Full text of "Rock stratigraphy of the Silurian system in northeastern and northwestern Illinois"

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STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

Rock Stratigraphy of the
Silurian System in Northeastern
and Northwestern Illinois

H. B. Willman

ILLINOIS GEOLOGICAL
""SURVEY * 10RM*
APR 3H986

ILLINOIS STATE

John C. Frye, Chief
CIRCULAR 479

GEOLOGICAL SURVEY

Urbano, IL 61801
1973

CONTENTS

Page

Abstract 1

Introduction 1

Time-stratigraphic classification 3

Alexandrian Series 5

Niagaran Series 5

Cayugan Series 6

Regional correlations 6

Northeastern Illinois 6

Development of the classification 9

Wilhelmi Formation 12

Schweizer Member 13

Birds Member 13

Elwood Formation 14

Kankakee Formation 15

Drummond Member 17

Offerman Member 17

Troutman Member 18

Plaines Member 18

Joliet Formation 19

Brandon Bridge Member 20

Markgraf Member 21

Romeo Member 22

Sugar Run Formation . . „ 22

Racine Formation 24

Northwestern Illinois 26

Development of the classification 29

Mosalem Formation 31

Tete des Morts Formation 33

Blanding Formation 35

Sweeney Formation 36

Marcus Formation 3 7

Racine Formation 39

References 40

GEOLOGIC SECTIONS

Northeastern Illinois 45

Northwestern Illinois 52

FIGURES
Figure

1 - Distribution of Silurian rocks in Illinois 2

2 - Classification of Silurian rocks in northeastern and

northwestern Illinois 4

3 - Correlation of the Silurian formations in Illinois and

adjacent states 7

4 - Distribution of Silurian rocks in northeastern Illinois (modified

from State Geologic Map) 8

lis. 5 - Silurian strata in northeastern Illinois 10

^- 6 - Development of the classification of the Silurian System in

|§ northeastern Illinois 11

7 - Distribution of Silurian rocks in northwestern Illinois (modified
from State Geologic Map) 2 7

8 - Silurian strata in northwestern Illinois 28

9 - Development of the classification of the Silurian System in
northwestern Illinois 30

10 - Index to stratigraphic units described in the geologic sections • • 46

o
CO

ROCK STRATIGRAPHY OF THE SILURIAN

SYSTEM IN NORTHEASTERN AND

NORTHWESTERN ILLINOIS

H. B. Willman

ABSTRACT

Silurian age rocks are exposed in northeastern and
northwestern Illinois in two large areas, which are sepa-
rated by a broad north-central area of older strata. On the
basis of fa unal correlations, the same formation names have
been used previously in the two outcrop regions. Inasmuch
as these correlations have been found to be partly in error
and the same names have been applied to units of greatly
differing lithology, the rock-stratigraphic nomenclature is
revised and several changes are made in both regions. The
Silurian formations have important economic uses, and a
number of reference sections are described to aid in the iden-
tification of the formations in each region.

INTRODUCTION

Silurian strata form the bedrock surface in large areas in northeastern
and northwestern Illinois (fig. 1). In both regions they consist largely of dolo-
mite and have a maximum thickness of about 500 feet. In the Chicago area an
understanding of Silurian stratigraphy has been significant in the interpretation
of well drilling records for building foundations, for water resources (Suter
et al., 1959; Zeizel et al., 1962), and most recently for the deep tunnel system
for floodwater and sewage disposal (Buschbach and Heim, 1972). Enormous
quantities of stone for building and road construction and for lime, flux, re-
fractories, and building stone have been produced from Silurian strata in north-
eastern Illinois (Krey and Lamar, 1925; Willman, 1943, 1944). In northwestern
Illinois, Silurian rocks have also been quarried in many places and in addition
are a reservoir for groundwater (Bergstrom, 1956).

The two areas of Silurian rocks in northern Illinois are separated by an
area of older formations along the Wisconsin Arch, the Ashton Arch, the La Salle
Anticline, and the Sandwich Fault Zone. South of their outcrop areas, the Si-
lurian rocks are overlapped by Pennsylvanian formations, and the only connec-
tion between the two areas occurs in subsurface south of the outcrop areas
(Willman et al., 1967, sub-Pennsylvanian geologic map).

2 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

Surface areas of
Silurian rocks

Silurian rocks
covered by younger
bedrock formations

Silurian rocks
eroded

1 M

0Tt\

Fig. 1 - Distribution of Silurian rocks in Illinois (modified from State Geologic Map)

ROCK STRATIGRAPHY OF THE SILURIAN SYSTEM 3

This report is largely concerned with the development of a rock- strati-
graphic classification of the Silurian strata to conform to present policies (Will-
man, Swann, and Frye, 1958; American Commission on Stratigraphic Nomen-
clature, 1961). It summarizes studies of the outcrop and quarry exposures and
establishes reference sections. The sections are given at the end of the re-
port, their locations are shown on figures 4 and 7, and the intervals described
are shown on figure 10. The report is based in part on outcrop data included
in an unpublished Illinois Geological Survey manuscript (1949) on the Silurian
of the Chicago region by H. B. Willman (outcrop stratigraphy), L. E. Workman
(subsurface stratigraphy), and H. A. Lowenstam (paleontology). The regional
correlations are based in part on field conferences in northeastern Illinois and
eastern Wisconsin with A. J. Boucot, G. M. Ehlers, H. A. Lowenstam, G. O.
Raasch, R. H. Shaver, D. H. Swann, and L. E. Workman.

Most of the Silurian rock units in both northeastern and northwestern
Illinois were originally differentiated by T. E. Savage (1912, 1913, 1914, 1926),
but the nearly identical sequence in Iowa across the Mississippi River from
northwestern Illinois had been differentiated earlier by Iowa geologists, in-
cluding Savage. Following the practice of his time, Savage named his units on
the basis of faunal correlations between the two regions and with other areas,
regardless of lithologic composition. Although some of the correlations were
in error, the differentiation of the rock units, with a few exceptions, was ef-
fective and has been widely used, However, changes in policy requiring de-
velopment of separate rock- and time -stratigraphic classifications makes nec-
essary a number of changes in nomenclature. The effect of the changes is to
restrict the usage of some names and to develop local rock-stratigraphic classi-
fications. This is not an objective, but it provides a base from which use of
names can be broadened as relations between the units become better known.
A need still remains for a more detailed time- stratigraphic classification based
on faunal zonation to show correlations between the areas. Only the major
aspects of time -stratigraphy are discussed in the following summary.

TIME-STRATIGRAPHIC CLASSIFICATION

The Silurian System (Murchison, 1835; Lapworth, 1879) in Illinois is
subdivided into the Alexandrian, Niagaran, and Cayugan Series (fig. 2). Berry
and Boucot (1970), who have published the most recent North American Silurian
correlation chart, accept the European classification of Llandovery, Wenlock,
Ludlow, and Pridoli Series. All of these except the Pridoli are based on type
sections in England and Wales; Pridoli is Czechoslovakian and is somewhat
controversial. Berry and Boucot indicate that the top of the Llandovery is low
in the Niagaran, the top of the Wenlock is about mid-Niagaran, and the top
of the Ludlow is near the top of the Niagaran. This classification, as shown in
their correlation chart (1970, pi. 2), does not provide well-defined faunal
criteria for differentiation of units of series rank in the Silurian of Illinois.
The biostratigraphic classification based on evolution of the graptolites is
highly significant in establishing world-wide correlation but has not proved
practical in delimiting time- stratigraphic units in the dominantly carbonate rocks
of Illinois. The conodonts promise to be more useful for regional correlations
(Rexroad, 1970; Rexroad and Nicol, 1971). Despite the inadequacies of the
present classification, there is merit in having a series classification based on
provincial units, which are more accessible for comparison.

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

NORTHWESTERN ILLINOIS

NORTHEASTERN ILLINOIS

THIS REPORT

Series

Formation

Member

Formation

Series

Z
<

rr
<

Racine

Z
<

rr
<

Sugar Run

Waukesha

Marcus

Joliet

Romeo

Markgraf

Brandon
Bridge

Joliet

Sweeney

Kankakee

Plaines
Troutman
Offerman
Drummond

Kankakee

z
<

_l
<

z
<
rr

z
<

_J
<

Kankakee

Blanding

Elwood

Edgewood

Tete des Morts

Wilhelmi

Birds
Schweizer

Mosalem

Fig. 2 - Classification of Silurian rocks in northeastern and northwestern Illinois

ROCK STRATIGRAPHY OF THE SILURIAN SYSTEM 5

Alexandrian Series

The Alexandrian Series was named by Savage (1908, revised 1916) for
exposures in Alexander County, Illinois. Savage found that fossiliferous strata
in the Mississippi Valley, previously included in the Niagaran, were older than
the faunas in the Clinton in New York, which contains the oldest fauna in the
type Niagaran. He established the top of the Stricklandia pyriformis Zone,
called Stricklandinia in some reports, and at present called Microcardinalia
triplesiana, as the top of the Alexandrian. This widespread zone occurs at
the top of the Brassfield, Kankakee, Sexton Creek, and Sweeney (new this re-
port) Formations, and recognition of it in effect defines, and in some areas
redefines, the base of the Niagaran. The Alexandrian Series, therefore, con-
sists of all the Silurian strata including, and older than, the Microcardinalia
Zone. Berry and Boucot (1970) correlate the Microcardinalia Zone with the base
of the upper Llandovery of Europe. Instead of "Alexandrian," some authors
have preferred the terms "Medinan" (Fisher, 1959) or "Albion" (Swartz et al.,
1942), both based on the New York section.

