in 1851 by the Mariposa Battalion, 
under the leadership of .Major James D. Savage, while 
pursuing the Indian tribe called the "U-zu-ma-ti" (mean- 
ing "grizzl\- bear"), led by Chief Tenaya. The members 
of the Battalion gave the valley the name that the\" 
understood to be that of the tribe; the Indians themselves 
called it "Ahwahnee", meaning "deep, grass\- valley". 
The valley achieved national fame through the writings 
of Dr. Lafe\ette Bunnell, James Hutchings, John .Muir, 
the Reverend Thomas Starr King, and others. In 1864 
an area that included the Yosemite \'alle\' and also the 
Mariposa Grove was set aside by the Federal Govern- 
ment as the Yosemite Grant; this area was administered 
by California until 1905, when it was included in the 
much larger Yosemite National Park, which had been 
established in 1890. 

Geologic investigations in the Yosemite X'alley region, 
most of which have been summarized by Matthes (1930, 
p. 4-7), were begun in the earl\' 1860's by J. D. Whitney, 
State Geologist of California, who was assisted by Clar- 
ence King. Whitney (1868) and King (1874) concluded 
that the valley was formed by faulting so recent that 
the valley walls had not been greatly modified by sub- 
sequent erosion. In the late 1860's and later this view 
was challenged by W. P. Blake (1867), John .Muir (his 
extensive bibliography was published in the Sierra Club 
Bull. vol. 10, p. 41-54, 1916, but see particularly .Muir, 
1874 and 1875), and Joseph LeConte (187.3), who at- 
tributed the formation of the valley to glacial and stream 
erosion. The general e.vtent of the Sierra Nevada com- 
posite batholith was described in the 19th century in 
geologic reports and folios by H. W. Turner, Waldemar 
Lindgren, and others, but detailed study of this vast in- 
trusive complex is still going on and may continue for 
years to come. Near the turn of the century Turner 
began to map the geology of the Yosemite and Mount 



Publicotlon authorized by the Director, U.S. Geological Survey. 



Lyell quadrangles, but was never able to complete this 
difficult assignment. Shortly before World War I, F. E. 
.Matthes and F. C. Calkins began a highly detailed study 
of the physiography and bedrock geolog\- of the Yosem- 
ite \'alle\- area and a less detailed mapping of surround- 
ing areas in the Sierra Nevada (1930), but the complex 
bedrock geology of the valley has not even \'et been 
fully mapped. Ernst Cloos (1936) plotted structural fea- 
tures in the granitic rocks of the Yosemite region. Black- 
welder (1931, p. 907-909; 1939) related the glacial stages 
to those of the eastern flank of the Sierra Nevada; and 
Curtis and others (1958) obtained potassium-argon ages 
of some of the granitic rocks. 

SIERRA NEVADA BATHOLITH 

The rocks in w hich the Yosemite \'alley was carved 
belong almost wholly to the great Sierra Nevada com- 
posite batholith, which extends continuously along the 
range for about 400 miles and has a maximum breadth 
of about 100 miles. The rocks of this batholith were in- 
truded into sedimentary and volcanic rocks of early 
Paleozoic to Mesozoic (Late Jurassic) age. As shown by 
Bateman and others (in press) it was emplaced along the 
axis of a s\ncIinorium in those la>"ered rocks. The pre- 
batholithic rocks are not represented in the Yosemite 
\'alley except b\' a few small masses not visible from 
the valle\' floor, but the roads from the west pass through 
them for many miles on the way to El Portal (see de- 
scription by Clark in this guidebook), and they are 
widely exposed on the east side of the batholith along 
the crest of the range. Rinehart and others, (1959), 
found that pre-batholithic rocks they mapped in the 
Devils Postpile and .Mount .Morrison quadrangles have 
a total thickness of about 60,000 feet. The lower half of 
the stratigraphic column there consists of sparsely fos- 
siliferous Ordovician to Permian(?) homfels, metamor- 
phosed calcareous sandstone, slate, and marble; these are 
overlain conformably by about the same thickness of 
metamorphosed pyroclastics and lavas, with a few minor 
interbeds of calcareous and tufTaceous rocks which have 
yielded fossils of Early Jurassic age. 



(:7) 



18 



California Division of Mines and Geology 



;Bu11. 182 



xtiVNb3ivno 



Sn033V13MD 



aaaio qnv oissvynr 



; ° J 



S 

^ 






S^ArsnJiul town 




o 



19621 



MiRci I) Canyon and Yosemite Valley 



19 



It needs to be emphasized tliat the Sierra Nevada 
batholith is co7nposite. Some people fail to realize how 
complex it is, and what a long and eventful histor\' it 
has had. At least seven irregular intrusive bodies such 
as are commonl\' called stocks, are exposed in the walls 
of Yosemite Valley and the Merced Gorge, and measure- 
ments of potassium-argon ratios indicate that the oldest 
and \oungest of these differ in age by about 12 million 
\ears, though ail of them were intruded in Cretaceous 
time. The stocks are cut, moreover, b\ dikes and irregu- 
lar sheets of at least half a dozen other kinds of rocks— 
not counting pegmatites and apiites. On the nearb\- up- 
lands, moreover, within the area drained by tributaries 
of the Merced and the headwaters of the Tuolumne, one 
can see exposures of many other intrusive bodies, large 
and small. If this area gives us an\thing like a fair sample, 
the Sierra Nevada batholith was formed by scores, if 
not hundreds, of distinct intrusions, and the making of 
it may have taken something like 15,000,000 years. 

Brief descriptions are given below of the intrusive 
rocks exposed in the valley and the gorge, and also of 
tw o others that are closel\' related to the rock forming 
the falls of the eastern part of the valle\-, but that could 
not be reached \\ ithout going eastward to the Tuolumne 
Meadows. The distribution of the major intrusive rock 
units in tiie ^'osemite \'alley area is roughly shown in 
figure 1. 

AGE RELATIONS OF THE INTRUSIVE ROCKS 
Principal Groups 

The rocks that form relatively' large bodies exposed in 
the Yosemite X'alley and in line with it to the east and 
west may be assigned to two series, the western and the 
Tuolumne intrusive series. The western intrusive series, 
which is the older, forms the walls of the western half 
of the valle\', and of the can\on of the Alerced down 
to El Portal. The younger rocks of the Tuolumne intru- 
sive series are exposed in the eastern half of the Yosemite 
\'alley and in the high countr\' still farther east, which 
is drained in part by the Tuolumne River. The rocks of 
both series are cut by apiites and pegmatites, which will 
not be described and which are not included in the 
group fo be considered next. 

Generall\' intermediate in age and position between 
the intrusive bodies, of large to moderate size, that have 
been assigned to the western and Tuolumne series, there 
are a great many dikes, sills, and small irregular bodies 
that do not clearly belong to either series. All of them 
are probabl\- \ounger than an\- rock of the western 
series. None is now known to be younger than any rock 
of the Tuolumne intrusive series, but one or two of them 
could be. Because of these uncertainties, combined with 
the fact that no potassium-argon ratios have been deter- 
mined for an\' of these minor intrusive bodies it seems 
expedient to put them in a separate group. In the petro- 
graphic summarv the\' are placed between the western 
and Tuolumne series. The>' are for the most part at least 
intermediate in age between the two, and are mainl\ 
exposed in the middle part of the valley. 



The extreme complexity of the intrusive pattern in 
the Cathedral Rocks and Kl Capitan suggests— though it 
does not prove— that a local subsidence occurred in the 
area crossed b\- the middle part of the valle\', perhaps 
shortly after the intrusion of the western series was com- 
pleted. Such a luechanism could help to account for the 
generall\- flat-l\ing attitude of the intrusive bodies of 
Bridalveil granite; withdrawal of support would have 
tended to open approximatel\- horizontal fissures into 
w hich magmas would be injected from a pluton that has 
not been identified and may not be exposed. But sub- 
sidence would also have opened steep fissures to be filled 
bv dikes. There is evidence, also, that sulfide-bearing 
solutions arose along some of the steep fissures, for here, 
and nowhere else in the Yosemite \'alley area, some of 
the rocks contain a little p\rite, the weathering of which 
produced iron oxides that have locally imparted a red 
color to the outcrops, especiall\' on the southwestern 
slopes of the Cathedral Rocks. In a few places, moreover, 
a very little mol\bdenite has been found. 

Western Intrusive Series 

The sequence of intrusion within the western series is 
not full\' known. Field relations prove that the Taft 
granite is its Noungest member, that it was preceded by 
El Capitan granite, and that El Capitan was preceded by 
both the diorite of the Rockslides and the granite of 
Arch Rock. No field evidence had been obtained, how- 
ever, regarding the relative ages of the granite of Arch 
Rock, the above-mentioned diorite, and the granodiorite 
of The Gateway. Potassium-argon ratios obtained by 
Curtis and others (1958) indicate that the granite of 
Arch Rock is older than the granodiorite of The Gate- 
w a\', and the latter rock older than El Capitan granite. 
This dating of the granite of Arch Rock seems question- 
able because it does not accord with the usual order in 
an intrusive series— that of increasing silica content. It is 
nevertheless provisionally- accepted in the petrographic 
summar\', in w hich the diorite of the Rockslides is as- 
sumed—again provisionalh — to be the oldest member of 
the series. 

Minor Intrusive Bodies 

The order in w hich the small bodies were intruded is 
not fully known and ma\- never be completely worked 
out. We do not even know how many different kinds 
of rock they consist of, and only a few are described 
herein, in an order that represents the best guess we can 
make regarding their relative age. The one that occupies 
the largest areas on the map (fig. 1) is the Bridalveil 
granite. This cuts nearly all of several rocks with which 
it is in contact, but although it is w idely exposed on the 
south side of the valle\", especially in the Cathedral 
Rocks and along Bridalveil Creek, it has not been posi- 
tively identified on the north side. The \ oungset of all 
these rocks may be the diorite forming the "Map of 
North America," on the face of El Capitan, w hich cuts 
across a dike, sloping upward toward the east, of a gray 
rock that is probabl\' Leaning Tower quartz monzonite. 



20 



California Division of Mines and Geology 



[Bull. 182 



Tliis diorite does not differ much from the diorite of the 
Rockslides except in being generally finer grained. As 
we have provisionally regarded the older diorite as the 
oldest member of the western series, the younger diorite 
is possibly to be regarded as the oldest member of the 
Tuolumne intrusive series; for the present, however, it 
is not described as such. 

Tuolumne Intrusive Series 

After having had to confess how much we don't know- 
about the order of intrusion in the western series and 
the minor intrusive bodies, it is a relief to come to the 
Tuolumne intrusive scries. For here the sequence of in- 
trusion is clearlx' shown b\' field relations, and confirmed 
by measurements of potassium-argon ratios in all four 
members. The order of intrusion appears, also, to be that 
of increasing silica content, which is commonly regarded 
as the normal order; this however is as yet uncertain 
because of a lack of anal\'ses. The outcrops of the four 
members, moreover, are roughly concentric in the cen- 
tral part of the area occupied by the series, the latest 
member being in the center. Because this series is so 
definitely a unit, we briefly describe it as a whole, even 
though only its two oldest and outermost members— the 
Sentinel granodiorite and the Half Dome quartz mon- 
zonite— are exposed in the Yosemite \'alley. In the 
Yosemite \'alley one cannot even see the porphyritic 
facies of the Half Dome quartz monzonite into which 
the non-porph\ritic facies, exposed around the head of 
the Yo.semitc \'alle\' and in Half Dome itself, grades near 
Tena\'a Lake. The porph\ritic Half Dome quartz mon- 
zonite is cut in that vicinity by dikes of the Cathedral 
Peak granite, and one can see from the valley floor, in 
the cliff west of the Royal Arches, flat-lying tongues of 
streak\- Half Dome quartz monzonite extending into the 
Sentinel granodiorite. A little farther west, in the Castle 
Cliffs, the granodiorite is intruded into the El Capitan 
granite in an extremely complex fashion, forming a pat- 
tern that has to be greatly generalized even at a scale of 
1 : 24,000 and could merely be suggested in figure I . Little 
of this pattern can be seen from the floor of the Yosemite 
\'alley; it is better exposed along Yosemite Creek above 
its falls and on the upland south of Glacier Point. 

CHARACTER OF THE INTRUSIVE ROCKS 

The following short descriptions do not aim to do 
much more than help those interested to distinguish the 
principal rocks from one another. The rocks described 
by Calkins (1930, p. 120-129), are here called by the 
names used in that publication. These names are largely 
based on megascopic rather than microscopic features, 
and some of them do not depend as much as some petro- 
graphers would like upon relative abundance of potas- 
sium feldspar and plagioclase. The dominant rock of El 
Capitan, for example, was called b\' H. W. Turner (1900, 
p. 304, 308) El Capitan granite, and that name has come 
into general use. It is based on the fact that the dominant 
facies of the rock contains abundant and conspicuous 
potassium feldspar and quartz, and onl\- a small propor- 
tion of its one ferromagnesian mineral, biotite. Roughly 



quantitative measurements of mineral composition indi- 
cate, however, that the dominant rock of this intrusive 
mass is a quartz monzonite if the feldspar ratio is made 
the criterion, though it contains more quartz than most 
quartz monzonites. 

The rocks are listed in what is regarded as their most 
probable order of age— the oldest first. Whenever the 
age of a rock has been estimated from the potassium- 
argon ratio by Curtis and others (1958, p. 7), the result, 
in millions of years, is given at the end of the description 
as "K/Ar age — m.y." 

Rocks of the Western Intrusive Series 

Diorite of the Rockslides. General color very dark 
greenish-gra\'. Texture varies from very coarse to me- 
dium-grained. Chief minerals plagioclase and hornblende, 
the latter being the more conspicuous; most specimens 
also contain subordinate quartz, potassium feldspar, and 
biotite, and some contain a little augite. The rock ma\' be 
in part a metagabbro. 

Granite of Arch Rock. Aledium-light-gray, medium- 
grained, non-porph\'ritic. Plagioclase predominates over 
potassium feldspar, which is generally' poikilitic, as can 
be seen by reflections from cleavage faces. Quartz moder- 
ately abundant. In most of the rock the only ferromag- 
nesian mineral is biotite (subhedral to anhedral), but a 
little hornblende is present in some places. 

K/Ar age 95.3 m.y. 

Granodiorite (or Qiuvtz Diorite) of The Gateway. 
Dark-gray, medium-grained. Potassium feldspar subordi- 
nate; some of the rock does not contain any. Biotite is 
fairly abundant, hornblende less abundant but every- 
where present. 

K/Ar age 92.9 m.y. 

El Capitan Granite. Light-gray, medium-coarse- 
grained. Some is vaguely porphyritic, with phenocrysts 
of potassium feldspar. Plagioclase is more abundant but 
in smaller grains. Quartz is conspicuous. Biotite, in mod- 
erate quantit\', is the only ferromagnesian mineral, though 
a little hornblende ma\' occur in marginal facies! 

K/Ar age 92.2 m.y. 

Taft Granite. \'ery light gra>', medium-grained. T\p- 
ical facies finer-grained and more uniform than El Capi- 
tan granite and not porphyritic, but a rock that may be a 
porphvritic facies of the Taft, exposed near the east 
portal of the Wawona tunnel, contains phenocr\'sts of 
potassium feldspar. Plagioclase, potassium feldspar, and 
quartz about equally abundant; biotite scarce. 

Rocks of Minor Intrusive Bodies 

Leaiitng Tower Quartz .Monzonite. Color medium- 
gray; texture medium-grained granular. Contains biotite 
and less hornblende; these are largeh' in clusters, about 
10 mm in maximum diameter, which give the rock a 
characteristic speckled appearance. 

Bridalveil Granite. iMedium-gra\-; the fresh rock has 
a slightly bluish tinge. Fine-grained, granular. Biotite 



19621 



Merced Canyon and Yosemitf X'allev 



21 



moderately abundant, in small, evenlv distributed flakes 
u liich give the rock a "pepper-and-salt" appearance. 

Quiirtz-Micj Diorite. .Medium-dark gra\-, mediuni- 
tine-grained, granular. All consists mainly of piagioclase, 
quartz, and biotite; most contains subordinate potassium 
feldspar and some contains a little hornblende. Not 
shown in figure 1. 

Diorite of the "Map of North Ainerica". Similar to 
the diorite of the Rockslides but finer-grained. Repre- 
sented by the same pattern as the diorite of the Rock- 
slides. 

Rocks of the Tuolumne Intrusive Series 

Sentinel Granodiorite. Generally medium-dark gray 
and medium-grained granular, but varies rather widely 
in both color and texture. Near contacts \\ ith El Capitan 
granite the rock tends to be darker than elsewhere and 
more or less foliated. Piagioclase predominates over po- 
tassium feldspar; quartz is inconspicuous. Biotite is fairly 
abundant and hornblende onl\' a little less so; both are 
in irregular grains tending to cluster together. 

K/Ar age 86.4 m.y. 

Half Do7ne Quartz Monzonitc. Lighter-colored and 
more uniform in both color and texture than the Sentinel 
granodiorite. Its potassium feldspar, though only about 
half as abundant as piagioclase, is more conspicuous be- 
cause in larger crystals. In a porphyritic facies exposed 
near Lake Tenaya— not seen from the Yosemite Yalley— 
there are numerous phenocrysts of potassium feldspar. 
Biotite and hornblende are less abundant than in the 
Sentinel granodiorite, and both tend to form discrete, 
fairl\- regular crystals. 

K/Ar age 84.1 m.y. 

Cathedral Peak Grajiite. A light-gray rock, charac- 
terized by numerous large phenocr\sts of potassium feld- 
spar (some as much as 2 inches long) in a medium- 
grained granular groundmass consisting of both feldspars, 
much quartz, a moderate amount of biotite, and a little 
hornblende. Not exposed in the area of figure 1, but 
boulders of this rock in the moraines of Yosemite \'^alley 
are among the evidences for glaciation of the Sierra 
Nevada. 

K/Ar age 83.7 m.y. 

Johvson Granite Porphyry. A porph\Titic rock, 
lighter-colored and finer-grained than the Cathedral Peak 
granite; contains a little biotite but no hornblende. Not 
exposed in the Yosemite X'allev. 

RELATION OF TOPOGRAPHY TO ROCKS AND 
STRUCTURE 

The major features of the Yosemite \'alley are due 
to erosion by streams and glaciers, whose handiwork 
was described at length in Matthes's classic paper (1930) 
and has been summarized in another section of this guide- 
book. But sculptural detail, in this area as in any other, 
depends to a large extent on the material that erosional 
agencies had to deal with— on what rocks the>" encoun- 
tered as thev worked do\\nward. If the bedrock in the 



upper-middle part of the Merced basin had been of uni- 
form composition and structure, erosion would never 
have produced a Yosemite \'alle\'. One reason for the 
astonishing variety of sculpture that causes the Yosemite 
. to stand unrivalled in the Sierra Nevada or an\ where 
j else for the magnificence of its falls, cliffs, and domes, 
all displa\ed within a distance of about 7 miles, is the 
varied nature of the rocks in which it was carved. The 
differences that matter in this regard are differences in 
susceptibility' to erosion. These are not due mainly to 
\ differences in hardness, which are not verv great. There 
are greater differences in resistance to weathering, but 
these again would not have had ver\- much effect if the 
rocks had all been jointed to the same extent. The great 
contrasts in topographic expression arise from the dif- 
ferent degrees to which the various rocks have been 
jointed. Broadl\' speaking, the more siliceous rocks of 
the Yosemite \'alle>' are less jointed than the less siliceous 
rocks. It has been thought, however, that in other areas 
texture rather than composition is the determining factor, 
the finer-grained rocks being the more closely jointed. 
The degree of jointing in the various intrusive rocks ma\' 
therefore be the resultant effect of both factors in com- 
bination. Kl Capitan, whose southeast face is one of the 
highest unbroken cliffs in the world, consists chiefly of 
two of the most siliceous rocks that form large intrusive 
bodies in this area— namel\' El Capitan and Taft granites 
—and these determine its character even though they are 
cut b\- man\- small bodies of less siliceous rocks. The 
Cathedral Rocks and the Leaning Tower also probably 
stand out as the\' do because, though of extremely com- 
plicated makeup, they consist mainl\' of siliceous rocks. 
El Capitan granite is one of the most abundant materials 
in them, and the minor intrusive bodies here consist 
mainl\- of Bridal veil granite. jHalf Dome, the greatest 
monolith of all, and also the other prominent domes 
overlooking the eastern part of the vallev, consist of 
the Half Dome quartz monzonite. This rock, judging 
from its mineral composition, appears to be a little less 
siliceous than El Capitan granite, but it is not cut b\' any 
rocks that are less siliceous, and this fact may help to 
account for its almost complete lack of joints. It is in- 
deed cut by many narrow dikes of pegmatite and aplite, 
but these are more siliceous and even more resistant than 
the dominant rock, so that great numbers of them stand 
out in relief on the southern slope of Half Dome. 