Savage (1926) reported the presence of Microcardinalia at the top of the
zone of cherty dolomite at Savanna and, therefore, correlated the cherty zone
with the Kankakee Dolomite of northeastern Illinois. The zone of Microcardinalia
was lost for many years, although Scobey (1938) reported one occurrence at
Sabula, Iowa. In 1950 Lowenstam and Willman found Microcardinalia in a zone
in the Joliet Formation 20 to 25 feet above the top of the Kankakee at several
localities near Savanna. As the strata in northwestern Illinois formerly called
Joliet, and herein named Sweeney, are very similar both in lithology and in
abundance and variety of corals to the Kankakee of northeastern Illinois, the top
of the Alexandrian is raised to the top of the Sweeney; however, it is recognized
that more detailed studies of the faunas are needed to confirm the position of
this time-stratigraphic boundary in northwestern Illinois.

The widespread occurrence of the closely related Platymerella (in Illinois
and areas to the south) and Virqiana (in areas to the north) provides a possible
basis for subdivision of the Alexandrian. In some localities as much as half
the Alexandrian occurs below the Platymerella Zone.

Niagaran Series

The Niagaran Series (Hall, 1842; Swartz et al., 1942), based on the
Niagara Falls Section in New York and restricted by assigning the pre -Clinton
strata to the Alexandrian Series, consists in northern Illinois of all the Silurian
strata above the Microcardinalia Zone. It contains the faunas characteristic
of the Clinton, Lockport, and Guelph of the New York type section. However,
the contact of the type Niagaran with the Cayugan above is not marked by a faunal
zone that permits definite correlation of the boundary. The Cayugan Series is
based on the Salina Group, which lacks marine faunas. Savage (1926) de-
scribed Guelph fossils in the highest Silurian strata (Racine) in both northeastern
and northwestern Illinois, and it appears that the northern Illinois Niagaran is
closely equivalent to the type Niagaran.

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

Cayugan Series

The Cayugan Series (Clarke and Schuchert, 1899; Fisher, 1959), named
for Lake Cayuga in New York, is thought not to be represented in the Silurian
rocks of northern Illinois . It has been advocated that the Racine reefs of
northern Illinois formed part of a barrier enclosing a basin, largely in Michigan,
Ontario, and Ohio, in which the Cayugan deposits, including thick evaporites,
accumulated (Briggs, 1958, 1962). The concept has merit in explaining the
origin of the evaporites, but, if true, it is not possible at present to identify
a time plane equivalent to the base of the Cayugan within the Racine Formation.
As the Racine is overlain unconformably by Middle and Upper Devonian rocks,
Cayugan equivalents, possibly reefs, could have been eroded from northern
Illinois. Cayugan strata are present in southern Illinois, where there was
essentially continuous sedimentation from Niagaran time to the early Devonian
and where the top of the Cayugan Series is in the lower part of the Bailey Forma-
tion.

Regional Correlations

The correlation of the Silurian formations of Illinois with those in ad-
jacent states is shown in figure 3.

NORTHEASTERN ILLINOIS

Silurian age rocks are the uppermost bedrock formations in a large area
in northeastern Illinois (fig. 4). The bedrock is largely covered by glacial drift,
and exposures are limited to the principal valleys and to quarries in the Chicago
Lake Plain (Fisher, 1925; Athy, 1928; Bretz, 1943; Willman, 1943, 1971). In
extensive areas the glacial drift is thick and there are no bedrock outcrops.
Silurian rocks are also at the bedrock surface in Ford and Iroquois Counties,
southwest of the area (fig. 1), but they are deeply buried by glacial drift.

In the outcrop area in northeastern Illinois the Silurian rocks generally
have an easterly dip from the Wisconsin Arch into the Michigan Basin. This
dip is interrupted by local structures, such as the Des Plaines Disturbance,
the Sandwich Fault, and the Herscher Dome. Because of the easterly dip, the
oldest Silurian formations are exposed along the west side of the outcrop area
along the Kankakee, Des Plaines, Du Page, and Fox Rivers. The youngest are
exposed along the Lake Michigan shore and near the Illinois-Indiana state line,
where the Silurian rocks are nearly 500 feet thick. The upper 150 feet is ex-
posed in a quarry at Thornton in Cook County. Although the Silurian rocks gen-
erally thicken eastward across the area, the thickness varies because of the
unconformity at the base, which has a relief of as much as 125 feet, and the
presence in the top of drift-filled valleys, which have a local relief of as much
as 150 feet (Suter et al., 1959).

The Silurian rocks unconformably overlie the Ordovician Maquoketa Group,
They generally rest on the eroded surface of the Brainard Shale, but locally they
cut entirely through it and overlie the Fort Atkinson Limestone. The Silurian
rocks are separated from the overlying Middle Devonian rocks by a major uncon-
formity. Middle Devonian limestone formations overlie the Silurian rocks in

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8 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

WISCONSIN

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NORTHEASTERN ILLINOIS 9

Indiana a short distance east of Illinois and also along the south edge of the
northeastern Illinois area (Willman et al., 1967). Upper Devonian black shale
occurs in local pockets in the youngest Silurian at Thornton (Bretz, 1939) and
also in even older Silurian rocks at Elmhurst (Weller, 1899; Alden, 1902). This
may indicate an overlap of Upper Devonian strata following erosion of the Middle
Devonian limestones, but the Upper Devonian shales could have been deposited
in joints extending through Middle Devonian strata now eroded.

The Silurian rocks of northeastern Illinois are almost entirely dolomite.
In a small area near Wilmington, Will County, the Kankakee Formation (fig. 5)
is limestone with dolomite mottling. Where the basal Wilhelmi Formation is
very thick, the lower 10 to 15 feet is largely a gray dolomitic shale but a thin
basal sandstone has been reported locally. A bed of shale as much as 2 feet
thick occurs locally in the Brandon Bridge Member of the Joliet Formation, and
small patches of shale occur on the flanks of some of the larger Racine reefs.
In general, the formations composing the lower half of the Silurian are argilla-
ceous or silty to moderately pure dolomite in units that have a distinctive lith-
ology and can be traced throughout the area. The overlying Racine Formation,
which forms the upper half of the Silurian section, consists of pure dolomite in
reefs surrounded by argillaceous, cherty dolomite, and no widely traceable
lithologic units have been found in it. Consequently, the strati graphic positions
of outcrops in the upper half of the Silurian are based largely on references to
some identifiable unit in the lower part of the Silurian.

Development of the Classification

Silurian rocks in northeastern Illinois were early correlated with the
Niagara Limestone (fig. 6), and the name "Niagara" was used in Illinois for many
years. In 1910 Savage reported pre-Niagaran fossils in a thin limestone exposed
about a mile south of Channahon, indicating the presence of the Alexandrian Series,
which he had earlier named for exposures in Alexander County, Illinois. By
1926 Savage had developed the classification that has been used with minor
changes ever since. However, in 1942, in the Silurian correlation chart,
Savage proposed, without adequate explanation or definitions, several changes
in the classification that were not accepted (Willman, 1943). By that time
studies in the Chicago area and elsewhere in the Midwest had established
the reef-interreef relations of some Silurian formations. For that reason, Will-
man restricted the Waukesha to the distinctive beds that underlie the reef zone
and have been quarried widely for building stone. The major part of the Waukesha
of Savage, the interreef beds, was included with the reefs in the Racine Forma-
tion. Savage's proposal to replace "Waukesha" with "Bellwood" was rejected
because his type Bellwood was largely referable to the interreef part of the
Racine. Savage (1942) proposed replacing "Port Byron" with the New York-
Ontario term "Guelph," but because no lithologic basis could be found for dif-
ferentiation of the Racine and the Port Byron, the Racine was extended to the top
of the Silurian. At the same time, Savage proposed restricting the Joliet by
differentiating unspecified basal beds as a new formation, the Rockdale, but
the name "Rockdale" was preempted and the unit was left in the Joliet. It
presumably included the strata herein differentiated as the Brandon Bridge
Member of the Joliet. Lowenstam (1949b) assigned all the Niagaran strata in

Pig. U- - Distribution of Silurian rocks in northeastern Illinois (modified from State
Geologic Map). The dashed line roughly outlines the areas where the Silurian
rocks have a thick cover of glacial drift and do not crop out. Sections are
described on pages U-5 to 55.

10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

FORMATION

MEMBER

COLUMN

THICK*
(ft)

GENERAL CHARACTER

Racine

300

Dolomite, pure, gray, vuggy, in reefs; and
argillaceous, silty, brownish gray and
greenish gray, cherty dolomite with beds
of relatively pure dolomite, between the
reefs

/;y/-/

Sugar Run

-/ ~ / A

- / - T^r
-/ - / ~

10-30

Dolomite, slightly argillaceous and silty,
light greenish gray, brown-weathering;
in smooth-surfaced medium beds; building-
stone beds

z z z

Romeo

A/ ,/

18-34

7 T

Dolomite, pure, I ight gray to white, mot-
tled gray and pink; in faint thin stylo-
tic beds

Joliet
40'-80'

Markgraf

Z A^ V

12-28

Dolomite, silty at base to slightly argil-
laceous at top, very light gray, cherty,
med ium-bedded

Brandon
Bridge

^ ' / -~ 7

-/ 7 I -T^-

11-25

Dolomite, argillaceous to shaly, gray, red,
green; contains a few pure beds; sili-
ceous foraminifera abundant

Ploines

"7" y

1.5-3

Dolomite, pure, white, massive; contains Microcardi-
nal ia and Pentamerus; pitted smooth surface on top

^P^r

Kankakee
20'- 50'

Troutman

T~TT

ZZZ5

11-29

Dolomite, pure, pinkish gray to greenish
gray; in thin wavy beds with green clay
partings; corals common

~r~7

Offermon

~T~T

2.5-11

Dolomite, as above, but slightly argilla-
ceous

Drummond

J^L

Dolomite, as above, but massive, vuggy, glauconiti
locally sandy; contains PI atymere I I a at base

/ ~ / A

Elwood

AAA
- / /

0-30

Dolomite, slightly argillaceous, brownish
gray; contains many layers of white chert

Birds

/ -/

0-20

Dolomite, argillaceous, gray, slightly
cherty

Wilhelmi
0-100'

-7 / ~ -h

Schweizer

0-80

Dolomite, very argillaceous, gray, and dolo-
m i 1 i c sha I e

/-/~/

-r t- -7-

*Where overlain by next younger unit.

Fig. 5 - Silurian strata in northeastern Illinois.

NORTHEASTERN ILLINOIS

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12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

northeastern Illinois to the Thorn Group, but this unit was part of a facies
classification and the name has not been accepted in rock-stratigraphic clas-
sification. Most of the units in the northeastern Illinois Silurian differentiated
in the present study were briefly described, but not named, in a previous report
(Willman, 1962).

In the present report, the term "Waukesha" is replaced by the local term
"Sugar Run," because these strata comprise only part of the section originally
included in the Waukesha at Waukesha, Wisconsin. Although the term "Edge-
wood" has long been used for the basal formation of the Silurian, correlation
with the type Edgewood in eastern Missouri was based entirely on fossils and
only part of the unit bears lithologic similarity to the type Edgewood. The Edge-
wood in northeastern Illinois includes two distinctive lithologic units that merit
recognition as formations, and therefore the term "Edgewood" is dropped. The
only other changes are the differentiation of members in several formations.

Wilhelmi Formation

The Wilhelmi Formation consists of argillaceous dolomite and dolomitic
shale that fills, or nearly fills, channels eroded in the underlying Brainard Shale
of the Maquoketa Group. In places, the channels cut through the shale to the
top of the Fort Atkinson Limestone. The Wilhelmi is as much as 100 feet thick in
the deeper channels but is absent or very thin in the areas between the channels.
It is overlain by the Elwood Formation, which also thins out in the interchannel
areas, with the result that in places the overlying Kankakee Formation overlaps
the Elwood to rest directly on thin phases of the Wilhelmi.

The Wilhelmi Formation was formerly the lower part of the Edgewood
Formation. It is named herein for Wilhelmi Airport in Will County, 4 1/2 miles
northeast of the type section, which is a railroad cut on the southeast side of
the Des Plaines River and is described as the Schweizer West Section.*

The Wilhelmi Formation includes the Channahon Limestone, which was
named for a few feet of dolomite, at present not exposed, about a mile south of
Channahon, Will County (Savage, 1910), and the Essex Limestone, which was
named for exposures along Horse Creek, 2 miles east of Essex, Will County
(Savage, 1912). Athy (1928) correlated the oolite below the Essex with the Noix
Oolite of Missouri and made the Noix and Essex members of the Edgewood. How-
ever, the oolite more recently has been correlated with the Ordovician Neda Oolite
of Wisconsin (Workman, 1950). The Essex, therefore, was equivalent to the
Edgewood, and neither "Channahon" nor "Essex" has been used for many years.

In outcrops, the Wilhelmi is thickest, about 40 feet, in the area of the
type section. It is either absent or represented by only a foot or two of argil-
laceous dolomite in exposures along the Kankakee River from near Ritchey south-
east to the Will-Kankakee county line (Cowan's Quarry and Kankakee River
Campground Sections). The Wilhelmi is about 15 feet thick in exposures along
Horse Creek between Essex and Custer Park in Will County. Along the Fox Valley,
it is absent in exposures south of Elgin (South Elgin Section), where the Kankakee
Formation rests directly on the shale of the Maquoketa Group. About 3 feet of
argillaceous dolomite assigned to the Wilhelmi Formation overlies the Fort Atkin-
son Limestone of the Maquoketa Group in an abandoned quarry east of Garden
Plain (SE SE NW 31, 44N-5E, McHenry Co., Genoa Quad.).

*The described sections and their locations are given at the end of the report. Locations of
sections mentioned, but not described in this report, are given in the text.

NORTHEASTERN ILLINOIS 13

The Wilhelmi Formation consists of two members: the upper, argilla-
ceous, dolomite is the Birds Member, and the lower, very argillaceous dolomite
anddolomitic shale, is the Schweizer Member. The Wilhelmi is differentiated
from the overlying Elwood Formation, which is much purer, is not shaly, and
contains much chert in persistent layers. Where it is very thick and the basal
part is a dolomitic shale, the Wilhelmi is not readily separable from the under-
lying shale of the Maquoketa Group, particularly in well samples (Ostrom, 1957).
However, the Wilhelmi is generally dark gray and the contact is placed at the
first appearance of the green shale characteristic of the Brainard Shale. In the
type section a thin conglomeratic dolomite bed is considered to mark the base of
the Wilhelmi .

The Wilhelmi Formation contains fossiliferous beds, particularly in the
upper part. Most of the fossils are dolomitic casts and molds, but Savage (1913)
described a large fauna from the Channahon and Essex exposures and Ross
(1962a) described graptolites from the Wilhelmi in the Schweizer West Section.

The Wilhelmi Formation is similar lithologically to the Mosalem Forma-
tion in northwestern Illinois and in Iowa, and two names may not be necessary
for these units. In southern Illinois the strata called Edgewood are somewhat
similar to the upper part of the Wilhelmi. North of Illinois in eastern Wisconsin,
basal Silurian strata similar to the Wilhelmi appear to be only locally present.
In a quarry in High Cliff Park overlooking Lake Winnebago, Calumet County
(SW SE 36, 20N-18E, Neenah Quad.), 10 feet of argillaceous dolomite like the
Wilhelmi overlies the Maquoketa Shale and underlies 20 feet of moderately pure
but very cherty dolomite like the Elwood. Both units are called MayvLlle in that
region. At Katell Falls, near Kolb, Brown County, Wisconsin (SE SE NE 32,
23N-21E, Denmark Quad.), the Mayville consists largely of relatively pure
Kankakee, but the lower few feet, overlying the Neda Oolite, is argillaceous
dolomite similar to the Wilhelmi.

Schweizer Member

The Schweizer Member of the Wilhelmi Formation consists of very argil-
laceous dolomite and dolomitic shale. It is locally as much as 80 feet thick
but is generally present only where the formation is more than about 20 feet thick,
It occurs, therefore, only in the deeper parts of the major channels eroded in
the underlying Maquoketa Group. It is overlain by the Birds Member.

The Schweizer Member is named herein for Schweizer School, a mile
east of the type section, the Schweizer West Section.

The only good exposures of the Schweizer Member are near the type
section, where the upper 15 feet is largely very argillaceous, silty, medium to
dark gray dolomite containing beds of dolomitic shale. A few thin beds are
fossiliferous. The lower 8 feet is dominantly medium to dark gray shale with a
few beds of argillaceous dolomite .

Birds Member

The Birds Member of the Wilhelmi Formation consists of 10 to 20 feet
of slightly to moderately argillaceous dolomite that underlies the Elwood Forma-
tion and overlies either the Schweizer Member or the Maquoketa Group.

14 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

The Birds Member is named for the railroad siding at Birds (Channahon
7.5-minute Quad.), 2 miles northwest of the type section, which consists of
two exposures: one, exposing the lower part, is the same as the type section
for the Schweizer Member (Schweizer West Section); the other, exposing the
upper part, is in a ravine a mile northeast and is described as the Schweizer
North Section. The two sections are connected by nearly continuous railroad
cuts.

The Birds Member is argillaceous and medium gray and contains only a
few scattered nodules of chert. It differs from the Elwood Formation above,
which is very cherty, much less argillaceous, and brownish gray. The Birds
contains several 2- to 3-inch beds of relatively pure dolomite that have a dis-
tinctly laminated surface where weathered. Some of the purer beds are fossil-
iferous .