The Half Dome quartz monzonite is mainly in huge 
masses almost free from joints, and these have disinte- 
grated for the most part by exfoliation, which occurs 
here on a grand scale. That is why this rock forms 
nearly all the domes; the single exception is Sentinel 
Dome, which consists of El Capitan granite. The Royal 
Arches reveal a cross section of exfoliation cracks in the 
quartz monzonite that are too far below the surface to 
form the tops of domes. 

The rock nearest in composition to the Half Dome 
quartz monzonite— on the less siliceous side— is the Sen- 
tinel granodiorite, which is cut by numerous joints. The 
lower part of the cliff east of Glacier Point consists of 



22 



California Division of Mines and Geology 



[Bull. 182 




Photo 1. Cliff face below Glacier Point, developed along vertical joints that trend almost due east. 



19621 



Merced Canyon and Yosemite V^alley 



23 



iiniointed Half Dome quartz monzonite, but this is over- 
lain, on an intrusive contact sloping gently w estw ard, by 
the granodiorite, which is considerabl\- jointed; in fact 
the e\e can trace the contact quite closely, from a view- 
point near the Ahwahnee Hotel, by noting this difference 
in structure. iMan\' of the joints in the granodiorite strike 
about east-northeast and are nearly vertical; joints of this 
character have mainly determined the form of Sentinel 
Rock. The sheer cliff below Glacier is developed along 
vertical joints that trend almost due east. In the zone 
w here there are complex intrusive relations between this 
granodiorite and El Capitan granite, the amount of joint- 
ing largely depends on which rock is the more abundant. 

The least siliceous of the principal intrusive rocks is 
the diorite of the Rockslides, and although rather coarse- 
grained on the average it is by far the most closely 
jointed. For this reason it is exposed in only one large 
area, above the lower part of the Big Oak Flat Road, 
where it is cut by countless irregular joints, both steep 
and flat-l\ing; many of the flat ones are injected with 
sheets of light-colored intrusive rock. The diorite is held 
up here by a backing of Taft granite, which forms the 
upland surface immediatel_\' to the north. Turtleback 
Dome and Elephant Rock, south of the river, consist 
mainly of El Capitan granite. 

The slope on the west side of the Merced Gorge con- 
sists mainly of the granite of Arch Rock. This is inter- 
mediate in composition between El Capitan granite and 
the diorite of the Rockslides, and it is likewise inter- 
mediate between them in the character of its jointing, 
though in both respects it resembles El Capitan granite 
more closely than it does the diorite. It is here cut by 
fairly numerous joints, most of which strike northeast- 
ward and are nearly vertical but somewhat irregular. 

The large taluses on the sides of the Yosemite Valley 
contribute greatly to the variety of its sculpture, because 
they present so striking a contrast with the cliffs and 
"points" in which the bedrock extends nearly to the 
valley floor. There is reason to believe that the taluses 
are largely underlain by rocks that are closely jointed, so 
that their surfaces receded more rapidly than those of 
rocks containing few joints. The moderately large talus 
around the mouth of Indian Canyon is presumably under- 
lain in large part by Sentinel granodiorite. The bedrock 
under the three largest taluses— the Rockslides and the 
taluses east and west of Bridalveil Canyon— is probably 
made up in considerable part of diorite. The Rockslides 
are flanked on the west by the largest exposures of the 
older diorite, and it seems likely that their eastern part 
covers an area in which the diorite receded all the way 
northward to its contact with El Capitan granite. Diorite 
is exposed in many places around the borders of the 
taluses on the south side of the valley east and west of 
Bridalveil Canyon; some of it can be seen near the east 
portal of the Wawona Tunnel. 

One of the strangest features of the Yosemite \'alley's 
topography is the manner in which the lower part of 
Bridalveil Canyon projects beyond the general course of 
the valley's southern wall. Bridalveil Fall springs from 



the end of what might almost be likened to a gigantic 
flume- though a very lop-sided one, since the Cathedral 
Rocks, on its northeast side, are of much greater bulk 
than the Leaning Tower, on its southwest side. This ab- 
normal relation -of relief to drainage appears to be partly 
explainable on the hypothesis that the rocks along Bridal- 
veil Creek are mucli more siliceous, on the whole, than 
those underlying the great aprons of talus to the east 
and west. 

So much for the topographic features whose character 
expresses jointing or the lack of it. But some notches and 
other depressions were eroded along more persistent frac- 
tures that may somewhat arbitrarily be distinguished as 
fissures.^ Fissures, and fissure zones, cut across all kinds 
of rocks, even those in which there are few joints. They 
were doubtless formed by local concentration of strain, 
and there was probably some movement along them, 
though none have been shown to be large faults. Since 
many of the features due to Assuring were well described 
by Alatthes (1930, p. 111-114) only a few of the more 
important will be noted here. 

The west side of El Capitan is bounded by a north- 
south fissure zone in the lower part of which there is a 
basic dike, and the next main drainage way must have 
been eroded along another fissure zone that strikes north- 
eastward. The middle Cathedral Rock is separated from 
the others by deep notches eroded along steep fissures 
that also strike about northeast, and the Cathedral Spires 
are probably bounded by vertical fissures. The Three 
Brothers, two miles northeast of El Capitan, which con- 
sist mainl\' of El Capitan granite, are separated from one 
another bv two fissures, or master joints, that dip about 
45° W. 

The great monolithic mass of Half Dome itself is 
bounded on the northwest by a smooth, nearly vertical 
2,000-foot cliff that must form the wall of a fissure, pre- 
sumably the southeasternmost in a fissure zone that deter- 
mined the course of Tenaya Creek. The Half Dome 
quartz monzonite is also cut by at least one gently dip- 
ping fissure that slants downward to the north in the 
low er part of the southwest face of Liberty Cap. 

Steep fissures probably determined the location of 
many of the cliffs bordering the Yosemite \'alley. The 
valley itself may have been eroded along a complex fis- 
sure zone which is now mostly concealed by alluvium; 
the two fissures that separate the Cathedral Rocks from 
one another may belong to this zone. 

References 

Bateman, P. B., Clark, L. D., Huber, N. K., Moore, J. G., and 
Rinehart, C. D., in press. The Sierra Nevada batholith— a synthesis 
of recent work across the central part: U.S. Geol. Survey Prof. 
Paper. 

* On the 1:24,000 topographic map of the Yosemite Valley, the word "Fis- 
sures" is printed a little southeast of "Taft Point", to designate a small 
group of notches along joints of northeasterly strike where they cross 
the brink of a steep cliff. These are shown by the contours on that 
large-scale map, but could not be shown on the small scale of figure 1. 
The word "Fissures" is used on the map in a somewhat different sense 
than the one defined above — to designate joint cracks widened by ero- 
sion. Many joints of the same system as those in the Fissures can be 
seen on the slope across the gulch to the northeast, but there they are 
expressed only by shallow cracks. 



24 



California Division of Mines and Geology 



[Bull. 182 



Black welder, Eliot, 1931, Pleistocene glaciation in the Sierra 
Nevada and Basin Ranges: Geol. Soc. America Bull., v. 42, p. 
865-922. 

Blackwelder, Eliot, 1939, Contribution to the history of glacia- 
tion in the Yosemite region (abstract): Geol. Soc. America Bull., 
V. 50, p. 1947. 

Blake, W. P., 1867, Sur Taction des anciens glaciers dans la 
Sierra Nevada de California et sur I'origine de la V'allee de Yo- 
semite: Compt. Rend., v. 65, p. 179-181. 

Calkins, F. C, 1930, The granite. rocks of the Yosemite Region, 
ill iVIatthes, F. E., Geologic history of the Yosemite Valley: U.S. 
Geol. Survey Prof. Paper 160, p. 120-129. 

Cloos, Ernst, 1936, Der Sierra-Nevada-Pluton m Californien: 
Neues Jahrb., B-B. 76, Heft 3, Abt. B, p. 355-450. 

Curtis, G. H., Evernden, J. F., and Lipson, J., 1958, Age deter- 
mination of some granitic rocks in California by the Potassium- 
Argon method: California Div. Mines Spec. Rept. 54, 16 p. 

King, Clarence, 1874, Mountaineermg in the Sierra Nevada, 
4th Ed.: Boston, James R. Osgood and Co., 308 p. 



LeConte, Joseph, 1873, On some of the ancient glaciers of the 
Sierras: .\m. Jour. Sci., 3rd ser., v. 5, p. 325-342. 

Matthes, F. E., 1930, Geologic history of the Yosemite X'alley: 
U.S. Geol. Survey Prof. Paper 160, 137 p. 

Muir, John, 1874 and 1875, Studies in the Sierra: Overland 
Monthly, v. 12, p. 393-403, 489-500; v. 13, p. 67-79, 174-184, 393-402, 
530-540; and v. 14, p. 64-73. 

Rinehart, C. D., Ross, D. C, and Huber, N. K., 1959. Paleozoic 
and Mesozoic fossils in a thick stratigraphic section in the eastern 
Sierra Nevada, California: Geol. Soc. America Bull., v. 70, p. 
941-946. 

Turner, H. W., 1900, The Pleistocene geology of the south 
central Sierra Nevada with especial reference to the origin of 
Yosemite Valley; Calif. Acad. Sci., Proceed., v. 3, p. 261-321. 

Whitney, J. D., 1868, The Yosemite book; a description of the 
Yosemite X'alley and the adiacent region of the Sierra Nevada, 
and of the big trees of California: California Geol. Survey, 116 p. 



THE GEOLOGY, GEOMORPHOLOGY, AND SOILS OF THE SAN JOAQUIN VALLEY 
IN THE VICINITY OF THE MERCED RIVER, CALIFORNIA 



By RODNEY J. ARKLEY 

lecturer and Specialist in the Agricultural Experiment Station 

Department of Soils and Plont Nutrition 

University of Colifornio, Berkeley, California 



The Merced River emerges frdni a V-shapcd gorge 
through the Sierra Nevada foothills into the San Joaquin 
N'alley at the ghost town of Merced Falls, 30 miles east 
of Turlock. From this point it flow s west-southwest for 
a distance of 40 miles, \\ here it joins the northwestw ard- 
flow ing San Joaquin River. On either side of the river is 
a series of geoniorphic surfaces rising in steps to the north 
and south almost 500 feet above the river (figs. 1, 3). The 
two oldest and highest of these land forms are pediments 
carved on a consolidated deposit of andesitic tuff. 1 he 
\<)unger land forms are the surfaces of a series of alluvial 
fans and related depositional stream terraces. The nature 
of these surfaces and associated geologic formations was 
first studied and reported briefly b\- the author in connec- 
tion w ith the soil surveys of eastern .Merced and Stanis- 
laus Counties (Arkley 1954, 1959). In these studies clear- 
cut relationships were established bet\\een land forms, 
geologic formations and soil series. Subsequently the con- 
clusions drawn with respect to the geology and geonior- 
phic histor>' of the area have been largely substantiated 
1)\ geologic investigations of Davis and Hall (1959) and 
Hudson (1960). These geologists used the work of the 
author extensively in their studies, although some dif- 
ferences in interpretation remain. 

Most of the exposed rocks of the area are poorh' con- 
solidated Cenozoic sediments, and all except the oldest 
are nonmarine. The structure of the rocks is simple. The 
Tertiar\- rocks are tilted westward throughout the area; 
the older Quaternary sediments are tilted slightly west- 
ward on the west side of a fault zone located about 10 
miles east of Turlock. A summary of the formations and 
related soils found in the area is given in table 1. The dis- 
tribution of the rocks exposed at the surface is shown in 
figure 2. 

PRE-CENOZOIC 

The oldest rocks of the area shown on the map (fig. 
2). designated as bedrock complex, are strongly folded 
meta-andesite and slate. These rocks are described by 
Clark in the section of this guidebook entitled Stmmiary 

of the pre-Tertiary geology of the Wester?; Sierra Ne- 



^\^di^ uietimiorphic belt. Cretaceous rocks are not exposed 
in the area but underlie the valle\-. thickening westward 
from a feather edge about 12 miles west of the bedrock 
foothills to a thickness of 9,500 feet under the west side 
of the valley. 



FIGURE 1. Schematic section (N-S) 3 miles west of Merced Foils. 



North 







c 
o 

o 

> 

kJ 



(25) 



26 



Mt J M 



California Division of Mines and Geology 



[Bull. 182 




M 
4> 

C 
3 

o 

c 
o 

D 



o 

X 



m 
o» 

c 

o. 
c in 

-•- >« 



a> 



— c 



Q. 

E 
o 
o 



u 
o 



CD 



o 
►5- 




1 



sa 



0> 

c 
a> 
u 
o 



V 


a> 


c 


c 


« 


a> 


o 


o 


o 


o 



u 

■5 

o 
v 



E 

3 



= ? ° 

♦- O jc 

«" J3 O 

0> «- O 

T3 <l> — 

2 q: I- 



T3 





a> 


S 



»- 






<N 


^C/) 






3 


jr 






8 




E 










5 




E 

6 



u 



c 

u 
o 

M 



1062 



Merced Canyon and Yosemite Valley 
Table 1 '. 



11 



Formation 



Age 



Dominant lithology 



Maximum 
thickness 



Dominant 
soil scries 



Alluvium 

Sand dunes ^ — 

Modesto 

Rivcrbank 

Turlock Lake 

North Merced 

(New) 
(Arroyo Seco?) 

China Hat ._ 

(New) 

Mehrten 

Valley Springs 

lune 

(Several formations) 



Recent 

Late Pleistocene and Recent 

Late Pleistocene 

Middle Pleistocene 

Early Pleistocene 

Early Pleistocene 

Late Pliocene 

Early, middle Pliocene 

Late Miocene 

Middle Eocene 

Late Cretaceous 

Late Jurassic 

Late Jurassic 



Granitic sand over gra\-el 

Fine sand ._ 

Granitic sand over stratified silt, sand.. 
Granitic sand over stratified silt, sand,. 
Granitic sand over stratified silt, sand.. 

Pediment gravel (mainly metamorphic) 

Pediment gravel (mainly metamorphic) 

Andesitic tufT and gravel 

Rhyolitic ash, clay 

Sandstone, clay 

Marine sandstone, shale 

Slate 

Meta-andesite. 



50 

30 

100 

200 

800 

30 

50 

1,200 

270 

. 200 

9,500 

unknown 
unknown 



Grangeville, Tujunga 

Delhi 

Hanford, Dinuba 

San Joaquin, Snelling 

Montpellier, Whitney, 
Rocklin 

Redding, Corning 

Redding (acid variant) 

PentE, Peters, Raynor 

Amador 

Hornitos 

In wells only; not 

exposed 
Daulton, Whiterock 
Auburn 



Modified from Davis and Hall (1959) and Hudson (1960). 



CENOZOIC 

lone F onuatioii . Resting unconformably on the bed- 
rock complex are the Eocene sandstone and kaolinitic 
clay of the lone formation described by Allen (1929). 
The sandstone has attractive hues of pink, yellow, red, 
and gray and has been quarried for building stone. This 
material is described as a fluvial and shoreline marine de- 
posit and contains fossils of Venericardia phviicosta at 
the Planicosta Buttes just south of the bridge at Merced 
Falls. The formation dips about 3'' to the west and dis- 
appears under more recent sediments a mile or two west 
of the bedrock foothills. The shallow, acid Hornitos 
sandy loam is the only soil mapped on this formation. 

Valley Sprhigs Fonnation. The \"alley Springs for- 
mation rests unconformablv on the lone sandstone. It 




consists of a fluvial sequence of rhyolitic ash, sandy 
clay and siliceous gravel. The material is acidic in reac- 
tion (pH 4..5 to .5.0) and pale \ellow to white in color. 
The formation is generall\' considered to be of .Miocene 
age, although Axelrod ( 1944) considers it to be Mio- 
Pliocene in age. This rock is exposed in an area only 
about a mile wide parallel with the mountains, just west 
of the lone formation. The distribution of the shallow, 
acid Amador soil coincides exactly with that of the \'al- 
ley Springs formation. 

Mehrten Fonnation. The Valley Springs is conform- 
ably overlain b\' the Alehrten formation, an easily recog- 
nizable sequence of dark sandstone, conglomerate and 
claystone beds of late Miocene and Pliocene age, accord- 
ing to Stirton and Goeriz (1942). The material is always 
more than 50 percent andesitic and often as much as 95 
percent andesitic. The formation dips westward with a 
slope of about 100 feet per mile and thickens to a maxi- 
mum of 1,200 feet under the center of the valley. It 
contains beds of moderately hard mudstone which stand 
out on eroded slopes as prominent ledges, giving rise to 
"haystack mountains" such as China Hat, a conical peak 
visible southwest of Merced Falls. 



ammmmf^^mmiiatiiu. 



Photo 1. lone formotion dipping 5° W.; hord sandstone capping 
kaolinitic clays. Recent alluvium in the foreground. View is toward the north. 



California Division of Mines and Geology 



[Bull. 182 





3 



Photo 2. Mima mound or "hog wallow" microrellef ond 
formation south of Merced Falls. View is toward the south. 



Amador soil formed on Volley Springs 



p..ja?3*w^"'- 



Photo 3. "Hoystock" hills formed by the erosion of alternately hard and soft beds of the Mehrten 
formation. Pentz soils on the hill sides; Peters cloy on the gentle slopes in the foreground. Three miles 
north of Snelling. 



The Aiehrten in the area shown in figure 2 contains 
few or none of the volcanic niudfiov\'s found farther 
north in the Sierra Nevada, but consists ahnost entirely 
of fluvial material rev\'orked from the volcanic deposits. 
As indicated in the section Geoinorpholo^y of Yosemite 
Valley in this guidebook, there is no source of iMehrten 
formation in the A4erced River drainage; its southernmost 
source was in the headwaters of the Tuolumne River. 
Therefore the Tuolumne River or some distributar\- of 
it must have flowed southward in A'lehrten time, and 
forced the Alerced River to a southerly course down- 
stream from Merced Falls. The Mehrten formation 
weathers easily and gives rise to three distinct dark-gra\' 
soils high in montmorillonite— the Pentz, Peters, and Rad- 
nor series; the last named is about 3 feet deep. 



China Hat Pcdhnent and Gravels. The China Hat 
pediment, named bv Hudson (1960) is an old erosion 
surface truncating the Mehrten formation. The only 
remnant of this surface is the high, flat-topped ridge 
south of the Merced River. It is 1 or 2 miles in widtii 
and e.vtends 9 miles west of its easternmost preserved 
point southwest of Merced Falls. The surface slopes 
westward from a high point 750 feet in altitude at 4.'> to 
68 feet per mile. The pediment is mantled with a 20- to 
40-foot layer of gravel, consisting largely of 2- to 6- 
inch pebbles and cobbles of vein quartz, quartzite and 
other hard metamorphic rocks eroded from the foothills 
of the Sierra Nevada. The pebbles, except for the quartz 
and quartzite, are strongly weathered; some are com- 
pletel)' decomposed. 