Elwood Formation

The Elwood Formation consists of 25 to 30 feet of pure to slightly argil-
laceous dolomite containing many layers of white chert. It overlies the Wilhelmi
Formation and is overlain by the Kankakee Formation. Both contacts are con-
formable. It is named herein for the town of Elwood, Will County, which is 5
miles south of the type section, a ravine on the southeast side of the Des Plaines
River, described as the Schweizer North Section. Although the Elwood is partly
covered in this ravine, its relation to both underlying and overlying formations
is well shown. Supplementary sections occur in railroad cuts northeast to the
Plaines West Section, where the upper 10 feet is well exposed.

The Elwood Formation was previously the upper, cherty member of the
Edgewood Formation, but it is lithologically distinct from the underlying argil-
laceous dolomite of the Wilhelmi Formation and therefore it is differentiated as
a separate formation. Although its equivalence to part of the Edgewood of the
type area in northwestern Missouri is indicated by its fossils (Savage, 1913,
1926), lithologically it is unlike the Edgewood.

Because of the strong eastward dip, the Elwood Formation is exposed
along the Des Plaines Valley only from the Schweizer School area northeast for
about 2 miles to the Plaines Station area. It is also exposed along the Fox
Valley north of Aurora (North Aurora Section), but it is absent south of Elgin
where the Kankakee Formation overlies the Maquoketa Group (South Elgin Sec-
tion). It is also absent along the Kankakee River at the Cowan's Quarry Section
and in quarries at Elmhurst and Hillside. Although absent as a cherty unit
along Horse Creek east of Essex, it may be laterally equivalent to argillaceous
beds at the top of the Wilhelmi, which contain Platymerella. In the quarry
east of Garden Plain, McHenry County, mentioned under Wilhelmi Formation,
the Wilhelmi Formation is overlain by 5 feet of dolomite that contains layers of
white chert and is assigned to the Elwood Formation.

The Elwood Formation consists of pure to slightly argillaceous, brownish
gray dolomite that is dense to slightly vesicular and is largely in 3- to 6-inch
beds. It contains dense white chert in layers 2 to 4 inches thick. Chert makes
up 40 to 50 percent of the upper part of the formation but is less abundant down-
ward. The Elwood is generally fossiliferous, and silicified corals are common.
Although Platymerella manniensis has been considered to mark the base of the
overlying Kankakee Formation, it also occurs, especially in chert layers, in the
upper few feet of the Elwood Formation .

NORTHEASTERN ILLINOIS 15

A similar cherty unit has been recognized in subsurface south of the
outcrop region and westward to La Salle County (Elwood Atherton, personal
communication). It closely resembles the Blanding Formation in northwestern
Illinois, but its continuity with the Blanding has not been established. Its
northward continuity has not been traced, but 15 to 20 feet of dolomite con-
taining chert layers in the Mayville south of Pewaukee Lake, Waukesha County,
Wisconsin (quarry NE NE SE 23, 7N-18E, Hartland Quad.), is similar to the
Elwood. Farther north in Wisconsin 20 feet of cherty dolomite in the Maysville
Formation, lithologically like the Elwood, is exposed in High Cliff Park over-
looking Lake Winnebago, as mentioned under Wilhelmi.

Kankakee Formation

The Kankakee Formation consists largely of greenish gray to pinkish gray
relatively pure dolomite that occurs in thin wavy beds separated by green clay
partings. It varies from 20 to 50 feet thick, overlies the Elwood Formation
conformably, and is separated from the Joliet Formation above by a diastem.

The Kankakee Formation is named for the Kankakee River (Savage, 1916),
and the type section is described in the Cowan's Quarry Section. The Kankakee
Formation was described as the strata including the Platymerella manniensis
Zone at the base and the Stricklandia pyriformis Zone at the top. Stricklandia
pyriformis is now referred to Microcardinalia triplesiana, and the zone is called
the Microcardinalia Zone. These zones embrace a distinctive lithologic unit,
which justifies retention of the Kankakee as a formation. Savage, in 1942, re-
placed "Kankakee" with the older name "Brassfield" on the basis of the correla-
tion of the faunal zones with the Brassfield of Kentucky, but this use has not
been followed.

The Kankakee Formation, about 40 feet thick, is exposed along the Des
Plaines Valley from 3 miles below Brandon Bridge (Schweizer North Section) to
the south side of Joliet (Joliet— Lincoln Quarry Section), where the east dip
carries it below the Joliet Formation. However, the Kankakee is exposed in
deep quarries at Joliet (Joliet— National Quarry Section), at Lockport, and west
of Chicago (Elmhurst and Hillside Quarry Sections).

The formation thins southward from the Des Plaines River and is only
about 20 feet thick where exposed along the Kankakee River from near Custer
Park to Warner Bridge (Cowan's Quarry and Kankakee River Campground Sections).
It is at least 30 feet thick where exposed along the Fox River in widely scattered
areas from Oswego, Kendall County (quarry in NW SW NW 16, 37N-8E, Aurora
South 7.5-minute Quad.), to North Aurora (South Elgin and North Aurora Sections).
About 7 feet of the Kankakee Formation is exposed in a shallow quarry 2 miles
southeast of Capron, Boone County (NE NE NE 24, 45N-15E, Harvard Quad.).

Although dominantly a relatively pure, greenish gray to pinkish gray,
fine- to medium -grained dolomite in thin beds with green clay partings, the
Kankakee Formation is subdivided into four members with slightly differing
lithologies. A 2- to 3 -foot thick bed at the top is pure, massive, nearly white
dolomite, and is here differentiated as the Plaines Member. The major and
most characteristic part of the formation is the thin-bedded, pinkish gray Trout-
man Member, which underlies the Plaines. Below it is a thinner bedded, slightly
argillaceous unit, the Offerman Member. The basal member is also a pure massive

16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

unit and is named the Drummond Member. The members are all recognizable
from Elmhurst and Hillside southward to the Kankakee River, a distance of about
50 miles. The type sections are all in the exposures at Plaines Station, 2 miles
southwest of Brandon Bridge, where the members are best developed. The
members are all present and more accessible, although thinner, in the Kankakee
type section (Cowan's Quarry Section).

The Kankakee Formation contains little chert. A few scattered nodules
occur in the Troutman Member, and a few large nodules and lenses occur in the
middle and lower parts of the Drummond Member. A few grains of glauconite
occur throughout the formation, but they are common only in the Drummond,
which also contains scattered grains of quartz sand in some localities . The
formation is dolomite in the outcrop area, except in a locality 1 mile east of Wilming-
ton, Will County, where at least the upper 12 feet is a very fine grained lime-
stone (quarry in NW NE NE 31, 33N-10E, Symerton 7. 5 -minute Quad.).

The Kankakee Formation is generally fossiliferous, and a large fauna has
been listed by Savage (1913, 1926) and Athy (1928). Corals are common through-
out the formation and silicified corals are particularly abundant in the Drummond
Member. Platymerella manniensis occurs in the lower 1 to 2 feet of the Drummond,
particularly in chert lenses. Pentamerus oblongus and Microcardinalia occur
near the top— in the Plaines Member and the upper part of the Troutman Member.

Although it has been suggested that an unconformity occurs at the base of
the Kankakee Formation (Berry and Boucot, 1970), there is no physical evidence
of a break at the Kankakee-Elwood contact, which occurs within the Platymerella
manniensis Zone. The top of the Kankakee has frequently been interpreted as
an unconformity because of its widespread smooth surface (Savage, 1926, 1942;
Fisher, 1925; Athy, 1928) and because of missing faunas (Berry and Boucot, 1970).
The distinctive smooth surface with its abundant deep pits filled with green
clay is similar to corrosion surfaces that occur abundantly in the Ordovician
Galena Group formations . The surfaces in the Galena generally truncate only
an inch or two of section, and they are considered to indicate minor diastems—
intervals of solution rather than deposition (Templeton and Willman, 1963).
Because the smooth surface on the Kankakee Formation extends for 50 miles
without truncating the 2- to 3 -foot Plaines Member, or even showing undulatory
relief, it is difficult to interpret the surface as an unconformity. The major
unit that appears to be missing at this position is the Marcus Formation of north-
western Illinois (the Schoolcraft Formation of Wisconsin), which contains
abundant shells of Pentamerus oblongus . The Marcus Formation could have
been deposited in northwestern Illinois during the short interval of nondeposition
represented by the smooth surface in northeastern Illinois.

In western Illinois (Calhoun and Jersey Counties), the name "Kankakee"
is also applied to the strata that include the Platymerella and Microcardinalia
Zones. In that area the formation is a very fine grained limestone, but it has the
same bedding characteristics as'in the type region and a similar pitted smooth
surface on the top. The Kankakee Formation is also correlated with the Sexton
Creek Limestone in southern Illinois. It is similar to the Sexton Creek litholog-
ically, and separate names are probably unnecessary. South and southeastward
from Illinois, the Kankakee is equivalent to the Brassfield Limestone in southern
Indiana, central Kentucky, and western Tennessee.