1962] 



Merced Canyon and Yosemite V^alley 



29 



The China Hat pediment is clearly post-Mehrten, 
therefore \ounger tli;in mid-Pliocene. Studies of remnants 
of a similar surface in Stanislaus County indicate that the 
China Hat pediment pre-dates the Tuolumne Table 
Mountain lava flow which has been dated as late Plio- 
cene or earl\" Pleistocene b\' Taliaferro and Solari (1949). 
Therefore, the China Hat surface is assigned a late Plio- 
cene age. The relativel\' steep gradient of the China Hat 
pediment (fig. 3) suggests that it was, tilted during the 
last major uplift in the Sierra Nevada, and this also is 
in agreement w ith a late Pliocene age for the pediment. 
Hudson ( 1960) attempts to relate this land form to the 
Broad \'alley stage of Matthes (1930), but a direct con- 
nection cannot be found through the lower foothills. A 
correlation with the Mountain \'alley stage seems more 
probable, but more work is needed to establish the 
relationship. 

The surface of the China Hat gravel is red, acid (pH 
4.5-5.0), infertile soil with an iron-silica cemented hard- 
pan, an acid variant of the Redding soil series. It has a 
pronounced micro-relief of "hogwallows" or Mima 
mounds which are the result of the activity of pocket- 
gophers (Arkley 1954). 

North Merced Ped'mient and Gravels. On both sides 
of the Merced River and also south of the remnant of the 
China Hat pediment are remnants of another broad 
westward-sloping erosion surface mantled with 10 to 
20 feet of gravel similar to the China Hat gravel. This 
pediment has an altitude of 500 feet at .Merced Falls, of 
220 feet 18 miles to the west. Its gradient is much less 
than that of the China Hat pediment and decreases west- 
\\ard from 22 to 15 feet per mile. The pediment trun- 
cates the lone, Valley Springs, and Mehrten formations, 
and is thought to be of the same age as the Arroyo Seco 
pediment identified by Gale et al. ( 1939) 40 to 70 miles 
to the north. Soil surveys indicate that the surface is 
well represented in the intervening area. Gale considered 



the Arroyo Seco pediment to have formed after the last 
important uplift of the Sierra Nevada. The evidence in 
this area is in agreement with this idea. The soil formed 
on the North Merced gravel is the Redding series similar 
to that formed on the China Hat gravel, but is less acid 
and infertile. This soil is also mapped on the .\rroyo Seco 
gravel to the north. 

Turlock Lake Formation. The Turlock Lake forma- 
tion is a fan deposit of dominantlx' granitic alluvium cov- 
ering the westw ard extension of the North .Merced pedi- 
ment and resting directly on /Mehrten formation where 
the North Merced gravel had been eroded away between 
North Merced and Turlock Lake time. The uppermost 
la\'er of this formation is a coarse sand from 10 to 30 feet 
thick which has been weathered to form the Montpellier 
soil, with a thick red, sandy clay loam subsoil. This layer 
is underlain by a light gray, thinly laminated deposit of 
silt and very fine sand 10 to 40 or more feet in thickness. 
Logs of wells drilled in the Turlock Lake formation and 
a few outcroppings indicate that at least one buried red 
soil similar to the .Montpellier soil formed from coarse 
sand lies beneath the silty layer. This weathered sand is 
in turn underlain by silt\- material. This clear-cut re- 
peated sequence of silty material overlain by decidedly 
coarse sand ( 1-4 mm) was not brought out by Davis and 
Hall (1959) because of an unfortunate choice of type 
section. The relationship was quite evident during the 
soil-survey investigations. Shallow, weakly developed 
W'hitne)' soil of fine sand\- loam texture and Rocklin 
soil with a thin silica-cemented hardpan are found where 
erosion has exposed the silt\ material. The strongly 
weathered buried soil at the top of the lower sequence 
suggests that the Turlock Lake formation was actually 
laid down during two periods of deposition separated by 
a long interval of weathering. 

Within the Turlock Lake formation is a wedge of blue, 
diatomaceous lacustrine clav which was correlated bv 




Photo 4, Undulating surface of the 
Turlock Loke formation with Montpellier 
coarse sandy loom soil. Twelve miles 
northeast of Turlock. 




-Jt- 



■1h,' 



30 



California Division of Mines and Geology 



[Bull. 



(Tpch-south of Merced River) 

Parenthesis = geomorphic surface 
vertical exoggeration 10- 5X 







> 
Ul 



Miles 



FIGURE 3. Sdiematic cross-section (E-W) north of Merced River. 



Davis and Hail (1959) with the Corcoran clay of Frink 
and Kuess (1954) (tig. 3). Davis and Hall place this 
"blue clay" within the Riverbank formation (1959, pi. 4) 
ba.scd upon its position well below a "red clay" layer 100 
feet beneath the town of Turlock. This red layer was 
assumed to be the surface soil of the Riverbank forma- 
tion. However, the westward extension of the surface of 
the Turlock Lake formation coincides with this layer, 
while the Riverbank surface is actually encountered 
within 10 feet of the surface at Turlock. Therefore the 



Corcoran clay is a member of the Turlock Lake fori 
tion (fig. 3). 

Riverbank Formation. The Riverbank formation c 
ers the western extension of the Turlock Lake format 
and also extends eastward through it to the bedrock f( 
hills as a depositional river terrace in a valley entrenc 
through the older formations. This material is cle: 
younger than the Turlock Lake formation, and hi 
similar but less clear-cut lithologic sequence of fine m 
rial capped \\ ith coarse sand. The dominant soil on 




Photo 5. Riverbank surface with 
Joaquin hardpan soils in the 
ground; Turlock Lake surface on hi! 
in the background. View is north 
East Avenue, 8 miles east of Tui 



\ 



i2] 



Merced Canyon and Yosemite Valley 



31 



irse sand is a nioderarcly developed brown soil (tiie 
filing series) and on tiie fine-textured and stratified 
dbeds is a reddish-brown soil (the San Joaquin series) 
1 a brown soil (the Ahidera series), both with strongly 
nentcd silica hardpans. 

Modesto Formation. The Modesto formation buries 
; westward extension of the Riverbank formation and 
3 extends eastw ard to the foothills through the older 
niations as a depositional river terrace. The sequence 
laminated silty material capped by sand evident in 
: Turlock Lake formation is clearl\' repeated in the 
jdesto formation. These materials are onl\' slightl\' 
athered with either no subsoil development (Hanford 
I) or a very weakly developed subsoil (Greenfield 
j Dinuba soils). These soils are only slightly leached 
J are among the most fertile in the state. In the vicinity 
Turlock and south into Merced Count)', the sand has 
en reworked into dunes by wind action (figure 2). 

Alhivmm. The Merced River crosses the Pleistocene 
rmations in a flat-floored valley bordered by bluffs 

to 60 feet high. The valley is underlain by alluvium 
lich consists chiefly of granitic material of fine sandy- 
im texture. Where the river has cut down to the 
ehrten formation, the lower part of the alluvium con- 
ts of coarse gravel with pebbles and cobbles of granitic 
d metamorphic rocks. These gravels have been dredged 
r gold, resulting in the large piles of gravel tailings 
)ng the Merced River. The river has been widening 

floodplain in this area (figure 2). This suggests that 
new era of pediment cutting has begun in recent time. 

GEOMORPHIC HISTORY 

On the basis of the soil-survey investigations and the 
ork of Davis and Hall (1959) and Hudson (I960), the 
quence of events in the Turlock-Merced Falls area is 
terpreted to have been as follows: 

. Erosion of the Eocene lone formation into a surface 
of strong relief was followed by deposition of the 
Valley Springs and the Mehrten formations which 
flooded the valley during Alio-Pliocene times with 
a thickness of 800 feet at Merced Falls, and nearly 
1,500 feet in the center of the valley. 

I. The China Hat pediment was cut by the Alerced 
River or by local streams flowing across the Alehrten 
and was mantled with metamorphic gravel derived 
from the Sierra Nevada. 

5. Tilting and uplift of the Sierra Nevada extended 
westward into the valley increasing the gradient of 
the China Hat pediment from an estimated original 
20 feet per mile to the present 45 to 68 feet per mile. 
Accelerated stream flow resulted in the erosion of 
most of the pediment. 

4. As the streams approached a more stable gradient, 
the North Merced (Arroyo Seco?) pediment was 
formed and mantled with gravel derived from meta- 
morphic rocks. 



5. In late Pliocene oi early Pleistocene time, renewed 
erosion resulted in the destruction of a portion of 
the North Merced pediment and incision of the liver 
to a lower level. 

6. At the beginning of the glacial epoch, glacial flour 
in the form of silt and ver\' fine sand was deposited 
in the valley, forming the lower portion of the Tur- 
lock Lake formation. As glacial activity- died out, the 
coarse sand layer was deposited. This sequence of 
events was apparentTy repeated at least once in Tur- 
lock Lake time, with an intervening period of weath- 
ering of considerable duration. 

Corcoran Lake appears to have existed in the val- 
ley during a period of rapid subsidence west of Tur- 
lock during the deposition of the Turlock Lake for- 
mation. 

7. Following a period of erosion and further entrench- 
ment of the river, renewed deposition of glacial out- 
wash gave rise to the Riverbank formation. 

8. The Modesto formation was deposited in a similar 
fashion during the last glacial period. 

9. In recent time the river has entrenched the Pleisto- 
cene formations to a depth of 20 to 60 feet, and, like 
the Tuolumne River to the north, has widened its 
floodplain in the eastern portion of the area where 
the river bed rests upon the Mehrten bedrock. 

10. Subsidence of the valley west of a fault zone 12 to 
18 miles west of the foothills continued until late in 
the Pleistocene and resulted in tilting the Turlock 
Lake and older formations slightly westward. 

Rejerences 

Allen, V. T., 1929, The lone formation of California: Univ. 
California, Dept. Geo!. Sci. Bull., v. 18, p. 347-448. 

Arkley, R. J., 1954, Soils of eastern Merced County, California: 
Univ. California, College of Agr. Soil Survey no. 11, 174 p. 

Arkley, R. J., 1954, The origin of Mima mound (hogwallow) 
micro-relief in the far western states: Soil Sci. Soc. of America 
Proc, V. 18, no. 2, p. 195-199. 

Arkley, R. J., 1959. Soils of eastern Stanislaus County, Cali- 
fornia: Univ. California College of Agr. Soil Survey no. 13, 197 p. 

Axelrod, D. I., 1944, The Pliocene sequence in central Cali- 
fornia: Carnegie Inst. Washington, Pub. 553, p. 207-224. 

Davis, S. N., and Hall, F. R., 1959, Water quality of eastern 
Stanislaus and northern Merced Counties, California: Stanford 
Univ. Pubs., Geol. Sci., v. 6, no. 1, 112 p. 

Frink, J. W., and Kues. H. A., 1954, Corcoran clay, a Pleisto- 
cene lacustrine deposit in the San Joaquin X'alley, California: 
Am. Assoc. Petroleum Geologists Bull., v. 38, p. 2353-2571. 

Gale, H. S., Piper, A. M., and Thomas H. E., 1939, Geology 
and ground-water hydrology of the Mokelumne area, California: 
U. S. Geol. Survey Water-Supply Paper 780, 230 p. 

Hudson, Frank S., 1960, Post-Pliocene uplift of the Sierra Ne- 
vada. California: Geol. Soc. America Bull., v. 71, p. 1547-1575. 

Matthes, F. E., 1930, Geologic history of the Yosemite \'alley: 
U. S. Geol. Survey Prof. Paper 160, p. 1-37. 

Stirton, R. .\., and Goeriz, H. F., 1942, Fossil vertebrates from 
the superjacent deposits near Knights Ferry, California: Univ. 
California Dept. Geol. Sci. Bull., v. 26, p. 447-472. 

Taliaferro, N. L., and Solari, A. J., 1949, Geology of the 
Copperopolis quadrangle, California; California Div. Mines Bull. 
145 (map only). 




Photo 6. The Yosemite Falls in April. Photo by Mory Hil 



GEOMORPHOLOGY OF THE YOSEMITE VALLEY REGION, CALIFORNIA 



By CLYDE WAHRHAFTIG 

U.S. Geological Survey, Menio Pork, California 

and University of California, Berkeley, California 



INTRODUCTION 

Yosemite \'alley lies in the Sierra Nevada, a strongly 
asymmetric mountain range with a broad gentle western 
slope that rises about 13,000 feet in a distance of 60 miles 
(200 feet per mile) and an abrupt eastern escarpment 
that drops 5,000 to 6,500 feet in 5 to 8 miles (about 1,000 
feet per mile). The eastern escarpment is the result of 
displacement in later Tertiary and Quaternary time along 
a complex system of en echelon and step faults, inter- 
spersed with ramps and tilted structures (Lindgren, 1911, 
p. 39-49; Matthes, 1930, p. 24-25; P. C. Bateman, written 
communication, 1960; Rinehart and Ross, in press). The 
western slope, the back slope of the tilted fault block 
that is the Sierra Nevada, is a broad upland surface of 
moderate local relief— generally less than 2,500 feet— into 
which west-flowing streams have incised narrow steep- 
walled canyons 1,000 to 7,000 feet deep. Yosemite Valley 
is the headward segment of one of these canyons— that of 
the iMerced River— somewhat widened and greatly deep- 
ened by glacial erosion. 

The geomorphic history of the Yosemite Valley— and 
of the western slope of the Sierra Nevada on which it 
lies— involved: (1) the development of the upland sur- 
face; (2) tilting of that surface and carving of deep 
canyons; and (3) modification of the headward parts of 
these canyons by glaciers. 

UPLAND SURFACE OF THE SIERRA NEVADA 

The upland surface of the Sierra Nevada has had a 
long and complicated histor>-, which is as yet imperfectly 
understood. Much of the evidence for this history lies in 
the northern part of the range, where the crystalline bed- 
rock is overlain by a complex of Cretaceous and younger 
formations, called the Superjacent series by Lindgren and 
Turner (1894); other evidence lies in the Great Valley 
of California, where much of the material eroded from 
the Sierra Nevada was deposited. 

Parts of the upland surface may be as old as middle 
Cretaceous. It is probable that most of the erosion 
that led to the exposure of the Jurassic and Cretaceous 
granitic rocks of the Sierra Nevada was accomplished 
before Late Cretaceous time, for along the western margin 

* Publication authorized by Ihe Director of the U.S. Geological Survey. 



of the northern Sierra scattered patches of the fossil- 
iferous marine sandstone of the Upper Cretaceous Chico 
formation rest on an irregular surface of moderate relief 
cut on granite and other basement rocks. This sandstone 
dips southwestward at angles of 2° to 4° (Lindgren, 1911, 
p. 22-23; Allen, 1929, p. 368; Creely, 1955, p. 96-117). 
Apparently the shoreline of the Late Cretaceous sea 
coincided closely with the present edge of the Great 
Valley sediments, for the Cretaceous rocks north of 
Oroville appear to correlate with coarse river-channel 
deposits in the foothills immediately to the east (Creely, 
1955, p. 96-117). Wells 10 miles west of the edge of the 
foothills have encountered, at depths of a few thousand 
feet below the surface, 2,000 to 5,000 feet of rocks cor- 
related with the Chico formation (Creely, 1955, pi. 3; 
Piper and others, 1939, p. 87; Davis and Hall, 1959, pi. 3). 

The Eocene lone formation and its correlative 
rocks, the auriferous gravels, indicate with certainty 
that by early Tertiary time the granitic rocks of the 
Sierra Nevada had been exposed to about their present 
depth, and that the mountain range that resulted from 
the orogeny that accompanied the intrusion of the gran- 
ites had been reduced to a surface of moderate to low 
relief. The lone formation consists of interbedded quartz 
sand, kaolin clay, kaolinitic sandstone, and thin beds of 
lignite, that dip 2° to 5° westward along the base of the 
Sierra Nevada, and that rest on deeply weathered meta- 
morphic and igneous rocks, or on a deep-red to chalk- 
white tropical soil developed on these rocks (Allen, 
1929). The formation is predominantly continental, but 
at Merced Falls and elsewhere it contains intercalated 
sandv beds with marine fo.ssils, indicating that the shore- 
line of the sea was then close to the present base of the 
Sierra Nevada (Allen, 1929). At the time the lone ac- 
cumulated California had a humid tropical climate, in 
which laterit°s and kaolin were formed; at other times 
in the Cretaceous and Tertiary the climate of California 
appears to have been cooler, for montmorillonite rather 
than kaolinite is the common clay mineral of the other 
sedimentary formations, and feldspar is a common con- 
stituent of their sandstones. 

The auriferous gravels, which grade into the lone for- 
mation along the western margin of the foothills (Allen, 



(33) 



34 



California Division of Mines and Geology 



[Bull. 182 



;^»0mK^-' 




Photo 1. View of Yosemite Volley from the top of Half Dome, showing the <heer valley walls incised 
into the gently rolling upland surface of the Sierra Nevada. The cliff at the left is Glacier Point, its 
unjointed lower port composed of Ho If Dome quartz monzonite, and the upper port, broken by vertical 
joints, of Sentinel granodiorite. The Glacier Point hotel con be seen at the top of the cliff. Sentinel Dome 
is the white dome rising from the forest directly above the hotel. El Capiton is the cliff on the north wall 
of the valley opposite the Cothedrol Rocks. Washington Column is in the extreme lower right. Photo by 
U.S. Notionol Pork Service. 



1929, p. 395-402; Creely, 1955, p. 142-163; Lindgien, 
1911, p. 33-37, 40, 86-89), were presumably laid down 
as stream gravels in a series of well-defined channels or 
valleys that coursed down the western slope of the Sierra 
Nevada (fig. 1). Remnants of these deposits now lie on 
broad spurs between tlie present west-flowing streams, 
having been preserved beneath cappings of lava or vol- 
canic mudflows. Because much placer gold was obtained 
from these gravels during the last century by hydraulic 
and drift mining, we know a great deal about their ex- 
tent (Lindgren, 1911). The streams by which they were 
deposited no longer exist— the drainage pattern of that 
period was completely buried by later outpourings of 
volcanic debris— but we know that in general these 
streams flowed westward from sources near the present 
crest of the Sierra Nevada, or even farther east. The 
minimum local relief of the Sierra Nevada at that time 
in the present foothill region, as estimated from the pres- 
ent height of hills of bedrock above nearby segments 
of the Tertiary river channels, was about 1,500 to 2,500 
feet (Turner and Ransome, 1897 and 1898; Turner, 1894; 
Lindgren, 1911, p. 37-39, 197-199, 218-219). According 
to Lindgren (1911, p. 37-39) the Tertiary topography 
consisted of three elements: a discontinuous line of 
abrupt ridges 1,000 to 2,500 feet high, extending north- 
westward near the present foothills and held up b\' re- 
sistant metavolcanic rocks (Logtown Ridge, Gopher 
Ridge and Bear Alountain, Penon Blanco, and .Mount 
Bullion, fig. 1); northea.st of the.se ridges, a broad north- 



west-trending valley cut in the soft slates of the Mother 
Lode belt; and still farther northeast, a rolling plateau 
cut chiefly on granitic rocks, in which broad Eocene 
valle>'s were incised 800 to 2,000 feet. 

The Merced River is not bordered by Eocene river 
gravels, so we do not know what its profile was in early 
Tertiary time; the blanket of volcanic debris, which pre- 
served the auriferous gravels north of the Tuolumne 
River apparently did not extend as far south as tl^r basin 
of the Merced. Alatthes (1930, p. 31-50), hwvever, was 
able to reconstruct the early landscape along the upper 
Merced River by a stud_\' of the present topography. He 
noted that the upper reaches of lateral tributaries of the 
Merced River flow with gentle gradient in broad shallow- 
valleys to an abrupt nickpoint, from which their lower 
courses cascade to the Merced River, which is incised in 
a narrow canyon several hundred to 3,000 feet below the 
level of those valleys (fig. 2). By projecting the profiles 
of the tributaries downstream to the points where the\' 
joined the Merced, he was able to reconstruct the profile 
that the upper part of the river followed at this early 
stage in the Yosemite region; and Hudson (1960, p. 1554- 
1557) has now extended this profile down to the Great 
\'alley (fig. 3). The reconstructed profile defines a broad 
valley incised 800 to 1,500 feet below adjacent mountains 
such as Half Dome, and having about the same depth as 
the Eocene valleys farther north. It therefore seems rea- 
sonable to assume that most of the cutting of the broad 



19621 



Merced Canyon and Yosemite Valley 

I2I°00' 



35 



Area covered by fhe Mehr- 
ten. and Volley Springs 
formotio n, ond ourtferous 
gravels 



Edge of the lone formafioi 
olong border of Greof 
Valley 



Iniervolcanpc (Pliocene ?) 
river chonne Is 



Terf I or y fiver c honnels 
( pro bob I y Eocene ) 



Mountoins that stood obove 
ibe Eocene nver volleys 




After Jenkins. 1932 ( com- 
piled fr-Dm Lindgren.l9M 
pM,ond other sources) 
wr t h oddi f tons 
By Wahrhoftig 



I20®00" 



FIGURE 1. Tertiary river channels of the Sierra Nevada. 



36 



California Division of Mines and Geology 




-as/Ma 030H3n • 





" 1 \ Vt\ 


si 

!f| 
?l 

11 

^1 


IS ill 



3 



19621 



Merced Canyon and Yosemite Valley 



37 



10,000 




40 60 80 

FIGURE 3. Longitudinal profiles of the Merced River (after hludson, 1960, p. 1551). 



SEA LEVEL 



100 MILES 



valley took place during or before the deposition of the 
auriferous gravels in the northern Sierra Nevada.* 

The lone formation and the auriferous gravels are 
overlain by extensive bodies of white rhyolite tuff and 
associated gravel, which constitute what is called the 
Valley Springs formation (Allen, 1929, p. 410-419; Piper 
and others, 1939, p. 71-80). These are dated as probable 
late .Miocene (Davis and Hall, 1959, p. 9). The eruptions 
that produced the rhyolitic debris initiated the volcanic 
outpourings that buried the Eocene surface of the north- 
ern Sierra Nevada. The relief of this surface ma\' have 
been reduced by erosion of its more upstanding portions 
in Eocene, Oligocene, and Miocene time, but the surface 
itself could not have been uplifted and dissected at that 
period, for Eocene gravels of the rivers that drained 
that surface are preserved beneath the Miocene and 
Pliocene volcanic cover. 

The most extensive volcanic outpourings in the north- 
ern Sierra Nevada occurred in late Miocene or early 
Pliocene time (Piper and others, 1939, p. 61-71; Lind- 
gren, 1911, p. 31-33). These constitute the Mehrten for- 
mation of Miocene and Pliocene age, a deposit of an- 
desitic volcanic mudflows interbedded with conglomerate 
and sandstone that consist mainly of andesitic debris. 
The Mehrten formation has been correlated with the 
Neroly formation of the San Pablo group in the north- 
ern Diablo Range (Davis and Hall, 1959, p. 9). i'he 

• Hudson, on the other hand (1960, p. 1555), places the Broad Valley stage 
of downcutting in the late Pliocene. Piper and others (1939, p. 78) 
correlate it with the Valley Springs formation of late Miocene (?) age. 



sources of the volcanics in the Mehrten are breccia dikes 
along the present crest of the Sierra, from which they 
were largel>- extruded in the form of breccia (Curtis, 
1954). The eruptions buried the northern part of the 
range under a volcanic blanket that was about 1,500 feet 
thick at the west foot of the mountains apd 4,000 or 
5,000 feet thick along the crest of the range, and that 
completely obliterated the old drainage of the Sierra 
Nevada. A new westward-flowing drainage was then de- 
veloped upon the constructional surface of the volcanic 
rocks (tig. 1 ). The southernmost of the mudflows of the 
Mehrten formation followed the course of the Tuolumne 
River (Lindgren, 191 1, p. 30-32, 218-219). None reached 
the Merced drainage, so the .Merced today is probably 
in about the same place that it was in early Tertiary 
time.* 

EROSION OF THE MERCED CANYON 

Matthes (1930, p. 31-50), in his restoration of the 
ancient profiles of the Merced River, recognized two 
stages in the cutting of the .Merced canyon, which fol- 
lowed the Broad Valley stage. His determination of the 
profiles of these stages was based on the same technique 
that he used for the Broad Valley stage, but here he used 
intermediate .segments of the tributary streams, with 
nickpoints at their upper and lower ends (see figs. 2 and 
4 for examples of the profiles). During the earlier of 

* For the distribution of the Mehrten formation beneath the Great Valley, 
and its probable influence on the lower course of the Merced River, see 
tlie paper by R. J. Arkley in this guidebook. 



38 



California Division of Mines and Geology 



[Bull. 182 



El C opilo n 




and 
'alley 



FIGURE 4. Projection, on a north-south plane, of the cross-profile of Yosemite Valley between El Capitan and Cathedral Rocks {solid line) 
of the profiles of Ribbon and Bridolveil Creeks (dashed lines). Elevations of Merced River of the Brood Valley stage (6700'). Mountain V , 
stage (5800'), and Canyon stage (4600') of downcutting, and profiles on which they are based (dot-dashed lines) are from Matthes (1930, fig. 
12, p. 42). Preglocial cross-profile of the valley (dotted line) from Matthes (1930, fig. 27, p. 87). Dota for bedrock surface from Gutenberg 
Buwolda, and Sharp (1956, fig. 8, p. 1072). Horizontal and vertical scales are the same. 



these stages, wliich he called the Mountain Valley stage, 
the river flowed in a fairly rugged narrow valley, 1,600 to 
2,200 feet below the surrounding uplands and about 600 
feet below the level of the Broad Valley stage. The later 
stage, which he called the Can\on stage, determined the 
present level of the Merced River in the unglaciated 
canyon below El Portal and also determined the original 
preglacial profile in the glaciated part of the can\'on 
above El Portal. The cutting of the China Hat and 
Arroyo Seco pediments, described by R. J. Arkley in 
this guidebook, may have occurred during the Moun- 
tain Valley and Canyon stages. 



Photo 2. View down the lower Merced Canyon from the top of El 
Capitan. The canyon is incised in valleys of the Broad-Valley and Moun- 
tain-Valley stages of canyon cutting; these stages are indicated by the 
gentle slopes descending to brood flats from the surrounding level ridge 
tops. Photo by F. C. Calkins, U.S. Geological Survey. 




UPLIFT AND TILTING OF THE SIERRA NEVADA 

The Merced River and other west-flowing streams of 
the Sierra Nevada cut their deep canyons because the 
range as a whole was uplifted and tilted to the west. 
Estimates made by several men of the amount, nature, 
and time of this deformation differ widely, partly, no 
doubt, because they have been based on different kinds 
of evidence. 

The direct estimates involve determination of the 
amount of deformation of the bedrock profiles of the 
early Tertiary rivers. The present deformed profiles, de- 
termined either from the slope and altitude of channel 
segments beneath the gold-bearing gravels or by recon- 
struction from existing topography, have been deter- 
mined with fair accuracy. The original profiles, from 
which deformation to the present is measured, must in 
large part be- reconstructed from our knowledge of the 
factors that control the gradients of debris-carrying 
streams. Recent investigations on this general problem 
(Leopold and Maddock, 1953, p. 48-51; Leopold and 
VVolman, 1957; Hack, 1957, p. 53-74) indicate that the 
auriferous gravels will have to be studied further before 
these original profiles can be reconstructed with a pre- 
cision that is up to modern standards. 

Indirect estimates of the deformation include (1) in- 
ferences drawn from fossil floras regarding former cli- 
mates, (2) measurements of displacement on the faults 
along the east side of the range, and (3) measurement, 
through subsurface investigations, of the tilting of sedi- 
mentary rocks in the eastern half of the Great Valley. 
The bearing of the second and third lines of evidence 
is open to question, for neither the faulting on the east 
side of the range nor the tilting of the sediments in the 
Great Valley was necessarily quite contemporaneous 
with the uplift and tilting of the mountains between 
them, or of the required magnitude; their aggregate 
effect ma\' have been either greater or less than that. . 



19621 



Mkrcf.i) Canyon and Yosk.mite Vallky 



39 



This section, therefore, will nierel>- describe the views 
of several writers regarding the amount and time of this 
deformation without attempting to choose between 

them. 

Lindgren (1911, p. 46) concluded that the Sierra Ne- 
vada had been tilted essentially as a rigid block, although 
he recognized several faults with as much as 500 feet 
of displacement cutting the auriferous gravels on its 
western slope. He determined (1900, p. 10) that the 
tilting at the latitude of Donner Pass amounted to be- 
tween 60 and 70 feet per mile, which corresponds to 
an uplift at the crest, now 7, .500 to 8,000 feet above sea 
level, of about 4,500 or 5,000 feet. In his opinion this 
uplift took place during the eruption of the volcanics 
which constitute the Alehrten formation, that is, in Mio- 
cene and Pliocene time. 

Lindgren based his calculations largely on the bedrock 
profile "of the Tertiary Yuba River, which apparently 
flowed southwestward from the crest of the range for 
about 35 miles, then turned to flow northwestward, 
parallel to the presumed axis of tilting, for about 45 
miles, and turned again to flow southwestward for about 
30 miles to a mouth near the present site of Wheatland, 
at the east side of the Sacramento Vallew The gradients 
of the reconstructed southwesterly reaches are 80 to 100 
feet per mile, and those of the northwesterly reaches 
20 to 30 feet per mile (Lindgren, 1911, pi. 10). Lindgren 
assumed that the northwest reach had not been tilted and 
therefore had the average gradient of the stream before 
tilting; he estimated the tilting, b>- subtraction, to be 
between 60 and 70 feet per mile. 

Matthes (1930, p. 44), using a similar method on the 
reconstructed profiles of the iMerced River, but with 
more assumptions since there were no long northwest- 
flowing reaches of that stream, concluded that total tilt- 
ing since the Broad \'alley stage, which he placed at 
the end of the Miocene, had amounted to 72 feet per 
mile, and that the total uplift of the crest of the Sierra 
Nevada in Yosemite National Park had been 9,000 feet. 
He estimated that 6,000 feet of the uplift, and tilting of 
60 feet per mile, had occurred since the end of the 
Mountain Valley stage, which he placed at the end of 
the Pliocene. The floor of Yosemite Yalley was, he in- 
ferred, 800 feet above sea level during the Broad Valley 
stage and 1,800 feet above sea level in the Mountain 
Vallev stage. 

Hudson (1955, 1960) has calculated the amount of 
tilting in the Sierra Nevada through use of the present 
gradients of differently directed adjacent reaches of the 
Tertiary streams, taken in groups of three. He assumes 
that the average gradient in all three adjacent reaches 
was originally the same, and computes the amount and 
direction of tilting necessary to produce the present 
gradients by solving three equations for three unknowns: 
the original gradient, the amount of tilting, and the di- 
rection of tilting. From these computations he has con- 
cluded that the Sierra Nevada was deformed not as a 
rigid block but in a very complex fashion, and, further- 
more, that the total uplift at the crest has amounted to 



less than 2,000 feet in the Lake Tahoe-Donner Pass re- 
gion and less than 4,000 feet in the Yosemite National 
Park region. From correlation of the Broad \'alley stage 
of the iMerced River with the China Hat pediment (Ark- 
le\, this guidebook) Hudson (1960, p. 1559-1560) places 
the uplift and deformation entircl\- within post-middle- 
Pliocene time. 

A.\elrod (1957) has shown that floras of Miocene and 
Pliocene age from the Valley Springs formation, the 
Mehrten formation, and their equivalents in west-central 
Nevada indicate similar climates, and that they do not 
indicate the existence of a rain shadow, such as the one 
now present on the east side of the Sierra Nevada. From 
his estimates of the range in altitude of the floras, he 
concludes that the mountains near Lake Tahoe could 
not have been higher than 2,000 or 2,500 feet in Miocene 
and Pliocene time, and therefore that the cre.st of the 
range has been uplifted 5,000 to 6,500 feet since then. 
There seems at present to be no way of reconciling 
Axelrod's views with those of Hudson. 

Hoots, Bear, and Kleinpell (1954, pi. 6) show that in 
the Chowchilla-Merced area of the Great Valley, just 
south of the Merced River, the base of the Pliocene sedi- 
ments slopes about 100 feet per mile west-southwest- 
ward. In their cross sections, the base of the Pliocene 
and the base of the Eocene are essentially parallel. If 
the evidence in the Great Valley can be projected east- 
ward to the Sierra Nevada, one may infer from it that 
there was no tilting of the range from middle Eocene 
to Pliocene time, and tilting on the order of 100 feet per 
mile since Pliocene time. As stated above, however, it 
has not \'et been proved that tilting in the Sierra Nevada 
was contemporaneous w ith that in the Great \'alley' or 
equal to it in amount. 

Somewhat similar evidence is found between Mono 
Lake and Bishop on the east side of the Sierra Nevada. 
According to Blackwelder (1931, p. 904) and C. D. 
Rinehart and D. C. Ross (oral communication), there 
has been about 3,000 feet of displacement on faults on 
the east side of the Sierra Nevada since the McGee stage 
of glaciation. Y^ertical displacements at least as great as 
this in the region immediately east of the fault have 
resulted in deformation of the Bishop tuff of Gilbert 
(1938), a glowing-avalanche deposit that rests on glacial 
till at least as old as the Sherwin stage; and that has been 
dated by the potassium-argon method as about one mil- 
lion years old (Evernden and others, 1957). This tuff, 
whose upper surface must originally have been nearly 
horizontal, now ranges in altitude from about 8,000 feet 
just south of Mono Lake to 3,300 feet in the alluvial 
sediments south of Bishop (Gilbert, 1938; Bateman, 1956, 
pi. 1, 2 and oral communication, 1957; Rinehart and 
Ross, 1957, and oral communication, 1960). The defor- 
mation just east of the Sierra Nevada may have taken 
place long after the tilting and uplift of the range was 
completed; it may represent the collapse of the Owens 
\'al ley-Mono Lake block, which may have involved 
either greater or less displacement than the uplift of 
the range. 



40 



California Division of Mines and Geology 



[Bull. 182 




^ ° 



ft: <r: :.. 







.< ^ 






< 
< 



c 
o 



o 
o 



^ 


o 


\ 


^ 


> 




° f 


<u 


>s 


•♦- 


D> 


/ 


,^ 


O 


• 


o i 


— 


; ^ 


o 


D 




o 




• 


E \ 


o 






•*~ 


/ 




a> 


• 


: > 


o 


■*- 


tn 


1 


^_ 


u 


• 




: 


1 > 


c 






c 




• • 


■*" 




' 


Cl> 


c 


1 


ID 




• • 


c 


o 


"4- 


-«- 


10 


\ 


0) 


• 
• 


0} 




o 


K 




X 


c 


• 


<= 


' E 




a> 




\ 


4) 


a> 


• 




c 


£ 






E 


o 




E 
o 


o: 


o> 


3 










V. 




w 


e 






3 

E 






Q. 




o 



^■i 



o >■ 



1 



o 
'5 

a 






CO 



o 
If) 

01 



3 



E 
o 



1962 



Mf.rcf.d Canyon and Yosf.mitk \'ai.ley 



41 



While the dates arrived at by these indirect means 
thus differ considerably, the writers quoted all place the 
niaior uplift and tilting of the range in late Pliocene and 
earh" Pleistocene time. In regard to the total amount of 
uplift, their estimates tend to agree more nearl\' with 
those of Lindgren and Matthes than with those of 
Hudson. 

GLACIAL EROSION OF THE YOSEMITE VALLEY 

In the summer-dry /Mediterranean climate of Califor- 
nia, the present snowfall on even the highest areas in tiie 
Yosemite National Park is barely sufficient to maintain 
a few small cirque glaciers on the shaded north sides of 
12,000- to 13,000-foot peaks. During the maximum extent 
of Pleistocene glaciation, however, snov\ accumulated on 
ridges no more than 9,.'>00 feet high, and, except on slopes 
that were too steep, the surfaces above that altitude \\ ere 
blanketed with snow and ice. The glaciers fed by these 
snowfields descended about 25 miles down the Merced 
Canyon to El Portal and be>ond, reaching altitudes as 
low as 2,000 or even 1,700 feet; they also went down 
the Tuolumne Can\on to like altitudes and for a distance 
of about 40 miles. At its maximum extent the Merced 
Glacier completely filled Yosemite \'alle\' and spilled 
slighth' over the rim onto the uplands on either side 
(Alattiies, 1930, p. 50-102) (fig. 5). 

Glaciers advanced down the .Merced Can>()n at least 
three and perhaps four times. During the later stages of 
glaciation, ice only partially filled the Yosemite \'alley, 
and glaciers of the latest major stage, the Tioga, did not 
even reach the valle\' but stopped just short of Nevada 
Falls (Blackwelder. 1931, p. 907). Little remains of the 
oldest moraines except scattered boulders of quartzite, 
schist, and other highly resistant rocks; any boulders of 
intrusive rocks that can still be identified are mostl\- de- 



composed to griiss (granite sand), and the bedrock sur- 
faces on the uphuuls that w ere glaciated have weathered 
down several inches or several feet, leaving glacial er- 
ratics perched on pedestals of disintegrated bedrock. The 
deposits of the youngest glaciation, on the other hand, 
are quite fresh, and large areas of bedrock that were 
covered by the latest glacier are so little weathered that 
the glacial polish on them still glistens in the sunlight. 
The stages of glaciation of the Yosemite region that have 
been recognized by various authors are shown in table 1. 
The glaciers profoundlx' changed the shape of the 
original Merced Canyon. They ground off projecting 
spurs, planed back parts of the walls to vertical cliffs, 
gouged a deep basin in the valley floor, and gave the 
valley the characteristic U-shaped cross-profile. .Accord- 
ing to Matthes (1930, p. 89-98) the glaciers eroded 
mainly by quarrying and plucking, joint blocks being 
lifted into the glacier by freezing of water in the cracks 
between them. Grinding and polishing appear to have 
plaved a relatively minor part in the erosion of the 
valle\'. Matthes has shown that the greatest deepening 
and widening of the valley occurred where the rocks 
were most closely jointed, and that many of the massive, 
almost unjointed monoliths were eroded slowly or not at 
all. As was pointed out in the paper in this guidebook 
by Calkins and Peck, the various intrusive rocks in which 
the canyon w as cut differ widely in the extent to which 
they are jointed, and the more mafic rocks are in general 
more closel\' jointed than the more siliceous rocks. Joints 
are indeed scarcest in the Half Dome quartz monzonite, 
which is less siliceous than the granites in £1 Capitan, but 
this may be because it forms an exceptionally large in- 
trusive bodv. Some depressions were eroded, however, 
along fissures that cut indifferently across all kinds of 
rocks, even across Half Dome quartz monzonite. Where 




Photo 3. Glacial polish, glacial striae, 
and erratics on Moraine Dome, Little 
Yosemite Valley. Photo by Ralph H. Ander- 
son, U.S. National Pork Service. 



42 



California Division of Mines and Geology 

Table 1. Correlation and characters of glacial stages in Yose?mte Valley. 



[Bull. 182 



Glacial stages 


Position of ice terminus 
on Merced River 


Weathering (after Birman, 1957, p. 55) 


Matthes 
(1930) 


Blaclcwelder 
(1931, p. 907-908) 


Birman 
(1957, p. 220-222) 


Till 


Bedrock 


"Little ice age" (Matthes, 
1942, p. 214) (2,000 
B.C. to present 






Matthes 
Recess Peak 
Hilgard(.?) 


Cirques on Mt. Lyell 
(includes modern gla- 
ciers). 


Unweathered (unstable 
on most recent mo- 
raines). 


(Glacial polish completely 
preserved.) 




Tioga 




Tioga 


Head of Nevada Falls. 


.Moraines fresh, undis- 
sected. 


Not much weathering of 
bedrock; much polish 
preserved. 


Wisconsin 


Tahoe 




Ciraveyard 


Foot of Bridalveil Falls. 


Moraines moderately 
weathered; dissected by 
gullies 10 to 40 ft. deep. 


Little or no erosion of 
bedrock; polish pre- 
served only on aplite 
pegmatite, and fine 
volcanics. 


El Portal 


Sherwin? 




Tahoe 


Few miles west of F.I 
Portal. 


Moraines weathered to 
10 ft.; dissected by 
gullies 10 to 70 ft. deep. 