The Kankakee Formation appears to be equivalent to the Sweeney Forma-
tion in northwestern Illinois, which it resembles lithologically. However, the

NORTHEASTERN ILLINOIS 17

Tete des Morts Formation in that area has many characteristics of the basal
Drummond Member of the Kankakee, and it is not impossible that the Tete des
Morts, Blanding, and Sweeney Formations all correlate with the Kankakee.
Previously only the Blanding Formation has been correlated with the Kankakee
(fig. 2).

The Kankakee Formation thickens northward and in eastern Wisconsin
it is equivalent, and similar lithologically, to much of the Mayville and to the
Byron and Hendricks Formations. In the type Mayville area in Dodge County,
Wisconsin, all of the Mayville appears to be Kankakee (quarry in SW 1, 11N-16E,
Horicon Quad.). At the type section of the Neda Oolite, also in Dodge County
(NE SE SW 12, 11N-16E, Harrison Quad.), the Mayville is all pure, massive
Kankakee resting directly on the Neda, as it does along the Kankakee River in
Illinois. The type of the Byron Formation, at Hamilton, Fond du Lac County,
Wisconsin (quarry in NE SW 10, 14N-17E, Campbell sport Quad.), is all Kankakee
All of the strata exposed in a quarry at Brillion, Calumet County, Wisconsin
(SE SW SW 24, 20N-20E, Chilton Quad.), which have been called Schoolcraft,
also appear to be Kankakee. As noted under Wilhelmi, the Mayville at Katell
Falls, Brown County, consists largely of Kankakee-type dolomite. The generally
massive, nearly white, pure dolomite of the Hendricks Formation in Wisconsin,
as much as 40 feet thick, may be equivalent to the few feet of the Plaines Mem-
ber of the Kankakee Formation in Illinois .

Drummond Member

The massive basal member of the Kankakee Formation is herein named
the Drummond Member for the village of Drummond, Will County, 5 1/2 miles
southwest of the type section, which is a railroad cut on the southeast side
of the Des Plaines River described in the Plaines West Section. It is 8 feet
thick in the type section, but it thickens northeastward to 11 feet at Joliet
(Joliet— National Quarry Section) . It thins southward to 5 feet along the Kanka-
kee River (Kankakee River Campground Section) and to only 1 foot where it rests
directly on the Maquoketa Shale Group (Cowan's Quarry Section). The member
is also exposed at the base of the deep quarries at Elmhurst and Hillside. It
is 8 feet thick in the Fox Valley (North Aurora Section).

The Drummond Member is relatively pure, vesicular, vuggy dolomite
similar to that in the Troutman Member, but it differs from the Troutman in being
massive and in containing scattered glauconite grains. A few grains of glauco-
nite occur on most surfaces an inch or more square. The unit also contains
scattered, well-rounded, medium grains of quartz sand like St. Peter sand.
Sand grains are less abundant than the glauconite grains and are not found in
some exposures. Silicified corals are abundant. The basal contact is tran-
sitional, within an inch or two, to the less vesicular, well-bedded, brownish
gray dolomite of the Elwood Formation.

Offerman Member

The Offerman Member of the Kankakee Formation, which overlies the
Drummond Member, differs from the Drummond and the overlying Troutman in
being slightly argillaceous, less vesicular, and thinner bedded and in having

8 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

a smoother, lighter colored weathered surface. It is named herein for Offer-
man School, which is 3/4 mile southeast of the type section at Plaines Station
(Plaines West Section).

The Offerman Member is thinnest, only 2 feet 6 inches thick, in the type
section. It is slightly thicker, 3 feet to 3 feet 10 inches, along the Kankakee
River, and is 6 to 11 feet thick in the quarries at Joliet, Elmhurst, and Hill-
side. It has been recognized only in northeastern Illinois.

Troutman Member

The Troutman Member, which overlies the Offerman Member, comprises
50 to 70 percent of the Kankakee Formation and conforms best to the generalized
description of the formation as pure, slightly vesicular, greenish gray to pinkish
gray dolomite in thin wavy beds with green clay partings. It is named herein
for Troutman Grove Cemetery, 2 miles south of the type section, which is in
the Plaines East and Plaines West Sections. The two sections, about one-fourth
mile apart, are correlated by intervening exposures and by a persistent bed
of nearly white clay that occurs in both sections .

The Troutman Member is 27 feet thick in the type section. The principal
thickness variations of the Kankakee Formation are in the Troutman, which thins
southward, somewhat unevenly, from a maximum of 29 feet at Elmhurst to 11
feet along the Kankakee River. The bed of white clay in the type section is
less than 1 inch thick, but it is present in nearly all exposures of the member
and makes a strong reentrant. One-half to three-fourths of the member occurs
above the clay. The part below the clay is very slightly argillaceous and a little
thicker bedded in some sections. The clay resembles bentbnite, but is a high-
illite clay (analyses by H. D. Glass).

The Troutman Member contains a few widely scattered small nodules of
chert. Corals are common throughout the member. It locally contains a few
sharply lenticular massive units as much as 2 feet thick, and 5 feet across,
which the adjoining beds bend over. These masses appear to have stood in re-
lief on the sea floor and are called "baby reefs.11

Plaines Member

The Plaines Member is the uppermost member of the Kankakee Formation
and generally is a massive bed only 2 to 3 feet thick. It is named herein for
Plaines Station, a railroad switching point on the south side of the Des Plaines
River, 1 1/2 miles southwest of Brandon Bridge in Will County. The name is
shown on early editions of the Wilmington 15-minute topographic map. The type
section is in the Plaines East Section, which is an abandoned quarry between
two railroads at the east end of a connecting switch. A more accessible supple-
mentary section is in the Lincoln Quarry, south of Brandon Bridge (Joliet — Lincoln
Quarry Section) .

The Plaines Member is well exposed at many places throughout the area,
and it was always found where the top of the formation is exposed. It is 2 feet
2 inches thick in the type section, but varies from 1 foot 6 inches in the Markgraf
Quarry at Joliet to 3 feet 4 inches in the National Quarry, less than a mile to
the southeast. In exposures from Elmhurst and Hillside to the Kankakee River,
it averages 2 feet 4 inches in thickness.

NORTHEASTERN ILLINOIS 19

The Plaines Member consists of nearly white, generally massive, mod-
erately vesicular, pure dolomite. In some localities the member, particularly
the lower half, contains very thin, green clay partings, as in the Troutman Member
below, and where long weathered, it has a bedded appearance. The contact
with the Troutman commonly is gradational.

The top of the Plaines Member is the smooth but deeply pitted surface
mentioned above and well described in previous reports (see illustrations in
Fisher, 1925, fig. 6, p. 28, and fig. 7, p. 29, and in Athy, 1928, fig. 13,
p. 45).

As previously noted, the Plaines Member is the Microcardinali a Zone.
However, Microcardinalia , along with Pentamerus oblongus, also occurs in
the Troutman Member below. The lower part of the Plaines Member is the princi-
pal position at which these fossils occur, but they are very erratic in occurrence:
clusters of hundreds of specimens occur in places, and none a few feet away.
In one locality previously mentioned, a mile east of Wilmington, the Plaines
Member is lithographic limestone, and there it contains abundant crinoidal debris
along with the pentamerids .

Joliet Formation

The Joliet Formation consists of 40 to 80 feet of dolomite that is shaly
and silty at the base, of intermediate purity in the middle, and pure at the top.
It overlies the Kankakee Formation and underlies the Sugar Run Formation through-
out the outcrop area. However, the latter is not present in subsurface in parts
of the region and there the Racine Formation rests directly on the Joliet. The
Joliet Formation is named for the city of Joliet (Savage, 1926), and the type
section is in a quarry on the south side of Joliet, Will County (Joliet— National
Quarry Section). Although Savage later (1942) proposed restricting the Joliet
by removing a basal zone, the change was not followed and Joliet is retained
in its original definition.

The Joliet Formation is 68 feet thick in the type section and is generally
70 to 80 feet thick where exposed along the Des Plaines Valley from about 2
miles below Brandon Bridge to a quarry in the valley floor about a mile west of
Lemont. However, the top is near the valley floor from Joliet to Lemont and the
formation is exposed only in quarries in the valley bottom at Joliet (State Prison
Quarry), Lockport (Material Service Corporation Quarry), Romeo (abandoned
quarries), and Lemont (Lemont West Quarry Section). Farther north and east,
the formation is covered by the Sugar Run Formation and is exposed only in the
deep quarries at Elmhurst and Hillside. The formation is partly exposed along
the Du Page River at and below Naperville, and along the Fox Valley at Batavia.
From the type area at Joliet it thins southward to about 40 feet along the Kanka-
kee River. The formation is exposed from the Cowan's Quarry Section, Will
County, southeast about 5 miles to Altdorf, Kankakee County. All except the
lower 10 feet is well exposed in the Rock Creek Canyon Section.

The formation consists of three members of approximately equal thick-
ness. A basal gray, red, and green shaly dolomite is differentiated as the
Brandon Bridge Member. Above it, the dolomite is nearly white, very silty
at the base but only slightly silty at the top. It is differentiated as the Mark-
graf Member. The Romeo Member, at the top, consists of nearly white, pure
dolomite. The names "Brandon Bridge," "Markgraf, " and "Romeo" were in-
advertently used without definition in a previous report (Graf, 1952).