Boulders perched 1 in.; 
pegmatite dikes pro- 
trude 3 in. Much 
weathering on surface; 


Glacier Point 






Sherwin 




Bedrock weathered to 6 
in. below surface and 
weathering pits 6 in. 
deep. 


weathering pits rare or 
shallow. 

Most boulders weathered; 
no morainal forms pre- 
served. 




McGee 




McGee 


(Not recognized on west 
slope of Sierra.) 


Rock pedestals and mafic 
inclusions more than 6 
in. high. 





the Merced Canyon was cut into rocks in which the joints 
are wideh' spaced, as it is in the gorge between El Portal 
and the mouth of Cascade Creek, its cross profile is still 
essentially V-shaped, although morainal deposits on the 
walls of the gorge show that it was occupied by the 



earlier ice. The relatively even longitudinal profile of the 
preglacial Merced River was converted by glacial pluck- 
ing into a series of giant steps separating glaciall)' scoured 
basins, the nearly vertical steps occurring where massive 
unjointed rock crosses the can\'on. 



FIGURE 6. Cross-profile of Yosemite Valley between North Dome and Glacier Point (after Matthes, 1930, p. 86, with corrections from Gutenberg 
and others, 1956, fig. 8). 



NE 



SW 



8000 



30 00 



Highest level of Yosemite Giocler m earlier stoges 
North Dome Glacier Point 




I MILE 



2000 + 



1962 



Merced Canyon and Yosemite Valley 



43 



The proportionate amounts of erosion accomplished 
in tiic preglacial can>on stage and during giaciation are 
indicated b\" the cross-profiles, (figs. 4 and 6), in which 
iMatthes" reconstructions of the can>on stage of the 
Merced River are superimposed on the present bedrock 
profile of the Yosemite \'alley. The glacier cut even 
deeper, however, in the valley itself, than Matthes 
imagined, for seismic exploration reported by Guten- 
berg, Buwalda, and Sharp (1956) has shown that the bed- 
rock surface beneath the valle\' floor is generally more 
than 1,000 feet below the surface of the alluvium and is 
fully 2,000 feet below it just south of the Ahwahnee 
Hotel. According to these authors (p. 1074) the remark- 
ably great deepening at this place was due to the great 
thickness and veIocit\' of the ice at the junction of the 
Tenaya and Merced glaciers, and to the fact that glacial 
quarrying was made possible b\' sheet spalling in other- 
wise massive rock, due to net decrease of load through 
erosion, even under several thousand feet of ice. 

The glaciers deposited extensive systems of morainal 
ridges and discontinuous patches of till and erratics on 
the uplands bordering the valley (Matthes, 1930, pi. 29). 
It has generally been possible to trace the outer limits of 
the various glacial stages by following the morainal de- 
posits along the rim. There are only three places where 
moraines can no\\' be seen on the valley floor or in the 
canyon of the .Merced below it. This is partly because 
some other moraines were washed away by swifth' flow- 
ing glacial meltwater in the narrow Merced gorge, but 
partly, perhaps, because some of them are buried under 
lake sediments on the valley floor. The moraines that we 
can see, however, are important for the interpretation of 
the history of the Yosemite X'alley. 

A set of six moraines between Bridal veil Meadow and 
El Capitan Bridge protrudes above the lake sediments and 
may have been in part responsible for holding in the lake. 
These are taken by Matthes to mark the maximum ad- 
vance of the Wisconsin ice in the \'alley (the Tahoe 
glaciatiOn of Blackwelder's terminolog\). A similar set 
of moraines encloses the lower end of Little Yosemite 
Valley, about half a mile above Nevada Falls. According 
to Blackwelder (1931, p. 907-908), these are much fresher 
than the moraines at Bridalveil .Meadow and mark the 
maximum extent on the Merced River of the ice of his 
Tioga stage. A third set of moraines, as yet unexplained, 
may be seen at the head of the Yosemite \'alley near the 
junction of Teneya Creek and the Merced River. 

During the glacial advances, the Merced River, over- 
loaded with great quantities of coarse debris delivered to 
it by the Merced Glacier, built an extensive valley train 
on the floor of its canyon, aggrading its bed several score 
of feet. Remnants of this valley train can be recognized 
as patches of outwash gravel preserved against the walls 
of its canyon downstream from El Portal. The outwash 
gravel stands out from the prevailing dark-hued meta- 
morphic rocks of the canyon walls because of the light 
tone of the granitic boulders of which it is composed. 



Outwash gravel of at least two ages can be recognized: 
in the older gravel the large granodiorite boulders and 
nearly all the small granitic pebbles have decomposed to 
sand; in the \oungcr gravel, which in some places over- 
lies the older, the pebbles and boulders are still relatively 
fresh. The fine sand and silt in the glacial debris were 
carried out to the floor of the Great \'alle\', where thev 
constitute the Turlock Lake, Riverbank, and Modesto 
formations of Davis and Hall (1959) described by R. J. 
Arkley in this guidebook. 

Gutenberg, Buwalda, and Sharp (1956, p. 1064-1069) 
recognized three layers in the fill beneath Yosemite Val- 
ley. The lowest la\er, about 1,000 feet thick, has a seis- 
mic velocit>' of 3 kilometers per second, and they regard 
it as a lake deposit of the Glacier Point stage or older 
(p. 1077). The intermediate layer, which is about 600 
feet thick and has a seismic velocity of 2 Yz kilometers per 
second, they regard as a glacial-lake deposit of the El 
Portal stage. The upper layer, which is about 300 feet 
thick and has a seismic velocity of 1 '4 kilometers per 
second, extends dow nvalley only as far as Bridalveil Fall. 
They regard this as a glacial-lake deposit of Alatthes' 
Wisconsin stage. / 

POSTGLACIAL AND INTERGLACIAL SCULPTURING 
OF THE CLIFFS 

Postglacial and interglacial sculpturing of the clifi^s has 
taken place by two different but somewhat allied pro- 
cesses: by pr\-ing out of roughly equant blocks bounded 
by joints (which are more numerous in some rocks than 
in others) and by spalling of huge curved sheets in joint- 
free rocks, thought to have resulted from expansion due 
to release of the weight of overlving rocks (Matthes, 
1930, p. 114-117; Jahns, 1943). the^ most thoroughly 
jointed rocks of the valley are the diorite near its western 
end, and cliffs of this rock have disintegrated to form the 
massive talus aprons called the Rockslides, across from 
Wawona Tunnel. The least jointed rock is the quartz 
monzonite exposed in Half Dome and other domes over- 
looking the head of the valle_\', which are almost literally 
monolithic and whose tops were shaped by spalling. Sen- 
tinel Dome is the only one near the valley that consists 
of a different rock, namely the El Capitan granite. The 
domes achieved their rounded forms long before the ad- 
vent of giaciation; they are not roches Diotitonees, for 
they were never overridden by ice. 

The diorite of the Rockslides and the quartz monzonite 
are at opposite extremes in character of jointing. The 
valley walls berween them consist of rocks that are in- 
termediate in this respect, as has been pointed out in the 
paper b\' Calkins and Peck in this guidebook. The 
taluses, three of which are of great size, were probably 
formed mostly in postglacial time, by the breaking down 
of diorite and other closely jointed rocks that are now 
for the most part covered with talus. Those east of the 
Cathedral Rocks and east of El Capitan contain some 
glacial debris. 



44 



California Division of Mines and Geology 



[Bull. 182 




a o> 



c o 

a. a- 



^1 



1962 



Merced Canyon and Yosemite Valley 



45 









-* ^ 




Phofo 4. Northeast side of Half Dome, 
showing exfoliation on a gigantic scale. 
The arrow points to two men half way up 
the slope. In the foreground is an old 
shell disintegrating to granitic sand. Photo 
by F. C. Calkins, U.S. Geological Survey. 



A sheeting that is flat rather than curved, induced ap- 
parently by wetting and drying or freezing and thawing, 
and controlled in part by joints, has led to the steepening 
of many of the cliffs of sparsely jointed rocks over which 
the waterfalls plunge, converting what may once have 
been foaming cascades to the free-plunging falls we see 
today. Alost of the falls, particularly at their lower ends, 
are in recesses that could not have been carved by ice 
moving down the A4erced River (note particularly Rib- 
bon Fall and the base of Bridalveil Fall). The walls of 
these recesses are marked by closel\' spaced fractures 
which indicate that slabs of granite, parallel to the rock 
surface, have broken awav from beneath the waterfalls. 
These fractures are particularly common near the lower 
ends of the falls. Talus cones, also, are piled beneath 
some of these falls, the debris of the fallen slabs. An 
example of this process, steepening a cascade to form a 
vertical waterfall, can be seen on the west side of Sen- 
tinel Rock. 

References 

Allen, V. T., 1929, The lone formation of California; Univ. of 
California Dept. Geol. Sci. Bull., v. 18, pp. 347-448. 

Axelrod, D. I., 1957, Late Tertiary floras and the Sierra Nevada 
uplift: Geol. Soc. America Bull., v. 68. pp. 19-46. 

Batcman, P. C. 19.';6, Economic geology of the Bishop tungsten 
district, California: California Div. .Mines Special Rept. 47, 87 pp. 

Birman, J. H., 1957, Glacial geology of the upper San Joaquin 
drainage. Sierra Nevada, California: Univ. of California (Los An- 
geles), Ph. D. thesis, 237 p. 

Blackwelder, Eliot, 1931, Pleistocene glaciation in the Sierra 
Nevada and Basin Ranges: Geol. Soc. America Bull., v. 42, pp. 
865-922. 

Creely, R. S., 1955, Geology of the Oroville quadrangle, Cali- 
fornia: Univ. of California (Berkeley), Ph. D. thesis. 



Curtis, G. H., 1954, Mode of origin of pyroclastic debris in the 
Mehrten formation of the Sierra Nevada: Univ. of California 
Pubs. Geol. Sci., v. 29, pp. 453-502. 

Davis, S. N., and Hall, F. R.. 1959, Water quality of eastern 
Stanislaus and northern Merced Counties, California: Stanford 
Univ. Pubs. Geol. Sci., v. 6, pp. 1-112. 

Evernden, J. F., Curtis, G. H., and Kistler, Ronald, 1957, Potas- 
sium-argon dating of Pleistocene volcanics: Quaternaria, v. 4, pp. 
1-5. 

Gilbert, C. iM., 1938, Welded tuff in eastern California: Geol. 
Soc. America Bull., v. 49, pp. 1829-1862. 

Gutenberg, Beno, Buwalda, J. P., and Sharp, R. P., 1956, Seismic 
explorations on the floor of Yosemite Valley, California: Geol. 
Soc. America Bull., v. 67, pp. 1051-1078. 

Hack, J. T., 1957, Studies of longitudinal stream profiles in 
V'irginia and Maryland: U.S. Geol. Survey Prof. Paper 294-B, 
pp. 45-97. 

Hoots, H. W., Bear, T. L., and Kleinpell, W. D., 1954, Geo- 
logical summary of the San Joaquin Valley, California, in Chap. 2 
of Jahns, R. H., ed.. Geology of Southern California: California 
Div. Mines Bull. 170, Chap. 2, pp. 113-129. 

Hudson, F. S. 1955, Measurement of the deformation of the 
Sierra Nevada, California, since middle Eocene: Geol. Soc. 
.\merica Bull., v. 66, pp. 835-870. 

Hudson, F. S., 1960, Post-Pliocene uplift of the Sierra Nevada, 
California: Geol. Soc. .\merica Bull., v. 71, pp. 1547-1575. 

Jahns, R. H., 1943, Sheet structure in granites, its origin and use 
as a measure of glacial erosion in New England: Jour. Geology, 
V. 51, pp. 71-98. 

Jenkins, O. P., 1932, Report accompanying geologic map of 
northern Sierra Nevada: California Div. Mines Rept. 28, pp. 279- 
298, and geologic map. 

Leopold, L. B., and iMaddock, Thomas, Jr., 1953, The hydraulic 
geometry of stream channels and some physiographic implications: 
U.S. Geol. Survey Prof. Paper 252, 57 pp. 

Leopold, L. B., and \Volman, M. G., 1957, River channel 
patterns— braided, meandering, and straight: U.S. Geol. Survey 
Prof. Paper 282-B, pp. 39-85. 

Lindgren, Waldemar, 1900, Description of the Colfax quad- 
rangle, Colfax, California: U.S. Geol. Survey Geol. Atlas, Folfo 66. 



46 



California Dimsion ok .Mini s and Gk()I.O(;y 



[Bull. 182 



Lindgren, W'aldemar, 1911, The Tertiary gravels of the Sierra 
Nevada of California; U.S. Geol. Survey Prof. Paper 73, 226 pp. 

Lindgren, W'aldemar, and Turner, H. \\'., 1894, Description of 
the gold l)clt, hi Description of the Placerville quadrangle I Cali- 
fornia]: U.S. Geol. Survey Geol. Atlas, Folio 3. 

Matthes, F. E., 1930, Geologic history of the Yoscmitc \'alley: 
U.S. Geol. Survey Prof. Paper 160, 137 pp. 

Matthes, F. E., 1942, Glaciers, in Meinzer, O. E., ed.. Physics of 
the earth, Pt. 9, Hydrology: New York, iVlcGraw Hill Book Co., 
pp. 149-219. 

Piper, A. .M., Gale, H. S., Thomas, H. E., and Robinson, T. W., 
Jr., 1939, Geology and ground-water hydrology of the Mokelumnc 
area, California: U.S.- Geol. Surve>' \\'ater-Supply Paper 78U, 
230 pp. 



Rinehart, C. D., and Ross, D. C, 1957, Geology of the Casa 
Diablo Mountain quadrangle, California: U. S. Geol. Survey Geol. 
Quad. .Map GQ-99. 

Rinehart, C. D., in press. Geology and mineral deposits of Mt. 
.Morrison quadrangle, California: U.S. Geol. Survey Prof. Paper 
385. 

Turner, H. W'., 1894, Description of the Jackson quadrangle 
I Sierra Nevada, (California I: U.S. Geol. Survey Geol. Atlas, Folio 
II, 6 p. 

Turner, H. W'., and Ransome, F. L., 1897. Description of the 
Sonora quadrangle I California I : U.S. Geol. Survey Geol. Atlas, 
Folio 41, 5 p. 

Turner, H. \V., 1898, Description of the Big Trees quadrangle 
ICalifornial: U.S. Geol. Survey Geol. Atlas, Folio 51, 8 p. 



Part ll-ROAD LOGS 



FROM HAYWARD THROUGH YOSEMiTE VALLEY VIA TRACY, 
PATTERSON, TURLOCK, AND MERCED FALLS 



CONTENTS 

Page 

Road log 1— U.S. Highway 50 from Hay ward to Tracy, California, by Holly 
C. Wagner and Frederic R. Kelley 51 

Road log 2— TracN- to El Portal, via Patterson and Turlock, California, by 
Clyde Wahrhaftig and L. D. Clark -- 55 

Road log 3— El Portal to Wawona and a circuit of Yosemite Valle\-, Cali- 
fornia, by Dallas L. Peck, Clyde Wahrhaftig, and Frank C. Calkins 61 

PLATES 

Plate 1. Geologic map and section of the southern part of the 
western Sierra Nevada metamorphic belt- In pocket 

Plate 2. Guide map to Highway 50, Hayward to Tracy, California In pocket 



Frontispiece, Part II, opposite. Winter, Yosemite Valley, California, by 
Ansel Adams. 



(48) 




i^K 



;j|fc-jj J*-! ; 










^ 



ii 



'4jk' 




A^ 




...tuM '•^- -SS&L 



.-star- 



,.,.«* " 






'<> 




ROAD LOG 1 



U.S. HIGHWAY 50 FROM HAYWARD TO TRACY, CALIFORNIA * 



By HOLLY C. WAGNER 

U.S. Geological Survey, Menio Park, California 

and FREDERIC R. KELLEY 

California Division of Mines and Geology 

San Francisco, California 



Plate 2, Guide map to Hig/iwoy 50, Hayward to Trocy, California, accompanies this road log. 



Between San Francisco and Hayward the field trip route follows U. S. Highway 50 across the Bay Bridge and 
turns onto the Nimitz Freewa\- (State Route 17), by-passing Oakland. As the Bay Bridge is crossed toward Yerba 
Buena Island, Alcatraz Island may be seen about } miles to the left, past the San Francisco waterfront. The build- 
ings of the Federal Penitentiary are built on sandstone referred to the Franciscan formation and similar to the 
sandstone passed through at the entrance to the tunnel on Yerba Buena Island. This sandstone also serves as 
the anchorage foundation for cables suspending the western part of the Ba\' Bridge. From the east side of Yerba 
Buena Island, the Berkeley Hills can be seen on the skyline ahead. Jurassic, Cretaceous, and Tertiary rocks, which 
have been complexly folded and faulted, underlie the Berkeley Hills. The nearl\- straight west-facing front of the 
hills generally coincides with the Hayward fault zone, the locus of several earthquake epicenters in historic time. 
The route traverses the bay-margin flats to the Tracy turnoff (19.6 mi. from the west abutment of the Bay 
Bridge) where it leaves the Nimitz Freeway and turns eastward. 



jMileage 

0.0 Directly beneath the first overhead east of the Nimitz Free- 
way (Foothill Blvd. overhead— about 2.1 mi. east of the 
Nimitz Freeway) where the route rejoins U. S. Highway 
50. This point is in the Hayward 7/2' quadrangle, about 
1'. miles east of the west border (see pi. 2). The Hayward 
fault zone, whose west limit was crossed about 0.2 mi. back, 
is about one-fourth mile wide in this area; the east limit of 
the fault zone crosses U. S. 50 just beyond this set of over- 
heads but is not visible. 

0.1 The gray exposure in the roadcut on the left is east of the 
Hayward fault zone and is largely mafic igneous rock, 
probably of Jurassic age (Ji on pi. 2). Aluch of the igneous 
rock is coarse grained and locally appears to be composed 
entirely of saussuritized feldspar; altered pyroxenes are 
common in small areas. The exposed rock is intensely 
sheared, and serpentine marks many of the shear surfaces. 
The outcrop may be a multiple intrusion, and the several 
fine-grained "glassy" pods and zones that crosscut the body 
may represent selvages along the borders of separate small 
intrusives. The brownish zone at the top of the exposure 
reflects the depth of weathering. 



' Publicotion authorized by the Director, U.S. Geologicol Survey. 



Mileage 

0.3 Hill with eucalyptus trees to left is capped by Pleistocene 
Leona rhyolite (Ql on pi. 2). Arenaceous shales of the 
Upper Jurassic Knoxville formation are here brought in 
contact with the mafic igneous rocks by the West Chabot 
fault, which is older than, and does not offset, the capping 
rhyolite. 

0.5 Brownish outcrops to right are shales of the upper part of 
the Knoxville formation (Jku). The sequence consists of 
nearly vertical thin beds of weakly metamorphosed shale 
and sandstone. The beds average about 1 inch in thickness 
and are locally sheared and brecciated. They are well 
bedded and have interlaminations of silicified(?) siltstone. 
Low hill to left is underlain by Lower(?) Cretaceous Oak- 
land conglomerate. The alluvium-covered trace of the East 
Chabot fault, which separates the Oakland conglomerate 
from the Chico formation, is crossed about one-half mile to 
the east (pi. 2). 

2.0 Gravels are exposed in lower parts of roadcuts to left and 
directly under, and just beyond the Castro Valley Boule- 
vard overhead. These gravels have been dated Pleistocene 
on the basis of vertebrate fossils found to the north. 



(51) 



52 



California Division of Mines and Geology 



[Bull. 182 



Mileage 

2.3 Leaving Castro Valley. Fairly good exposures of steeply 
dipping sandstone beds of the Lower (?) and Upper Cre- 
taceous Chico formation (Kc) can be seen in roadcuts for 
the next few miles. 

4.1 Steeply dipping beds of dark-bluish (brownish-weathering) 
shale of the Chico formation on left. 

4.6 Thin-bedded, gray (brownish-weathering), nearly vertical 
sequence of shale and siltstone in roadcut on left is in fault 
contact with massive grayish-brown sandstone near east 
end of cut. This fault is the northern extension of the 
Stonybrook fault mapped by Hall (1958) in the adjacent 
Dublin quadrangle. It has been revealed here by new road- 
cuts made since Robinson (1956) mapped the Hayward 
quadrangle. Folding in Tertiary rocks shown on plate 2 
just beyond this point is not visible from highway. 