20 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

The Joliet Formation is separated from the Kankakee Formation below
by a diastem, as previously described. The upper, pure dolomite (Romeo Mem-
ber) is separated from the typically silty Sugar Run lithology above by an ex-
ceptionally uniform gradation through 6 to 7 feet. It is most convenient in
field studies to place the boundary at the lowest bedding plane that has a
slightly shaly surface. This practice puts the transition zone largely in the
Sugar Run Formation.

No lithologic unit comparable to the Joliet Formation occurs in the north-
western Illinois area, but the pure Romeo Member conceivably is equivalent to
the massive upper part of the Marcus Formation in that area. The lower part of the
Marcus is much purer than the lower Joliet.

Northward from Illinois, the basal red shaly Brandon Bridge Member is
exposed in quarries at Burlington, Walworth County, Wisconsin (SW NE NE 36,
3N-18E, Burlington 7.5-minute Quad.), but was not found north of there. The
pure, nearly white Romeo Member underlies building -stone beds equivalent to
the Sugar Run Formation in a quarry at Waukesha, Waukesha County, Wisconsin
(SE SW 26, 7N-19E, Waukesha Quad.). The Joliet lithology was not noted in
sections examined north of Milwaukee, but the Joliet may be equivalent, at
least in part, to the Cordell Formation.

The southernmost exposure of the Joliet Formation in northeastern Illi-
nois is in the Lehigh Quarry Section about 6 miles south of the Kankakee River,
where the white, pure dolomite in the base of the quarry is the Romeo Member.
Farther south, the Joliet is recognized in subsurface as a white dolomite (Elwood
Atherton, personal communication). In the western Illinois outcrop area (Calhoun
County), the strata overlying the Kankakee Formation have been called Joliet,
but the basal shaly and silty zones are missing and the thick-bedded, strongly
crinoidal limestone has slight resemblance to the type Joliet. Southeastward
from the Joliet area, the basal shaly Joliet has long been correlated with the
shaly Osgood Formation of southern Indiana (Dunn, 1942), and the overlying
Laurel Formation is a nearly white limestone similar to the upper Joliet.

The Joliet is not abundantly fossiliferous, but Savage (1926) listed
several fossils. Siliceous foraminifera of considerable variety are abundant
in the Brandon Bridge Member, but they decrease in abundance upward as the
formation becomes less argillaceous and less silty. They are much more
abundant than in other Silurian formations .

Brandon Bridge Member

The Brandon Bridge Member, the shaly zone at the base of the Joliet
Formation, is named herein for Brandon Bridge, which crosses the Des Plaines
River on the southwest side of Joliet. The type section is in the quarry just
south of the bridge (Joliet— Lincoln Quarry Section). The Brandon Bridge is at
least partly equivalent to the zone that Savage (1942) removed from the Joliet
and named Rockdale. "Rockdale" was preempted, and the name has not been
used.

The Brandon Bridge Member is 25 feet thick in the type section and with-
in a foot or two of that in other sections at Joliet and in the deep quarries at
Hillside and Elmhurst. It is partly exposed in the Du Page Valley a mile south

NORTHEASTERN ILLINOIS 21

of Naperville, Du Page County (quarry in NW NW NE 30, 38N-10E, Naperville
Quad.), and in the Fox Valley a mile south of Batavia, Kane County (SE NW 27,
39N-8E, Aurora North 7.5-minute Quad.). The Brandon Bridge Member thins
southward from the type locality, and it is only 10 feet 6 inches thick in a small
ravine on the north side of the Kankakee River, a quarter of a mile east of
Warner Bridge in Kankakee County (NW SE SW 31, 32N-11E, Herscher Quad.).

In the Joliet area, the Brandon Bridge is separated into two parts by a
green to black shale, locally as much as 1 foot thick, slightly above the mid-
dle of the member. The lower part is more shaly and contains beds of red
crinoidal dolomite and greenish gray, very argillaceous dolomite. Some gray
beds are mottled with red. The strata above the shale are less shaly and are
largely gray, mottled green and red. The upper contact is placed at the highest
strong shaly parting. In other areas, the red crinoidal beds are less common
and some of the very argillaceous beds are dark red, as at Lemont and Elmhurst.
The member is thin bedded, and many bedding planes have fucoidal markings.
Except for the siliceous foraminifera, the Brandon Bridge is not very fossiliferous,
but large specimens of the trilobite Bumastus are occasionally found, par-
ticularly in the middle shaly zone .

Markgraf Member

The Markgraf Member, which overlies the Brandon Bridge, consists of
light gray to white, silty to slightly argillaceous dolomite. It is named herein
for the Markgraf Quarry on the north side of the Des Plaines River in the south-
west part of Joliet, which contains the type section (Joliet— Markgraf Quarry
Section). This long-abandoned quarry is now partly filled, but the Markgraf
Member is above the fill.

The Markgraf Member is 22 feet thick in the type section, but it varies
from 20 to 28 feet in other quarries in the area. In the Du Page Valley only 3
feet are exposed above water level in a quarry on the south side of Naperville,
Du Page County (SE SW SE 13, 38N-9E, Naperville 7.5-minute Quad.). The
Markgraf is at least 23 feet thick in the Fox Valley, where it is partly exposed
in a quarry half a mile south of Batavia, Kane County (NE NE 27, 39N-8E,
Aurora North Quad.). Like other Joliet members, it thins southward from Joliet,
and it is only 12 feet 3 inches thick in the Rock Canyon Section along the Kanka-
kee River.

The Markgraf Member consists of three units, which are well exposed in
the type section and the other two quarry sections at Joliet. The lower unit, 4 to
5 feet thick, is thick-bedded to massive, very light gray, very fine grained,
silty dolomite containing about 10 percent insoluble residue. The middle unit,
8 to 9 feet thick, is medium bedded, more argillaceous than silty, and contains
7 to 8 percent insoluble residue. Small nodules of white chert occur in five to
seven discontinuous bands. The upper unit, 9 to 12 feet thick, is similar but
slightly vesicular and purer, containing only 3 to 5 percent insoluble residue.
It contains seven discontinuous bands of chert nodules. Although the three
units show a progressive upward increase in purity from the more shaly Brandon
Bridge to the pure Romeo, they are sharply defined and not internally gradational.
Along the Kankakee River, where the Markgraf is only half as thick as at Joliet,
it is all similar to the upper unit but does not contain chert.

22 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

Romeo Member

The Romeo Member, the uppermost member of the Joliet Formation, con-
sists of about 25 feet of nearly white, relatively pure dolomite. It is named
herein for the village of Romeo in the Des Plaines Valley, 6 miles north of loliet.
Although the member was formerly well exposed in the quarries at Romeo, the
quarries are now water filled, and the exposure in the National Quarry in Joliet
is designated the type section.

The Romeo Member is 20 feet thick in the type section, but it ranges
from 34 feet in the Elmhurst Quarry Section to 18 feet in the Rock Creek Canyon
Section in the Kankakee Valley. In the Du Page Valley about 8 feet of the Romeo
is exposed in the abandoned quarry on the south side of Naperville. Although
not observed in the Fox Valley, it probably forms the top of the Silurian section
in the bluffs on the east side of the valley, just south of Batavia.

The Romeo consists of very light gray to white, fine-grained, vesicular
dolomite. It is a relatively pure dolomite, containing only 1 to 2 percent in-
soluble residue. It has a massive appearance on fresh quarry faces, but on
weathered surfaces tight, stylolitic bedding planes at 2- to 4-inch intervals are
prominent. It commonly has medium to dark gray mottling, and pink mottling is
present in many exposures. In the quarries at Joliet three units are recognized:
the lower 5 to 6 feet is pink mottled, the overlying 10 feet contains beds and
nodules of chert, and the top 6 feet is thicker bedded and contains large vugs.
Elsewhere the Romeo contains little or no chert. It is fossiliferous, but the
fossils are poorly preserved. Corals are common.

Sugar Run Formation

The Sugar Run Formation, which overlies the Joliet Formation, consists
of the well-bedded dolomite, about 25 feet thick, that has been the source of
most of the building stone quarried in the Chicago area. It is named for expo-
sures along Sugar Run, a tributary of the Des Plaines River, in the south part
of Joliet. The type section is in the Joliet— National Quarry Section, which is
on the south side of Sugar Run. Although almost all of the formation is exposed
in the quarry, the contact with the overlying Racine is not present. This con-
tact is well exposed on the north side of Joliet in the abandoned quarries north
of the state prison, particularly north of State Highway 4A, where it ascends
the east bluff of the Des Plaines Valley. In that locality about 20 feet of inter-
reef Racine, which has varied lithologies, overlies 6 feet of the more uniform,
evenly bedded Sugar Run (NW NW SE 34, 36N-10E, Joliet 7. 5 -minute Quad.). The
name "Elwood, " from a manuscript, was inadvertently used for this unit in a
previous report (Graf, 1952).

The Sugar Run Formation was long known as Athens Marble from quarries
at Lemont (formerly called Athens) and Joliet Marble for the stone quarried at
Joliet. Savage (1926) included the building-stone beds and the overlying inter-
reef beds in the Waukesha Formation, and Willman (1943) restricted the Waukesha
to the building-stone beds. However, examination of the type Waukesha at
Waukesha, Wisconsin, showed that it is equivalent to parts of the Joliet and
Waukesha Formations in northeastern Illinois, and the name "Waukesha" is
replaced in Illinois with "Sugar Run."

The Sugar Run Formation is present in the lower part of the Des Plaines
Valley bluffs, in parts of the valley floor from Joliet north to Lemont, and in

NORTHEASTERN ILLINOIS 23

quarries in the valley floor from Lemont to Sag Bridge. The principal areas
of quarrying were along the south side of the valley from Romeo to Sag Bridge
and along the east side of the valley at Joliet, particularly on the southeast
side of the city. It is also exposed in the deep quarries at Elmhurst, where it
is only 10 feet thick, and at Hillside, where it is thickest, 29 feet; at Hill-
side it is exceptionally silty and more massive than usual. It does not extend
as far west as the Du Page and Fox Valleys, where it and younger Silurian strata
have been eroded. It extends southward from Joliet, and along the Kankakee
River it is exposed from the Rock Creek Canyon Section southeastward for about
5 miles (Eggleston Section). It has been quarried for building stone on the
north side of the Kankakee Valley about 3 miles west of Bourbonnais, Kankakee
County (NE SW 15, 3 IN- HE, Kankakee Quad.).

The Sugar Run Formation has a basal transition zone, 6 to 7 feet thick,
in which the insoluble residues gradually increase upward from about 2 percent
at the base to about 20 percent. The upper 15 to 20 feet of the formation com-
monly has about 20 percent insoluble residue. The tight, stylolitic bedding
planes at the top of the underlying Romeo gradually give way to smooth sur-
faces at the top of the transition zone; and the smooth, even bedding of the
upper part of the formation makes it readily quarriable for building stone.

The building- stone beds consist of light gray dolomite, which is usually
slightly greenish on fresh surfaces but weathers yellowish brown, locally red-
dish brown. The dolomite is dense, very fine grained, and silty. Most beds
contain very thin, wavy, green, argillaceous streaks, which are prominent on
long-weathered surfaces . The argillaceous streaking is very prominent at the
north, in the Elmhurst and Hillside quarries, where the insoluble residues are
as high as 30 percent, but it decreases southward and is inconspicuous along
the Kankakee River, where the insoluble residues are down to 10 to 15 percent.
Farther south, in the Lehigh Quarry Section, the unit is even purer and is dif-
ferentiated from the Romeo below largely by the presence of smooth bedding
surfaces. The beds increase in thickness from 4 to 6 inches at the base to 2 to
3 feet in the middle and upper parts. The Sugar Run generally contains little
chert, but chert is abundant in the Hillside and Elmhurst Quarry Sections .

Although the Sugar Run is consistently present in the outcrop areas,
studies of samples from borings in the Chicago area indicate that it is absent
in parts of the region. Some of the Racine reefs may have started to grow during
deposition of the Sugar Run. In areas close to reefs, beds equivalent to the
Sugar Run would have the varied characteristics of the interreef rocks and, there-
fore, would be included in the Racine. The Sugar Run is similar lithologically
to many individual beds in the interreef rocks, but its uniformity and continuity
through a large area justify its differentiation as a separate formation. South-
ward it becomes purer and nearly white; and in subsurface south of the outcrop
area, it may be included in the Joliet Formation.

Northward, in Wisconsin, strata with a similar lithology are recognized
as far as Milwaukee and have been extensively quarried for building stone,
called the Lannon Stone. Exposures at Waukesha and Lannon show that the
building- stone beds overlie the relatively pure Joliet Formation. The Sugar Run
is most impure in the Chicago area, and it may be a westward tongue equivalent
to the Rochester Shale of New York.

Fossils are not common in the Sugar Run, but locally some beds contain
excellent specimens of Calymene niagarensis, particularly in quarries about a
mile northeast of Lemont. Lowenstam (1948) described a fauna from these beds
at Lemont.

24 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

Racine Formation

The Racine Formation consists of the reef and interreef dolomite overlying
the Sugar Run Formation. It is the uppermost Silurian formation and has a maxi-
mum thickness of about 300 feet. It was named by Hall (1861) for exposures in
quarries at Racine, Wisconsin. The name "Racine" was introduced into Illinois
by Savage (1926), but he applied the term only to the reefs (fig. 6). He did
not recognize the reef-interreef relations and called the interreef rocks Waukesha.
He also called Waukesha the strata that are beneath the reefs and have been quar-
ried for building stone and are here called Sugar Run. At the top he differentiated
reef strata that contained fossils characteristic of the Guelph of Ontario and
New York as the Port Byron after exposures in northwestern Illinois. Willman
(1943) restricted the Waukesha to the building-stone beds at the base, included
both reef and interreef strata in the Racine, and extended the Racine upward
to include the lithologically similar strata Savage had differentiated as Port Byron.
The name "Racine" is continued with that definition. Although the reef and in-
terreef strata are very unlike lithologically, they are not separable as rock-
stratigraphic units. The reefs are of all sizes, extend through various strati-
graphic intervals, and in places have complex overlapping relations.

The Racine Formation is exposed in the bluffs along the Des Plaines
Valley from the southeast side of Joliet to Sag Bridge and from there east in
scattered exposures along the Sag Channel to Blue Island and along the main
valley to Riverside. Elsewhere in the Chicago area, the Racine is exposed in
low hills that project through the glacial drift (Bretz, 1939, 1943; Willman,
1943, 1971). Many of the hills, called klintar, or individually a klint, are
reefs. The reef rock is more resistant to weathering than the surrounding inter-
reef rock, which has been more deeply eroded, in part by glacial scour. Be-
cause of the eastward dip, the youngest Racine strata are exposed in klintar
along the Lake Michigan shore and at Stony Island, Thornton, and Chicago
Heights.

Along the Kankakee River, the Racine is exposed from three miles north-
west of Kankakee to Momence. It is also exposed, or at a shallow depth, in a
large upland area from Kankakee north to Manteno (Willman, 1943). The Racine
Formation has been eroded from the area of Silurian rocks along the Du Page
and Fox Rivers.

The reefs consist of pure dolomite that is medium to light gray, dark gray
mottled, gray weathering, medium grained, and vesicular to highly vuggy. Some
of the reefs are more than a mile across, and the reef at Thornton is at least
150 feet thick. The central part, or core, of the larger reefs is poorly or irregu-
larly bedded or massive. The marginal, or reef- flank, beds are well bedded and
are largely 4 inches to 1 foot thick, although some are 2 to 3 feet thick. The
reef- flank beds dip steeply away from the central core at angles as much as 30
degrees. They represent lateral growth of the reef and consist partly of storm
debris from higher parts of the reef. Smaller reefs are massive and do not have
reef-flank deposits. The reefs are remarkably free of clay and silt impurities.
The insoluble residues are generally less than 1 percent, and in meny samples
there is almost no residue, or only pellets of the asphaltum that fills vugs in
some reefs. Chert was not found in any of the reefs. It is almost never found
in the high-purity rocks nor in the highly silty or argillaceous rocks.

NORTHEASTERN ILLINOIS 25

The interreef rocks in northeastern Illinois are largely silty, argillaceous,
cherty, light gray to brownish gray dolomite that weathers brown. They are
well bedded, but the bedding varies in thickness. Marginal to the reefs the
strata are highly varied in composition. Some massive, silty, argillaceous,
greenish gray beds contain 30 to 40 percent insoluble residue. Some nodular
beds grade into nodules in a clay matrix. The impure beds are separated by
pure, vesicular, gray dolomite like that in the reefs. Before dolomitization
these beds may have been calcarenite consisting of debris washed from the
reefs. The interreef rocks more distant from the reefs are more uniform and
commonly are slightly argillaceous or argillaceous, dense, brownish gray,
cherty dolomite . The character and origin of the reef and interreef rocks in
northeastern Illinois have been described in several reports (Bretz, 1939; Will-
man, 1943, 1962; Willman, Lowenstam and Workman, 1950; Lowenstam, 1950,
1957; Lowenstam, Willman, and Swann, 1956; Ingels, 1963).