4.9 Massive, grayish-brown sandstone in contact with thin- 
bedded, brownish-weathering sequence of sandstone and 
siltstone in large roadcut on left. Weathering has empha- 
sized shearing and crenulations in the thin-bedded sequence 
in upper part of exposure. Diflfercnce in attitude of beds in 
west half of cut suggests fault. 

6.0 Entering Dublin IVi minute quadrangle (pi. 2). Terminol- 
ogy in this quadrangle is somewhat different from that in 
the Hayward quadrangle. Cross sction B-B' on plate 2 
depicts structure one-half mile ahead. 

6.5 Exposure of middle Miocene Sobrante sandstone (Monte- 
rey group) in roadcuts to left. Lower Cretaceous Niles 
Canyon formation of Hall (1958) underlies hills to right. 

7.3 Summit. Middle .Miocene Oursan sandstone (Tmo) exposed 
in roadcuts. Outcrop on skyline about one-fourth mile to 
left is hard conglomeratic sandstone of the middle member 
of the upper Miocene Briones sandstone (Tmbm). One- 
half mile to the east this conglomeratic sandstone caps 
prominent ridge to left of highwa>-, and at end of ridge 
the strike swings northwest around the axis of a northwest- 
plunging synchne. 

8.1-8.3 Small outcrops of the lower sandstone member of the 
Briones (Tmbl) in roadcuts to left and right. 

8.4 Dublin fault crosses road. Exposures poor. 

8.7 Middle (conglomeratic) member of Briones (Tmbm) vis- 
ible on hilltop to left. Lower (sandstone) member of 
Briones (Tmbl) in roadcuts. 

9.0 The Livermore Valley may be seen stretching ahead. This 
valley is apparently a structural depression. On the west the 
valley has been downdropped along the Calaveras-Sufiol 
fault: it may also have been downdropped along several 
northwest-trending faults that extend into the valley from 
the east, under the alluvial cover. Drilling for oil and gas 
has shown the presence of faults below the surface and a 
measure of the relative downward movement is supplied 
by the thickness of 8,000 to 10,000 feet of conglomerate of 
the Pliocene Orinda formation indicated by Hall (1958, 
pi. 2). 

9.6 Passing under Foothill Road (State Hwy. 21) overhead. 
Calaveras-Suiiol fault crosses road at this point but is con- 
cealed by alluvium. Cross section C-C is 0.1 mi. ahead 

(pi. 2). 

10.1 Mount Diablo (elev. 3,849') can be .seen on skyline to left. 
This mountain was a guiding landmark in early Spanish 
days, and since 1851 has been the point of origin for land 
surveys of a large part of northern California and Nevada. 
On its south flank are steeply dipping Cretaceous and Ter- 



Mileage 

tiary beds; its core is composed of Jurassic rocks of the 
Franciscan formation. On the northeast side the Cretaceous 
and Tertiary sequence contains coal beds and white sands 
of economic interest in rocks of Eocene age. 

10.9 Pastel-painted buildings to left are housing for Camp Parks, 
U. S. Army. 

12.4 Alameda County Rehabilitation Center on left. To the 
right in the middle distance are stockpiles of processed 
gravel recovered^rom the alluvial deposits of Arroyo del 
Valle and Arroyo Mocho southwest of Livermore. Much 
of the gravel used in the Bay area comes from large gravel 
pits in this vicinity. 

12.9 Entering the Livermore IVi minute quadrangle. 

13.5 Low hills on left are underlain by nonmarine conglomer- 
ate and sandstone of the Pliocene Orinda formation. One 
and one-half miles ahead Hall's map (1958) shows dips of 
80°-85° in these beds; some are overturned. 

16.5 Crossing Hall's cross section F-F' (pi. 2). 

19.8 Entering the Altamont 7/; minute quadrangle. Livermore 
gravels of Clark, 1930 in low hills on both sides of road. 

20.5 Livermore road iunction. Lawrence Radiation Laboratory, 
a division of the University of California, about two miles 
to right. The Livermore facility was established in July 
1952 and covers more than 600 acres. The program at the 
Laboratory (1961) includes investigations concerning phys- 
ics and engineering of fission and thermonuclear devices; 
nuclear ram-jet propulsion (Project Pluto); critical assem- 
bly and reactor research; controlled release of thermonu- 
clear energy (Project Sherwood); basic nuclear-particle 
research with various types of accelerators; and industrial 
applications of nuclear explosives (Project Plowshare). 

23.0 Crossing over Western Pacific and Southern Pacific rail- 
roads. First roadcut on right exposes sandstone and some 
thin pebbly sandstone beds of the Cierbo formation. The 
Greenville fault crosses the road just beyond this roadcut 
(pi. 2) but is not exposed. 

23.2 Roadcuts for next 3 miles expose westward-dipping beds 
of sandstone and shale of the Panoche formation of Late 
Cretaceous age. The unconformity with the overlying 
upper Miocene Cierbo is poorly shown. 

25.2 Summit of Coast Ranges, here only about 1,040 feet in 
elevation. 

26.2 Good exposure of west-dipping well-bedded shale of the 
Panoche formation on right. 

26.5 Approximate axis of the AltamorHr anticline, .\head the 
beds dip to the east. 

27.2 Passing under Western Pacific Railroad overhead. Mileage 
checkpoint. 

27.5 Entering the .Midway 7'; minute quadrangle. 

28.4 Small fault cuts sandstone and shale beds of the Panoche 
formation in roadcut to left. Beds to east of fault dip more 
steeply than those to west. 

28.7-29.0 Excellent exposures of sandstone and shale of the 
Panoche formation. 

29.1 On hillslopes to right can be seen discontinuous outcrops 
of several hard beds of sandstone of the Panoche forma- 
tion. Hill to left of road in near distance is capped by the 
hard basal conglomerate of the upper Miocene Neroly for- 
mation (Tnss). These beds unconformably overlie the 
Panoche formation (see cross section A-.A', pi. 2). 



19621 



Merced Canyon and Yosemitk Valley 



53 







'-lap'i- 









F^K«^* 



^.Jttamjiil&^lSile^-'' 



Photo 1. Unconformity between the Panoche formation of Late Cretaceous age and the Neroly formation of late Miocene age; view north. 
SoncJstone and shale beds of the Panoche ore overlain by less steeply dipping conglomerate and sandstone beds of the Neroly. 



Mileage 

29.5 STOP I. Byron Turnoff. Because of freeway traffic we 
wish to stop the buses off the main road where the exit 
road turns off. Passengers REMAIN IN BUS: geology is 
pointed out through bus windows. Buses will then go 1,000 
feet ahead to a small side exit, turn around, and return to 
the freeway. 

Unconformity between Neroly formation (upper Mio- 
cene) and Panoche formation (Upper Cretaceous). The 
sandstones and shales of the Panoche formation strike about 
N. 20° W. and dip about 20° NE. The sandstone beds arc 
generally yellowish gray, 1 "2 to 4 feet thick, and locally 
show prominent cross bedding and convolute bedding. The 
sandstones are niediuni-to coarse-grained, and are made up 
predominantly of subangular to rounded, fairly well-sorted 
quartz grains. Feldspar, biotite, chlorite, and dark grains 
are present but not abundant. The shale beds of the Panoche 
are thin (Is" to 2"), are light gray to yellowish orange, 
and are slightly silty and carbonaceous. Thin (';") sand- 
stone beds are commonly intercalated. Tjhe moderately 
fissile finely micaceous shale is predominantly clay and is 
closely jointed, breaking to small rhomboid fragments. 

The Panoche is overlain unconformably by conglomer- 
ate of the Neroly formation which lies upon the trun- 
cated edges of the sandstone and shale at an angle of about 
15°. No weathered or humic zone occurs at the top of the 
Panoche, and the surface is grooved in a N. 25° E. direc- 
tion. The grooves are filled with conglomerate in which 
the pebbles range in size from !;" to 3" but average 1" to 
1/4". All pebbles are well rounded and are mostly vol- 
canic rocks of andesitic composition held in a matrix of 
medium- to coarse-grained sandstone. Lenticular stringers 



Mileage 

of sandstone occur in the upper part of this basal conglom- 
erate, which grades upward into the overlying sandstone. 
The sandstone is medium gray and is essentially a medium- 
to coarse-grained lithic wacke containing quartz and many 
dark grains. It is about 30 feet thick and is locally cross 
bedded. The sandstone unit lenses eastward to about 10 feet 
in thickness, and an overlying pebble-conglomerate unit 
nearly merges with the basal pebble-conglomerate bed. The 
upper conglomerate is locally cemented and breaks across 
the pebbles. Generally, however, the pebbles weather out 
in relief from the matrix. Locally the sandstone has a dis- 
tinct purplish-blue cast. 

29.7 .Midway fault crosses road and repeats conglomerate at the 
base of tbe Neroly formation on east side of valley. Cross 
section A- A' crosses road at this point (pi. 2). 

29.9 Upper (shale) member of the Neroly formation (Tnsh). 

31.1 Roadcut on left shows lake clays in sharp contact with 
sands and gravels (mapped as Tulare formation of Pliocene 
and Pleistocene (?) age by Reiche, 1950, p. 4). Gravels 
contain subangular to rounded pebbles of finely veined 
brownish-red chert, white quartz, reddish-brown volcanic 
porphyry, and several varieties of greenstone. Crossbedded 
sands are interbedded with the gravel, but the lake beds, 
above, are nearly horizontal. 

31.5 Delta-Mendota Canal, a major water transportation facility 
in the northern part of the San Joaquin N'alley. This canal 
carries water from the delta area of the Sacramento and 
San Joaquin Rivers southward to supply the west side of 
the San Joaquin X'alley with irrigation water. At the main 
pump station, w hich is 4 miles north of here, the water is 



54 



California Division of Mines and Geology 



[Bull. 182 



Mileage 

lifted about 150 feet and flows southward nearly 120 miles 
to Mendota. The grade of the canal is about ? inches per 
mile and near the south end regional subsidence of land 
has changed the grade and created flowage problems. Sub- 
sidence has amounted to about 10 feet since 1937 and in the 
area of maximum depression along the canal, about 15 
miles northwest of Mendota, is about '/2-foot per year. The 
subsidence is apparently a result of withdrawal of deep 
groundwater. 

32.7 San Joaquin Valley ahead. Coast Range-Diablo Range runs 
southeastward to the right. Mt. Oso is the highest promin- 
ence in the hills to the right and forms part of the Fran- 
ciscan core of the Diablo Range. Several manganese de- 
posits occur in the vicinity of Mt. Oso in the Ladd-Buckeye 
area. The principal producer has been the Ladd mine, which 
began operations in 1867. Intermittent production to 1950 
totalled an estimated 30,000 to 50,000 tons of manganese 
ore. The manganese occurs as gray rhodochrosite and be- 
mentite in massive white chert lenses in rocks of the Fran- 
ciscan formation. Eight miles beyond Mt. Oso is the Red 
Mountain magnesite district, which yielded approximately 
870,000 tons of magnesite between 1905 and 1945. The mag- 
nesite occurs as replacements of serpentine in shear zones 
in a large ultramafic intrusive body. 

34.0 Town of Tracy ahead. At the northeast edge of town is 
the Tracy Gas Field. This field was the first commercial 
gas field found in northern California and the first Cali- 
fornia field to produce gas commercialK' from Cretaceous 
strata. The discovery well was completed in August 1935 
and produced from the Tracy gas sand in the Panoche 
formation at a depth of about 4,000 feet. Maximum annual 
production from the field was more than 3 trillion cubic 
feet in 1936. The field is currently (1961) "shut-in". 

34.4 Leaving strip-map area (east boundary of Midway 7'/> 
minute quadrangle). 

38.8 Tracy High School. 



Mileage 

41.6 Leave Highway U.S. 50. Turn right on State Highway 33 
toward V'ernalis and Patterson. The McMullin Ranch field, 
a new discovery, lies about 6 miles due east of this point. 
The discovery well was completed May 4, 1960 at 5,925- 
5,945 feet in sands of Late Cretaceous age. As of this writ- 
ing (February 1961) active development is in progress. The 
V'ernalis gas field lies east of Highway 33 about 7 miles 
ahead. This field was discovered in January 1941, and first 
commercial gas deliveries began in May 1942. The gas is 
produced from as many as 10 zones between 3,000 and 5,000 
feet depth. For further information see Manlove's article 
on the Vernalis gas field in Bulletin 181. A fourth produc- 
ing area nearby, called the Vernalis-Southwest field, lies 
about 5 miles southwest of the Vernalis field. The dis- 
covery well was completed in August 1959 with production 
from Upper Cretaceous sands at 4,560 feet. Payne (1962, 
pi. 17), shows correlations of the subsurface units in the 
Tracy and Vernalis gas fields with well sections farther to 
the .south. 

References 

Clark, B. L., 1930, Tectonics of the Coast Ranges of middle Cali- 
fornia: Geol. Soc. America Bull., v. 41, p. 747-828. 

Hall, C. A. Jr., 1958, Geology and paleontology of the Pleasan- 
ton area, Alameda and Contra Costa Counties, California: Univ. 
California Pub. Geol. Sci., v. 34, no. 1, 89 p. 

Huey, A. S., 1948, Geology of the Tesia quadrangle, Cahfornia: 
California Div. Mines Bull. 140, 75 p. 

Jenkins, Olaf P., ed., 1951, Geologic guidebook of the San Fran- 
cisco Bay Counties . . . : California Div. Mines Bull. 154, 392 p. 

Pa\nc, M. B., 1962, Type Panoche group (Upper Cretaceous) 
and overUing Moreno and Tertiary strata on the west side of the 
San Joaquin Valley: California Div. Mines and Geology Bull. 181, 
pp. 165-175. 

Rciche, Parry, 1950, Geology of part of the Delta-Mendota 
Canal near Tracy, California: California Div. Mines Special Rept. 
2, 1: p. 

Robinson, G. D., 1956, Geology of the Hay«ard quadrangle, 
California: U. S. Geol. Survey Geol. Quad. Map GQ-88. 



ROAD LOG 2 



TRACY TO EL PORTAL VIA PATTERSON AND TURLOCK, CALIFORNIA * 



By ClYDE WAHRHAFTIG 

U.S. Geological Survey, Menio Park, California, and 
University of California, Berkeley, California 



and L. D. CLARK 

U.S. Geological Survey, MenIo Pork, California 



Plate 1, Geologic map and section of the southern parf of the western Sierra Nevada metamorpbic be/t, accompanies this paper. 



Mileage 

0.0 Junction of Highways U.S. 50-California 33. Turn right 
(south) on Highway 33. 

9.1 Junction California 33 and 132. 

17.9 Junction California 33 and road to Grayson and Modesto. 

24.5 Turn left (east) onto Patterson-Turlock Road. 

27.4 Cross San Joaquin River. 

40.2 Center of Turlock (intersection Main and U.S. 99)— turn 
right (south) to first street past stoplight. 

40.3 Turn left (east) off U.S. 99 on East Ave.- road to Snclling. 

40.2 The road crosses the depositional surface on top of the 
to Pleistocene Modesto formation of Davis and Hall (1959), 

46.0 equivalent to the upper part of the late Pleistocene Victor 
formation (Piper and others, 1939). The Victor formation 
is an alluvial fan. For a geologic description of the area 
between Turlock and Merced Palls, see the paper by R. J. 
Arkley in this guidebook. 

46.0 The road leaves the smooth depositional surface and enters 
slightly rolling topography carved in the Quaternary Riv- 
erbank formation of Davis and Hall (1959), equivalent to 
the lower part of the Victor formation. Outcrops of red- 
dish sandy alluvium along the road at mile 47.8 arc prob- 
ably of this formation. 

48.2 To the north can be seen a line of low, flat-topped hills 
carved in the Turlock Lake formation of Davis and Hall 
(1959). 

52.2 The low, flat-topped hill about a quarter of a mile to the 
southwest (right) consists of Mehrten formation, of Mio- 
cene and Pliocene age, capped by a thin layer of Arroyo 
Seco gravel of middle or late Pleistocene age (R. J Arkley, 
this guidebook). 

52.5 The road enters the area mapped as Turlock Lake forma- 
tion by Davis and Hall, characterized by rolling country 
carved in reddish sandy alluvium. 



Prepared cooperatively with the U.S. Geologicol Survey. 



Mil,eage 
54.9 North bank of Dry Creek. The roadcuts along the grade 
down to the creek expose two layers of thinly bedded fine 
sand and silt interbedded with coarse sand. These may rep- 
resent glacial lake deposits (glacial rock flour) laid down 
by the Merced River. The terrace traversed by the road 
beyond Dry Creek is underlain by the Riverbank forma- 
tion of Davis and Hall (1959). 

55.4 The China Hat pediment (see article by R. J. Arkley, this 
guidebook) can be seen across the Merced River, above the 
trees at 2:00 o'clock. 

Just beyond this point the road descends 20 feet to the 
terrace surface of the .Modesto formation. 

57.2 On the skyline to the east across the river can be seen the 
westward-sloping China Hat pediment. 

57.9 The road descends the terrace to the modern floodplain of 
the Merced River. The small outcrop at the base of the 
bluff is of thinly bedded fine sand and silt. This is overlain 
by 20 to 30 feet of well-.sorted medium-grained sand of 
granitic detritus. 

61.6 The abandoned grade visible at intervals on the right is 
that of the Vosemite Valley Railroad, which extended from 
Merced to I'.l Portal and operated from 1907 to 1945. Its 
chief business was freight, but the railroad carried many 
Yosemite-bound travelers during the summer. 

65.5 Snelling. Small white courthouse at Snelling, erected in 
1857, was the first in Merced County. 

Between Snelling and Merced Palls the road passes piles 
of dredge tailings. The gold mining dredges are large 
barges with chains of scoop buckets that gnaw away on 
one side of :he artificial pond on which the dredge floats. 
The buckets feed into a large trommel on the dredge that 
separates the coarse gravel from the fine gold-bearing sand. 
This sand passes from the trommel over a complicated 
series of riffles which trap the gold. The barren gravel is 
carried up a long stacking arm by an endless belt and 
dumped on the side of the pond opposite from where it 
was dug. By excavating at one end and back-filling at the 
opposite end, the dredge carries along the pond on which it 



(55) 



56 



California Division of Mints and Gfology 



I Bull. 182 



Mileage 

floats as it mines the gold. The dredge is anchored to the 
shore by cables and swings from side to side by pulling 
alternately on the cables, digging and stacking as it goes. 
This back and forth swinging from a fixed point gives the 
curious cross ridges of the dredge tailings; from the air 
these tailings piles look like stacks of coins that have fallen 
over. 

The gravel that has been dredged here is apparently post- 
glacial alluvium deposited in the shallow steep-walled valley 
that the Merced River carved in the Pleistocene Modesto 
and Riverbank formations. The large size of the boulders 
in this gravel is very puzzling, for the Pleistocene alluvium 
contains no boulders of this size so far out from the moun- 
tains. In addition to gold, a small amount of platinum was 
recovered from the dredgings. 

With luck, one can see to the south the profile of the 
China Hat pediment. 

72.0 Merced Falls. 

72.1 .Merced Falls dam and powerplant on right. The dam rests 
on the westernmost outcrop of bedrock, whicli consists of 
black slate with interbedded graywacke. 

72.3 Sawmill ruins on right. 

72.5 Cross abandoned grade of the Yosemite \'alley Railroad. 
E.xposures of steeply dipping Upper Jurassic slate. The 
metasedimentary rocks extending from here eastward to 
Stop 1 are on the western limb of a large anticline (pi. 1). 
.\lthough the beds are in places complexly folded, tops are 
westward in intervening areas where bedding is not 
crumpled. 

72.7 Continue straight ahead on Exchequer Dam road. 

72.8 Cleavage and bedding in slate at left dip steeply. 

73.0 Cleavage still dips deeply, but in minor folds bedding 
crosses the prominent cleavage. 

74.7 STOP 1. Lunch. Stream-polished exposures on the north 
bank of the .Merced River show details of the structure of 
much-deformed slate and graywackes. Some graded gray- 
wacke beds can be found. The most prominent structures 
here are minor folds that plunge southwest and southeast at 
angles of 70° and 80°. These small folds are superimposed 
on beds that already dipped steeply as a result of earlier 
folding about nearly horizontal axes. The Jurassic slate of 
this area differs from most of that exposed farther north in 
the abundance of minor folds and in the steep plunge of 
fold axes. 

Return to Snelling-Hornitos Road. 

75.7 Cross Merced River. 

76.7 STOP 2. The flat-topped hill just west of the road is 
capped by about 100 feet of coarse cross-bedded sandstone 
and conglomerate of the lone formation, which rests on 
deeply weathered Jurassic slate. .*\t the base of the hill can 
be seen outcrops of black un\\eathered slate. Near the top 
of the hill, the sandstone can be seen resting on bleached 
white slate. The sandstone contains pebbles of bleached 
slate, volcanic rocks, and quartz. It is largely a quartz- 
kaolin sandstone with a very low percentage of heavy 
minerals, all extremely resistant to weathering, and was ap- 
parently derived from the erosion of a terrain that had 
been deeply weathered in a tropical- climate. Casts of 
V ejiericardia planicosta have been reported from the sand- 
stone in this hill (.\llcn, 1929, p. 361). 

The flat-topped hills to the southeast, which slope gently 
southwestward, are also capped by about 100 feet of similar 
sandstone of the lone, resting unconformably on the up- 
turned edges of the Jurassic slate. If the hill crests are 
projected by eye eastward, they will be seen to coincide 
roughly in height with the even-topped ridges in the dis- 



Mileage 

tance to the east, the foothill ridges of the Sierra. These 
ridges are underlain by metavolcanic rocks. 

76.9 The road here passes tjirough rolling country surmounted 
to by the flat-topped mesas of the sandstone of the lone. On 

79.9 the lou , rolling hills can be seen the curious "hogwallow" 
microrelief of evenly spaced mounds 2 to 3 feet high, and 
about 20 to 40 feet across. See article by R. J. .\rkley in 
this guidebook, for a discussion of the origin of this micro- 
relief. 

79.9 W'hitish-ucathered schistose felsite, probably Upper Jur- 
assic. 

82.5 Road crosses into Jurassic metavolcanic rocks. 
82.7 Roadcuts in schistose metavolcanic rocks. 

83.3 Hornitos (take left fork). 

83.4 .\dobe and stone buildings, some in ruins but others still 
occupied. Hornitos was settled in 1850 by .Mexicans who 
uere invited to leave the town of Quartzburg, about 4 
miles to the northeast. Joaquin .Murietta, a bandit idolized 
by some in Mother Lode history, is alleged to have once 
escaped through a tunnel leading from a building here. One 
of the ruined buildings once housed the store of D. Ghirar- 
delli, \\ ho later went into the chocolate business. From the 
Wells Fargo office, established in 1852, gold shipments of 
$40,000 per day are reported, and the population of 
Hornitos reached a high point of about 15,000. The name 
"Hornitos" means "little ovens" and was derived from the 
dome-like bake ovens constructed here by a group of 
Germans. 

84.6 Schistose amphibolite in roadcuts. 

85.4 Old placer diggings in Burns Creek on the right. 

87.1 Quartzburg school. Site of the gold-rush town of Quartz- 



88.7 



burg. 

Take left fork of road. 



90.7 Hunter X'alley Road on left. Continue straight ahead 
tou ard Bear \'alley. 

91.1 Hunter \'alley extends to the northwest (left). Bedded 
L'pper Jurassic tuff dips about 60° northeastward in road- 
cuts on the right. 

92.7 \"iew ahead of Bear Valley and Bullion .Mountain. The 
floor of Bear X'alley, at an elevation of about 2,000 feet, is 
probabK- a surface of the Broad \'alle\- stage of the .Merced 
Ri\er. Bullion Mountain, which has a present elevation of 
4,200 feet, threfore probably stood at least 2,200 feet above 
the Broad X'alley stage of the .Merced River (approximately 
equivalent in age to the lone formation or the auriferous 
gravels). Bullion .Xlountain is held up by resistant meta- 
volcanic rocks while Bear X'alley is carved in soft slate of 
the .Maript)sa formation. The Xlariposa formation is sepa- 
rated from the metavolcanic rocks by the .Xlelones fault 
zone which is concealed by mass wastage debris on the 
lower slopes of the mountain. 

93.8 West contact of the .Xlariposa formation. 

94.4 Junction State Highway 49 at Bear X'alley. Turn left 
(north) on Highway 49. The town was the site of Col. 
John C. Fremont's headquarters after he had purchased the 
.Mariposa land grant in 1847. Fremont operated lode gold 
mines and a stamp mill until 1863. 

The road north is over the rolling upland surface of 
Bear X'alley at altitudes of 2,000 to 2,300 feet. From Bear 
X'alley to the Pine Tree mine (Stop 3), and back to Mari- 
posa, the route follows the Mother Lode, a geographic belt 
2 to 3 miles wide in which a system of discontinuous east- 
ward-dipping quartz veins crops out. Lode gold deposits 
are not restricted to the .Xlother Lode belt, hut within it 



Merced Canyon and Yose.mite Valley 



57 



Mileage 

the quartz veins and ore bodies are more numerous and can 
be followed farther in mining than elsewhere in the Sierra 
Nevada. The gold is associated with the quartz veins, but 
in man\- mines gold in the quartz is relati\cly scarce; the 
ore was formed by replacement of the rock adjacent to 
the veins. .Man\- Mother Lode veins and ore bodies are 
within the Meloncs fault zone, but they are related to 
smaller faults that arc apparently younger than the chief 
movements of the fault zone. 

95.8 Edge of the canyon of the Merced River. The topography 
drops abruptly away to Hell Hollow, the ravine directly 
below, and the .Merced River, 1,400 feet below and 2 miles 
north of this point. 

96.9 Contact between .Mariposa formation and serpentine. The 
Mariposa formation is sheared for a distance of about 100 
feet westward from the serpentine as a result of movement 
on the .Melones fault zone. The fault zone here includes 
the serpentine and the sheared part of the Mariposa forma- 
ton. 

97.2 STOP 3. Pine Tree mine. Walk out to point north of iron 
tanks while bus is turning around. North and east from 
this point can be seen the Merced Canyon and also the 
Broad Valley surface, which is marked by accordant ridge 
crests at or just above our level on the north side of the 
river. These accordant crests rise north of us to the base of 
a mountain (Buckhorn Peak), which is crowned by a flat 
area more than a mile across (Buckhorn Flat, altitude about 
3,400 to 3,500 feet) that is nearly 1,200 feet above the Broad 
Valley stage of the .Merced River. Buckhorn Flat is cut 
across steeply dipping metavolcanic rocks. Northeast of 
Buckhorn Flat, and several hundred feet below it, is an 
extensive gently rolling upland cut across the steeply dip- 
ping Paleozoic Calaveras formation. Patches of auriferous 
gravels have been found on this upland (Turner and Ran- 
some, 1897). 

Due east can be seen the High Sierra, with a few of the 
higher peaks (possibly Mt. Clark and Red Peak, 11,500- 
11,600 ft.) rising from behind the even-topped forest-cov- 
ered ridges of the Sierra upland that form the skyline. At 
the head of the Merced Canyon, the tops of El Capitan, 
Half Dome, and Clouds Rest, three of the famous monu- 
ments of Yosemite X'alley, can be seen rising slightly above 
the forested plateaus. If, in mind's eye, the can><)ns be 
filled in up to the level of the Broad X'allcy stage, a rough 
picture of the appearance of the Sierra Nevada in Eocene 
time can be obtained. 

The Pine Tree gold mine, at the head of the ravine to 
the south, recovered ore from the Pine Tree vein, discov- 
ered in 1849, and from the Josephine vein, discovered 
shortly thereafter. The mine operated intermittently from 
1849 to 1944, part of the time under the ownership of Col. 
Fremont, and has a recorded production of about $3,400,000 
from 8 miles of workings. The total production is prob- 
ably greater than 54,000,000. Mariposite, a bright-green 
chrome mica, is among the minerals found here. (Notes on 
mine statistics and history in this road log arc from Bow en 
and Gray, 1957.) 

Return to Bear Valley on California 49. 

100.0 Town of Bear \'alley. Ruins of adobe and stone buildings 
on left. 

105.1 Site of .Mt. Ophir .Mint. This was an officially sanctioned 
private mint established in 1851; for a short time it manu- 
factured hexagonal S50 gold slugs to ease a currency short- 
age. The white quartz vein and diggings at the top of the 
hill ahead mark the site of the Mt. Ophir mine. Discovered 
in 1849 or 1850, the mine was operated intermittently until 
1914. Recorded production is 585,703, but total production 
is estimated at about $270,000. 



Mileage 

106.7 Town of Mt. Bullion. The Princeton group of mines is on 
the Mt. Bullion-Cathay road at the southern outskirts of 
.Mt. Bullion. The Princeton mine, discovered in 1852, pro- 
duced about 55,000,000 in gold from workings that extended 
to a depth of 1,250 feet. Most of the ore carried between 
$4 and 57 per ton in gold, but near-surface ore yielded 
about 570 per ton. No sustained mining has been carried on 
since 1927. 

108.2 Contact between serpentine in the .Melones fault zone and 
greatly sheared slate and conglomerate of the Mariposa 
formation. The long axes of the elong.ite conglomerate 
pebbles are nearly- vertical. 

108.8 Road follows contact between serpentine on the right and 
metavolcanic rocks on the left for the next half mile. 

111.5 Junction California 49 and California 140. Turn right 
(southeast) into Mariposa. ".Mariposa" means butterfly. 

112.2 Rest stop. Turn around. Leave .Mariposa traveling toward 
Yosemite Valley on California Highway 140. 

112.9 Junction California 140 and 49. Continue ahead on 140. 
From here to mile 117.2 steeply dipping metavolcanic rocks 
with some interbedded slate are exposed in the roadcuts. 

1 14.4 Small body of talc-antigorite schist enclosed in metavol- 
canic rocks. 

116.5 Mariposa summit. 116.5 to 121.0: the highway passes down 
the valley of Bear Creek, a broad gentle valley probably 
graded approximately to the Merced River of the Broad 
X'alley stage (Hudson, 1960, fig. 2. p. 1551). 

117.2 Highway enters an area underlain by a granitic rock. 

117.8 Highway passes from the granitic area back to metavol- 
canic rocks. 

121.0 The grade of Bear Creek steepens somewhat, the canyon 
narrows, and the stream has carved incised meanders. The 
segment of the stream from 121.0 to 123.1 was probably 
graded to the Merced River of the Mountain Valley stage. 

122.8 Bridge over Bear Creek. 

123.1 STOP 4. Bear Creek here plunges over a fall held up by 
the resistant metavolcanic rock and descends the abrupt 
canyon ahead to the .Merced River. This is probably the 
nickpoint between the segments of Bear Creek graded to 
the Mountain Valley stage (above) and to the present 
stream in its canNon (below). 

123.7 Contact between steeply dipping metavolcanic and meta- 
sedimentary rocks, both parts of the Calaveras formation 
of Paleozoic age. 

123.7 Steeply dipping planar structure in Paleozoic slate exposed 
to on the left is cleavage. Bedding is in general nearly paral- 
125.4 lei to the cleavage, but often crosses it. 

125.4 Briccburg. 

125.5 STOP 5. Slate and thin-bedded chert (Calaveras forma- 
tion) are here greatly sheared and the only structures re- 
maining are the steeply plunging lineations and minor folds 
related to the last stage of deformation. From here to mile 
1 30.5, the Cplaveras formation consists of phyllite derived 
from siltstone that locally contained interbedded chert. 
Bedding is preserved in part of this interval, but in other 
parts has been destroyed by shearing as it is here. 

Granitic boulders are abundant in glacial outwash resting 
on water-polished bedrock in cuts on the .south side of the 
road at Stop 5. Since the gravel consists largely of well- 
rounded boulders of granite and granodiorite, it must have 
been transported by the river, for the bedrock for many 
miles upstream from this point consists of metasedimentary 
rocks. Over the outuash gravel is colluvium derived from 
the slope above. 



58 



California Division of Mines and Geology 



[Bull. 182 



Photo 1 . Canyon of Bear Creek. At the head of the canyon is a fall which is held up by resistant 
metovolcanic rock and which separates segments of the creek graded to the Mountain Valley stage (above) 
and to the present river in its canyon (below). 








3fsiajrjyww.Tr'' 











':^ 



'.V 



* 
.A- 



^■ 






'U 



'^^ -. 



\ 



Photo 2. Glacial out wash overlain by colluvium, near Brlceburg. Out wash consists largely of boulders 
of granite and granodiorite, tronsported by the river from bedrock exposures mony miles upstream. 



19621 



.Mkrcki) Canyon and Yosfmitf. V'allf.y 



59 



Mileage 

I2A.A Boudinapes in steeply-dipping mafic dike in large cut on 
the right. 

127.S STOP 7 (return trip). C)ut« ash gravel exposed in the 
roadcut may indicate two layers of outwash, separated by 
a period of dissection when ice retreated in the head- 
waters of the river. In the lower 10 feet of the gravel nian\ 
of the boulders of granodiorite and granite are rotted to 
granite sand; maximum size of the boulders is about 4 feet, 
but this is no larger than boulders now being moved !)>■ 
the .Merced River on the other side of tiie road. The upper 
30 feet of tlic exposure, separated from the lower part b\' 
a row of boulders slumped from the hillside above, con- 
sists of finer gravel (about I foot average size) composed 
of largel> un«eathered boulders of granite and granodio- 



.\lileage 

The folds all plunge steeply, but the bearing of the fold 
axes and attitudes of axial planes arc not consistent. 

Landslide debris in roadcut on the right. 1 he scarp at the 
top of the slide, not visible from this point, is near the 
crest of the ridge. Looking downstream from this point the 
\'-shapcd canyon of the Merced River is well displayed. 

Young glacial outwash gravel or hydraulic mine tailings 

on right. 

Bridge across the South Fork of the .Merced River. 

Waste dumps of the Clearing House mine can be seen to 
the cast across the river. The ore was discovered in 1860, 
and before mining ceased in 1937, the mine had yielded 
more than S.?, 350,000 in gold, some silver, and small 



134.2 



154.5 

135.0 
1 36.5 







■*••/ 'vf "*'' ^ - V 




/^■'' 




Photo 3. V-shoped canyon of the Merced River a mile below the mouth of the South Fork. On the 
top of the ridge on the skyline are preserved remnants of a gently rolling surface produced during an 
earlier cycle of erosion. 



rite. The upper gravel may be of Tiog^ or Graveyard (of 
Birman, 1957) age. probably the latter, and the lower gravel 
Tahoe or Sheruin in age (see table 1, VV'ahrhaftig). 

129.6 Crossing Feliciana Creek, one of Matthes' classic areas of 
the Broad X'alley stage. 

129.7 Quartz ladder veins in dikes to the right. 

I ?((.5 Thin-bedded nietachert of the Calaveras formation, locally 
contorted. The light-gray-weathering more massive rock 
is limestone, traceable from here to about 1 mile north of 
the ciuarr>- at mile 132.0. 

131.6 Small granitic stock to the north across the river. 

131.8 Inactive limestone quarry. Limestone from this quarry was 
used in Merced for the manufacture of Portland cement. 
The geologic structure on hillsides to the north (left) is 
brought out b\ the resistant nietachert and limestone beds. 

!.6 STOP 6. Geologic marker. Tightly and complexly folded 
nietachert and black ph\llite of the Calaveras formation. 



amounts of copper and lead. Graodiorite exposed on hill 
north of the mine. 
136.9 Southwestern contact of an isolated granitic pluton. 

137.1 Gold Star mine on right. The mine produced small 
amounts of gold from 1936 to 1952. 

137.3 On the spur on the canyon wall to the north (left) can 
be seen the tiace of the incline where log-laden flatcars 
were lowered from spur lines on the gently rolling upland 
surface above to the main line of the Yosemite X'alley Rail- 
road near river level. 

J 37.4 Dumps at the Rutherford gold mine are visible north of 
the river. The production is not recorded but there are 
several reports of high-grade ore. 

137.8 Outwash gravel in roadcut. 

138.2 Northeastern contact of granitic pluton. 

'38.3 SpheroidalK- weathered boulders of granitic rock. 



60 



California Division of Mines and Geology 



[Bull. 182 



Mileage 

139.7 Southwestern contact of a granitic body. The highway 
crosses metamorphic rock trom 139.7 to 140.2. 

140.2 Northeastern contact of a granitic body. 

140.4 The flat area to the left marks an abandoned meander of 
the river. 

140.6 The sheet-iron structure at the base of the hill on the far 
side of the river is a mill that processed tungsten ore from 
mines in this vicinity. 

141.1 At this point, the canyon of the Merced River widens out 
and is U-shaped. Downstream from this point the canyon 
is V-shaped and winding, and there is some question as to 
how far below this point ice actually extended. There is 
no question that ice reached this far downstream on the 
Merced River in the oldest glacial stages recognized on the 
west side of the Sierra. 

141.2 Tunnels in the sharp ridge north of the river are workings 
of the Kl Portal barite mine. Most of the nearly 400,000 
tons of barite produced was used in oil-well drilling mud. 
The mine has been idle since 1948. 

142.0 Bridge across Merced River. 

142.3 Metamorphic rocks across river to the right. 



Mileage 

142.6 El Portal. (Gasoline station at east end of town.) 

142.7 Contact between Calaveras formation and part of the Sierra 
Nevada batholith. 

Refereiices 

Allen, Victor T., 1929, The lone formation of California: Univ. 
California, Dept. Geol. Sci. Bull., v. 18, p. 347-448. 

Birman, Joseph H., 1957, Glacial geology of the upper San 
Joaquin drainage. Sierra Nevada, California: Univ. California at 
Los Angeles, Ph. D. thesis, 237 p. 

Bowen, O. I',., Jr., and Gray, C. H., Jr., 1957, Mines and mineral 
deposits of Mariposa County, California: California Jour. Mines 
and Geology, v. 53, p. 35-243. 

Davis, S. N., and Hall, F. R., 1959, Water quality of eastern 
Stanislaus and northern Merced Counties, California: Stanford 
Univ. Pub., Geol. Sci., v. 6, no. 1, 112 p. 

Hudson, F. S., 1960, Post-Pliocene uplift of the Sierra Nevada 
Cahfornia: Geol. Soc. America Bull., v. 71, p. 1547-1573. 

Piper, A. M., Gale, H. S., Thomas, H. E., and Robinson, T. W., 
1939, Geology and ground-water hydrology of the .Mokelumne 
area, California: U.S. Geol. Survey Water-Supply Paper 780, 
230 p. 

Turner, H. W., and Ransome, F. L., 1897, Geology of the 
Sonora quadrangle: U.S. Geol. Survey Folio No. 41. 



ROAD LOG 3 

EL PORTAL TO WAWONA TUNNEL AND A CIRCUIT OF 
YOSEMITE VALLEY, CALIFORNIA * 



By DALLAS L. PECK 

LI.S. Geological Survey, Menio Pork, California 



CLYDE WAHRHAFTIG 

LJniversity of California, Berkeley, Colifornio, and 

U.S. Geological Survey, MenIo Pork, California 



and FRANK C. CALKINS 

U.S. Geological Survey, MenIo Park, Colifornio 



EL PORTAL TO THE EAST PORTAL OF THE 
WAWONA TUNNEL 



lileage 

Kl Portal (Standard Oil Company Service Station). The 
contact between the Calaveras formation (late Paleozic) 
and the granitic rocks of the Yosemite area trends due 
north a few hundred feet east of here. The marginal in- 
trusive rocks include coarse diorite and some norite. 
Farther east is the V-shaped gorge of the Merced River, 
which, although glaciated during two prc-Wisconsin stages 
when the glaciers extended about one mile below here, was 
not glaciated during the Wisconsin. El Portal was formerly 
the terminus of the Yosemite \'alley Railroad. For refer- 
ence see the sketch map of the Yosemite Valley area (fig. 
2), the generalized geologic map (Calkins and Peck herein, 
fig. 1), and the glacial map (Warhaftig, herein, fig. 5). 

3.5 .\rch Rock Entrance Station, National Park Service. 

3.6 Arch Rock. Two large fallen blocks are in contact at the 
top but are separated at the bottom by enough space for 
passage of the old road. Talus of Arch Rock granite +. is 
exposed in a quarry on the north side of the road. The 
granite contains sparse inclusions of an unidentified darker- 
gray rock similar in appearance to the granodiorite at the 
Gateway. 

4.7 Elephant Rock is straight ahead. 

6.0 Junction u ith the Coulterville Road, the first road into 
Yosemite \'alley (completed as a toll road on June 17, 
1874). 

6.2 Wildcat Creek. El Capitan granite is exposed at the falls 
just west of here. 

6.5 Cascade Creek. El Capitan granite (probably Cretaceous) is 
exposed in a nearby cliff, and some large fallen blocks of 
it can be seen from the road. 



Mileage 
8.4 Junction with the Big Oak Flat Road. The original road, 
which lies farther up the slope, was completed one month 
after the Coulterville Road. At the road junction are ex- 
posures and talus of the older diorite (described as "diorite 
of the Rockslides" by Calkins and Peck). Here the diorite 
contains light-colored aggregates consisting mainly of 
plagioclase that probably formed as the result of meta- 
morphism by the nearb>- El Capitan granite. 

9.4 Turn right across Pohono Bridge. 

10.4 Turn right on Wawona Road. 

12.2 Stop in parking lot at the east portal of Wawona Tunnel. 
\'iew to the east of El Capitan, Sentinel Rock, Cathedral 
Rocks, the hanging valley of Bridalveil Creek, and Bridal- 
veil Fall (fig. 1). Nearb>' exposures of various granitic 
rocks and of diorite. 

The abundance of joints in the diorite in the opposite 
valley wall (directly north of here) contrasts strongly 



FIGURE 1. Some of the features seen from the east portal of Wawona 

Tunnel. 



* Publication authorized by the Director, U.S. Geological Survey. 
t For descriptions of this and other rock units in this section see paper by 
Calkins and Peck in this guidebook. 




(61 ) 



California Division of Mines and Geology 




[Bull. 182 



Photo 1 . North wall of the Yosemite 
Valley above the Church Bowl. Light col- 
ored, nearly flat-lying dikes of coarse 
pegmatite and Half Dome quartz monzo- 
nite intrude Sentinel granodiorite. 



Photo 2. Sentinel Cascade and its alcoves. Spoiling of sheets 
of granodiorite around the cascade has enlarged the alcove 
and steepened the cliff over which the cascade plunges. Photo 
by U.S. National Park Service. 




1962 



Merced Canyon and Yosemite Valley 



63 



Photo 3. Cothedral Rocks. Light-colored 
dikes of Bridolveil granite intrude o maze 
of older granitic and dioritic rocks. Photo 
by U.S. National Pork Service. 




with their scarcity in the massive cHffs of El Capitan and 
the Cathedral Rocks (composed mostly of El Capitan, 
Bridalveil, and Taft granites, all probably of Cretaceous 
age). The construction in the valley between El Capitan 
and the Cathedral Rocks may be due to the massive nature 
of the granitic rocks at this point. The great abundance 
of talus in the cliffs directly north of here, in contrast to 
the paucity of talus farther up the valley, is due to the 
close jointing of the diorite of the cliffs. 

The U-shape of Yosemite V'alley, in contrast to the V- 
shape of the gorge of the .Merced below El Portal, is well 
displayed here. The bottom of the U, however, is much 
flatter than in typical glaciated valle>s. According to 
Gutenberg, Buwalda, and Sharp (1956, pi. 5), the bedrock 
surface lies almost 1,000 feet beneath the floor of the valley 
between El Capitan and Cathedral Rocks, and what we see 
is essentially a plain floored by lake sediments. 

The top of the highest glacier in Yosemite N'alley, ac- 
cording to .Matthes (19.^0), reached about to the brow of 
El Capitan, and was about .^00 feet above the top of the 
Cathedral Rocks. The glacier swept around the flank of 
Sentinel Dome, but did not cover the dome. The upper 
700 feet of Half Dome, likewise, was unglaciated. These 
domes owe their form to concentric spalling of massive 
uniointed rock, not to glacial erosion. 

The steep lower course of Bridalveil Creek above Bridal- 
veil Fall is graded to the level established by the .Merced 
River during the most recent of three distinct stages of 
preglacial erosion (from oldest to youngest, the Broad 
\'alle\ , .Mountain X'alley, and Canyon stages of .Matthes, 
19U), p. 45-.50); hence it helps to define the amount of 
glacial erosion in Yosemite N'allcy. 

The \'-shaped form of the gorge of the creek, although 
t\pical of stream erosion, is preserved because the sloping 
walls of the gorge coincide with throughgoing joints in 
the otherwise nearly unjointcd rock. The upper part of 
Eireplace Creek, a little downstream from us on the op- 
posite wall, is graded to the .Mountain \'alley stage of the 
.Merced Canyon. Ribbon Creek, above the head of Ribbon 
Fall (which cannot be seen from here, but can be seen on 
the north wall of the can>'on from places a mile or two 



down the road), is graded to the Broad Yalley stage of the 
Merced (see Wahrhaftig, herein, fig. 4). 

The blasted rock face at the west end of the parking lot 
exposes a complicated mixture of diorite and El Capitan 
granite. The porph\ritic phase of the Taft granite is well 
exposed on the slope just to the west. El Capitan granite 
along the south side of the road contains blocks of partialh- 
assimilated diorite, and has a steeph- dipping foliation. 

Turn around and drive east along the south side of the 
Valley, across Sentinel Bridge and to the Ahwahnee Hotel. 

CIRCUIT OF YOSEMITE VALLEY 
Mileage 

Ahwahnee Hotel. The route is plotted on the sketch map 
of the Yosemite Valley area (fig. 2). 

03 Entrance to Ahwahnee Hotel, turn right. 

0.5 STOP 1. Church Bowl. View across valley of Glacier Point. 
In the cliff face west of Glacier Point, note the contrast 
between unjointed Half Dome quartz monzonite (probably 
Cretaceous) below, and jointed granodiorite above. In the 
center of the valle\' south of here as much as 2,000 feet of 
glacio-lacustrine debris overlies the bedrock (Gutenberg, 
Buwalda, and Sharp, 1956; see Wahrhaftig, herein, fig. 5). 
In talus and in cliff faces on the north side of the valley at 
this stop dark-colored Sentinel granodiorite is cut by 
gently dipping light-colored dikes of coarse pegmatite and 
Half Dome quartz monzonite at the margin of the large 
body of Half Dome quartz monzonite. In some of the dikes 
of quartz monzonite unequal concentration of dark min- 
erals produces a nearly horizontal la\ering. 

0.7 STOP 2. Parking lot at Yosemite National Park Head- 
quarters. Visit to Aluseum. 

1.7 View of Sentinel Rock across valley. Along the cascade of 
Sentinel Creek to the right (west) of the rock, can be seen 
recesses caused by spalling of the granitic rocks around the 
cascade. Enlargement of the recesses appears to be devel- 
oping vertical waterfalls from this sloping cascade. 

3.0 \'iew of El Capitan straight ahead. 



64 



California Division of Mines and Geology 



[Bull. 182 




ico^ v6oji pup 



2 
U 
O 

< 
o 



< 

< 

y 

UJ 



5 



Ui 

O 

>- 



l- 

u. 
O 

CL 

< 

a 

H 
m 



i: -5; 

It- o 

o _ 



1» 



o 



o 

<0 



q; 



.5 ^ 

O >- - 



o 
o 

Q) 



Q: C ^ <^ 






VsifS'/jBi^ oj," jC-o 



ro^ 



X 



1962] 



California Division of Minf,s and Geology 



65 



rt'^T^ 



Photo 4. The Yosemite Valley from Vol- 
ley View. At left is the great cliff of El 
Capitan, at the right the Cathedral Rocks 
and Br idol veil Foil. In the foreground is 
Bridalveil Meadow, which is underlain by 
almost 1 ,000 feet of Pleistocene glocio- 
lacustrine deposits. Photo by U.S. Notional 
Pork Service. 











Photo 5. Ribbon Fall and its alcove. The edges of cracks along which great 
sheets hove spoiled can be seen near the head of the alcove. Photo by U.S. 
Notional Pork Service. 



66 



California Division of Mines and Geology 



[Bull. 182 



Mileage 
3.9 Junction at nortii end of El Capitan Bridge: continue 
straight ahead. Brief stop for view, across valley, of Cathe- 
dral Rocks, Cathedral Spires, and Taft Point. On the faces 
of North and Middle Cathedral Rocks can be seen nearly 
horizontal light-colored dikes of Bridalveil granite cutting 
a maze of other granitic and dioritic rocks. 

4.6 STOP 3. (At V-7 sign.) The low ridge to the left is the 
easternmost of a series of Wisconsin terminal moraines, 
which held in the former Lake Yosemite. The lake was 
filled with as much as 300 feet of silt and sand deposited 
in advancing deltas by the Merced River and Tenaya 
Creek; this debris was possibly supplied by glaciers of 
Tioga age, which reached only as far downstream as the 
lip of Nevada Fall and the upper part of Tenaya Canyon 
(see Wahrhaftig, herein, fig. 5). Walk south along the old 
road at the east side of the moraine to the Merced River, 
a distance of about 800 feet. In the moraine at the river 
arc exposed boulders of Cathedral Peak granite, Half Dome 
quartz monzonite. Sentinel granodiorite (probably Creta- 
ceousj, and Bridalveil granite. The nearest exposure of 
Cathedral Peak granite is 12 miles east of here. Return to 
the main road and continue west. 

4.9 Turnout. Brief stop for view of Bridalveil Fall, Leaning 
Tower, and Cathedral Rocks across valley. 

5.8 Valley View turnout. Brief stop for view of El Capitan, 
Clouds Rest, and Sentinel Rock to the east and Cathedral 
Rocks and Bridalveil Fall to the southeast. The western- 
most of the Wisconsin terminal moraines lies a few hun- 
dred feet west of here. 

6.0 Turn left across Pohono Bridge. 

6.5 View of Rockslides across valley. 

6.7 Road again crosses the westernmost Wisconsin moraine. 

7.0 Junction with Wawona Road; turn right. 

7.1 STOP 4. Turn left into parking lot. Walk along trail 
(about 1,000 feet) to viewpoint at base of Bridalveil Fall. 
Near viewpoint are fallen blocks of Leaning Tower quartz 
monzonite (probably Cretaceous), Bridalveil granite, di- 
orite, and El Capitan granite. On the cliff face at the lip 



Photo 6. Rood cut through the terminal moraine on the south side 
of Yosemite Volley at the base of Cathedral Rocks. The large boulder at 
the right-hand end of the cut is of Cathedral Peak granite, the nearest 
exposure of which is 13 miles owoy. Photo by U.S. National Pork Service. 




Mileage 

of Bridalveil Fall can be seen a thick horizontal sheet of 
smooth-weathering Bridalveil granite. Underneath is red- 
dish rough-weathering Leaning Tower quartz monzonite, 
and to the east is dark diorite. Note that the lower part 
of Bridalveil Fall is in a slight recess flanked by buttresses. 
The borders of the recess are marked by the edges of 
slabs parallel to the surface, apparently the original con- 
tinuations of these slabs across the face of the fall have 
dropped from the cliff and are represented by the cone 
of talus extending up the clitf to an apex just west of the 
base of the fall. 

Looking across the vallc\- to the recess in which Ribbon 
Fall lies, one can see similar spalling in the upper part of 
this recess. Return to parking lot and drive east along the 
south side of the valley past the turnoff to Pohono Bridge. 
Bank on south side of road exposes bouldery terminal 
moraine. 

7.9 Road crosses the moraine examined at Stop 3. 

H.4 \'iew of El Capitan across valle>'. 

8.8 Junction with road to El Capitan Bridge. View of Sentinel 
Rock to the east, directly up the road. The forms of Sen- 
tinel Rock, Glacier Point, the north face of Half Dome, 
etc., are controlled b>' vertical joints trending east to 
northeast. 

9.0 Brief stop at V-33 sign for view of El Capitan and Three 
Brothers across the valley. On the face of El Capitan the 
irregular boundaries of an intrusive body of diorite form 




^^- -f^j^r^r 



' /ft^" 



Photo 7. Southeast face of EI Caplton. The irregular bound- 
aries of an intrusive body of diorite form a crude map of 
North America. 



1962] 



Merced Canyon and Yosemite Valley 



67 



''■'■:^l^' 














Photo 9. Half Dome from Glacier Point. The precipitous northwest face 
is bounded by the wall of a nearly vertical fissure. The rounded bock of 
the dome was formed by exfoliation of mossive quartz monzonite. Photo 
by U.S. National Pork Service. 



Photo 8. The Three Brothers. The west-dipping joints that give these 
monuments their characteristic shape are emphasized by a light fall of 
snow. Photo by U.S. Notional Park Service. 




Geologic strip map olong U.S. 50 in the Hoyword quadrangle 

QlifiTERNfifiV TEfiTlilRY 




^^v 



ALTAMONT 
•JJ!».ft ^^ QUADRANGLE 




mi^--" 



[ZG '^m m Esi i 



Alluvium L 



_ Netoly Nerely 

gravels fgrmalian fofmation 

lUppef- 1 (10 r«f rLoiHr-6(u» 101 



-^■^.^^iS;^:^.^i 



Kpss 






fl PleislBcene Pleisl 



^naiz] ES3 ES3ia SHca^ 



> along U S 50 in I 



ia«0» 7.S-minule q 



JURASSIC JURASSIC!?) 



□ 



'j'-^-^Ai.^. - 



•*;'^5^;.-^iS^vr;;-7;r.->.-r ._,,,_._ ___,,_^__^_^^ ___^ 



^l^'.X\ -A HAYWARD - "j" \'i3^A /^, 













CORRELATION OF GEOLOGIC UNITS 




QUATER NARY 
fteeeni Pleistocene PIm 



H iH ^ [^ I T,.!.. j'H..f ^.a [W] gjg |t^ ^ ^ 



lotmalion IUdb", 




ALTAMONT AND MIDWAY 

QUADRANGLES 
HueylTesIa Quad.), 1948 



"-^ 


TOI, 


gia.eK 


P.'::;... 


^ 


T->tn 


Upc* 


r 


Pgbig 


Wft' 
















Up 




Upp.r 



S ia FMP^ 






Vertical and noilionlal tcalci tqual OeOlDgi and nninanclaturi liDrn Hall. 1950, map h 'il < 

CtofOgic slnp map along U.S. 50 in the Oublfn and Livermore T.S-minute quodrongles. 



GEOLOGIC GUIDE MAPS ON 

HinHWAV RD-MAYWADn TD TDAPV PAI ICnDMIA 



DIVISION OF MINES AND GEOLOGY 
IAN CAMPOELL, CHIEF 



STATE OF CALIFORNIA 
DEPARTMENT OF CONSERVATION 




Ultromotic rocks 

Mostly serpentine .but includes 

some pendotile ond dunitc 



Melosedimenlory rocKs 
Mostly thin-bedded sillstone 
but includes some groywocke. 
tuff , and conglomerale Includes 
Mofiposo formotion 



Melo volconi c rocks 
Mostly pyroclostic rocks of on- 
desitic composihon,but inclu- 
des some pillow lovo ond 
metocfiert Possibly includes 
Paleozoic metovolconic rocks 
north of Moriposa 



Geology odopted by LD Clork from sources listed below 



IPcs.v 




Turner.H W , ond Ronsome.FL; 
1897 


Turner.H W, and S mil h,W S T , un- 
published 


Geologic mop of Cohfornio 
(1 250,000) ir> progress Son 
Jose sheel 


Cloos, 
Ernst, 
1932 


^^ 



Co lover OS formotion 
Pcs,Tioslly black corbonoceous phy- 
ihte with mterbedded metochert, 
Sut includes subordmole len- 
ticulor mefvolconic rocks and 
sparse sandstone and conglomer- 
ate-, 
fPcl, limestone ond dolomite; 
IPCv, ondesitic pyroclostic rocks with 
sporse pillow lowo 



Dip and sinke 



f oult zone 



Dip OnO sirihe 
of s c hislosily 



Strike of ver I icol 
sc hisfosi t y 



Direction of 
lops of beds 



GEOLOGIC MAP AND SECTION OF THE SOUTHERN PART OF THE WESTERN SIERRA NEVADA METAMORPHIC BELT 



68 



California Division of Mines and Geology 



[Bull. 182 




Photo 10. Glacial polish and slickensides on north wall of Yosemite 
Valley near Mirror Lake. 



(0.6 
II. 1 



11. 



Mileage 

a crude map of North America. This cuts an inconspicuous 
westward-sloping dike of gray rock, probably Leaning 
Tower quartz monzonite. The form of the Three Brothers 
is controlled by joints dipping obliqueh' westward. 

\'iew of Yosemite Falls across the valley. 

Road junction at south end of Sentinel Bridge. Continue 
east, not crossing bridge. South of here are remnants of 
the Old Village, the center of commercial activity in the 
valley between the late lS50's and 1917. 

Brief stop for a view to the east of North Dome, Half 
Dome, Royal Arches, Washington Column, and Glacier 
Point. The form of the Ro\al Arches, Half Dome, and 
North Dome is controlled by exfoliation of the Half Dome 
quartz monzonite, resulting from expansion due to unload- 
ing brought about by denudation. 

Road junction; continue straight ahead. 
Road crosses Happy Isles Bridge. 

Road crosses Wisconsin moraine. \'icw of North Dome 

straight ahead. 

Road junction; bear right. 

Road junction; bear right. 

Road crosses rock avalanche that dammed Tenaya Creek, 
forming .Mirror Lake. 



11.5 
12.2 

12.7 

12.8 
1.^.1 

13.5 



Mileage 
1.?.8 STOP 5. Parking lot at Mirror Lake. View of Mirror 
Lake, and of Tenaya Canyon to the east. Walk along trail 
to the northeast about 1,000 feet to see glacial polish and 
striae on Half Dome quartz monzonite. Note well-formed 
books of biotite in this rock. 

Return to parking lot and drive west. 

14.5 Road junction; bear right. 

14.7 "Indian Cave," north of road, is in coarse talus at the foot 

of the cliff. 
14.9 View ahead of sheeting in the Royal Arches, formed by 

exfoliation. 
15.3 Sugar Pine Bridge. 

15.7 Road junction; bear right. 

15.8 View ahead of flat-lying dikes of Half Dome quartz mon- 
zonite and pegmatite in Sentinel granodiorite. 

15.9 Entrance to Ahwahnee Hotel; turn right. 
16.2 Ahwahnee Hotel. 

References 

Gutenberg, Beno, Buwalda. J. P., and Sharp, R. P., 1956, Seismic 
explorations of the floor of Yosemite Valley, California: Geol. 
Soc. America Bull., v. 67, p. 1051-1078. 

Matthes, F. E., 1930, Geologic history of Yosemite Valley: 
U. S. Geol. Survey Prof. Paper 160, 137 p. 



Photo 11. The Royal Arches, North Dome, Washington Column, and 
Basket Dome, from near Glacier Point. These monuments ore carved from 
a neorly joint-free mass of Half Dome quartz monzonite. Thin exfoliation 
slabs can be seen near the top of North Dome. The Royal Arches can be 
seen in this photograph to be the edges of giant exfoliation sheets formed 
on the southwest side of this joint-free moss. Pho*o by U.S. Notional Pork 
Service. 




A51133 11-61 3,500 



pTinteJ in CALIFOKNIA STATE PRINTING OFPICB 



37' 



-500 
■10,00 



THIS BOOi' • - 



W ' 'Af ^ ' ■■ 



14 



8501 




& 



tH*l*B*«i*li 



'''^"'■^"■^"