Reefs that have a base low in the Racine Formation occur in the Elmhurst
Quarry Section; at Sag Bridge, Cook County (SE SW 13, 37N-11E, Sag Bridge
7. 5 -minute Quad.); along Fraction Run south of Lockport, Will County (SW NE SW
26, 36N-10E, Joliet 7.5-minute Quad.); and along the Kankakee Valley in Kanka-
kee County, especially along the lower part of Davis Creek (SE NW NE 23,
31N-11E, Kankakee Quad.) and in a small deeply incised ravine at the south-
west corner of Bourbonnais (N 1/2 NW NW 30, 31N-12E, Kankakee Quad.).
Reefs in the middle of the formation occur at La Grange, Cook County (in the
Federal and Superior quarries, NW 10, 38N-12E, Berwyn 7.5-minute Quad.)
and in quarries near Manteno, Kankakee County (NE SE SE 28, and SW SW SE 33,
31N-12E, Kankakee Quad.). Reefs in the upper part of the formation were for-
merly well exposed at Stony Island and Chicago Heights, but these exposures
are now largely covered. However, the reef at the Thornton Quarry Section is
exceptionally well exposed, and its relation to the interreef rocks has been
described (Willman, 1962). The interreef rocks are well exposed in the Hillside
and Elmhurst Quarry Sections, the Lemont East Bluff Section, and the Thornton
South Quarry Section. Many other exposures of reef and interreef rocks have
been described (Willman, 1943).

The Racine reefs contain a large and varied fauna in which corals, stro-
matoporoids, bryozoans, brachiopods, and crinoids are most common and trilo-
bites, gastropods, and pelecypods are not rare (Winchell and Marcy, 1866;
Meek and Worthen, 1868; Norton, 1895; Weller, 1900, 1907; Savage, 1926;
Fenton, 1931; Grubbs, 1939; Lowenstam, 1942, 1949a, 1950, 1952; Lowenstam,
Willman, and Swann, 1956; Ingels, 1963). Fossils are not common in the inter-
reef beds but occur locally in reef detritus. However, Lowenstam (1948) collected
a large fauna from a few localities, the fauna differing notably from that in the
reefs, with sponges, crinoids, and brachiopods most abundant. A local occur-
rence of very argillaceous black and green dolomite interbedded with dark brown
dolomitic shale in the upper part of the formation at Blue Island, mostly ob-
served from waste piles along the Calumet Sag Channel, has been informally
called the Blue Island beds or the Lecthaylus shale for the abundance of this
fossil, which is the remains of a marine worm. It also contains graptolites,
inarticulate brachiopods, and spores (Roy and Croneis, 1931; Lowenstam, 1948;
Ross, 1962b). The large brachiopod Conchidium is abundant on the south slope
of a hill 1.5 miles southwest of Manteno, Kankakee County (SW SW SE 20,
32N-12E, Kankakee Quad.), which is near or below the middle of the formation.

26 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

The Racine Formation is lithologically like the Racine at Racine, Wis-
consin. Farther north in Wisconsin, reefs appear to be more abundant, and the
interreef rocks are purer and less cherty, as in northwestern Illinois. The
Racine is essentially equivalent and lithologically similar to the Engadine in
Michigan. It corresponds to the Lockport and Guelph Formations in Ontario
and New York. Westward, the Racine is lithologically like the Racine in north-
western Illinois and the Gower in Iowa. Eastward, in Indiana, it is equivalent
to the Wabash Formation, which has reef and interreef strata, the latter more
argillaceous than in Illinois. The Racine probably also includes the Louisville
Limestone below the Wabash Formation. Southward from the Chicago area, the
lower Racine exposed in the Lehigh Quarry Section in Kankakee County is much
purer, more uniform, and less cherty than the interreef rocks in the Chicago
area. It is largely pink mottled, which relates it to the pink but very argilla-
ceous Moccasin Springs Formation, which occurs in the same position in southern
Illinois. However, the Moccasin Springs Formation probably extends much
higher in the section, because it grades without noticeable break upward into
the Lower Devonian Bailey Formation. The Racine reefs, however, extend south-
westward from the Chicago area to the East St. Louis area (Lowenstam, 1949b).

NORTHWESTERN ILLINOIS

The distribution of the Silurian rocks of northwestern Illinois is related
largely to the regional southwestern slope away from the Wisconsin Arch; the
slope is interrupted in Carroll County by the east-west Savanna Anticline (fig. 7).
The anticline is asymmetrical with a relatively steep north slope (about 10°).
There is little more than the regional slope southward from the anticlinal axis
and northward from the synclinal axis, which is close to the base of the steep
northern flank of the structure. The Silurian rocks are entirely eroded along
the axis of the anticline. North of the anticline the Silurian rocks cap the many
ridges and mounds that distinguish the topography of the Driftless Area (Trow-
bridge and Shaw, 1916; Willman, 1943). In the glaciated area, only a few
mounds persisted through glaciation. South of the anticline the area was gla-
ciated, the streams have not cut through the Silurian rocks, and the topography
is much less rugged.

As Devonian rocks overlie the Silurian only at the extreme south margin
of the outcrop area, the thickness of the Silurian is related largely to the re-
gional slope, roughly 20 feet per mile, to the topographic irregularities, and
to the conformation of the basal unconformity, where there is a relief of about
100 feet. Silurian rocks 50 to 75 feet thick cap some of the mounds in the north-
ern part of the area, and as much as 200 feet occurs in the trough on the north
side of the Savanna Anticline. Southward from the axis of the anticline, where
they are missing, the Silurian rocks thicken progressively to about 450 feet at
Port Byron. A short distance south of Port Byron, they are overlain unconformably
by Middle Devonian strata.

The rocks of Silurian age in northwestern Illinois are almost entirely
dolomite (fig. 8), the only exceptions being a minor amount of dolomitic shale
in the lower part, thin green shale partings in the middle part, and chert, which
occurs in nodules and in essentially continuous bands in the middle and lower
parts. The Silurian rocks are subdivided into two series— the Alexandrian Series
and the overlying Niagaran Series .

NORTHWESTERN ILLINOIS

WISCONSIN

Fig. 7 - Distribution of Silurian rocks in northwestern Illinois (modified from State
Geologic Map). The dashed line roughly outlines the area where the
Silurian rocks have a thick cover of glacial drift and do not crop out.
Sections are described on pages 45 to 55.

The Alexandrian Series was deposited on an uneven surface eroded on
the Maquoketa Shale Group, and it thins markedly over the Savanna Anticline.
It is as much as 200 feet thick in Jo Daviess County but is less than 100 feet
thick in Carroll County. The lower part of the Alexandrian rocks consists of
argillaceous dolomite, the middle part is massive, pure, cliff-forming dolomite,
and the upper part includes the interval with chert bands and the unit with green
shale partings at the top.

The Niagaran Series consists almost entirely of pure dolomite. The lower
part is a very massive dolomite characterized by the abundance of Pentamerus
oblongus , and the upper part is reef-type dolomite. Reef structures, which are

28 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

SERIES

FORMATION

COLUMN

THICK,
(ft)

GENERAL CHARACTER

Racine

r^=^

z_z

300

Dolomite, pure, gray, thin-bedded to mas-
sive; local reef structures; local areas
of brownish gray, argillaceous dolomite

/ 1 . /

zS^z

zzz

^S2

Marcus

35-45

Dolomite, very .pure, buff, vesicular, mas-
sive; contains Pentamerus in greaf abun-
dance i n lower 5'- I 5'

Sweeney

iJ=P^

I A I

45-55

rS=

Dolomite, pure, pinkish gray; in thin wavy
beds with green clay partings; corals
abundant; 3'-5' cherty zone near middle
contains Microcard i na I ia and Pentamerus

/ /

Blanding

A A /A A A
/ /

A A AAAAAA

a /-a -/;

35-50

Dolomite, pure, brownish gray; lower 3'-8'

slightly argillaceous; contains many

layers of white chert; silicified corals
abundant

Tete des Morts

0-24

zZt2

Dolomite, pure, gray, glauconitic, massive,
cliff-forming; persistent chert band in
upper part; silicified corals abundant;
pitted surface on top

Mosalem

T~TT

0-100

Dolomite; upper 20'-30' is argillaceous,
gray, cherty, medium bedded; lower part is
very argillaceous dolomite grading to dolo-
mitic shale at base

*Where overlain by next younger unit.

Fig. 8 - Silurian strata in northwestern Illinois.

NORTHWESTERN ILLINOIS 29

common in Iowa, are only locally present in northwestern Illinois, but the hor-
izontally bedded interreef rocks are also dominantly pure dolomite like that in
the reefs . Only a few exposures of argillaceous interreef rock have been noted
in the reef-bearing zone .

Development of the Classification

In the first geological survey of the upper Mississippi Valley, Owen
(1844) called the Silurian rocks the "coralline and Pentamerus beds of the Upper
Magnesian Limestone." Owen overlooked the Ordovician Maquoketa Shale
Group, which separates the Silurian from the Galena and Platteville Groups, which
comprise the lower part of his Upper Magnesian Limestone. Percival (1856)
used the appropriate but informal name "Mound Limestone" for the Silurian
rocks. Hall and Whitney (1858) correlated the Silurian rocks with the Niagara
Limestone of the New York section, and the name "Niagara" was used in north-
western Illinois by Worthen (1866) and Shaw (1873) (fig. 9).

Iowa geologists, in particular Wilson (1895), Calvin and Bain (1900),
and Savage (1906), made the first differentiation of the Niagara Formation.
They differentiated but did not formally name most of the units now recognized
in northwestern Illinois.

The Silurian strata were correlated with the Niagara, or the Niagara
and Clinton (Owen, 1844), until Savage (1914) recognized that the earliest
Silurian strata, which he differentiated into the Winston and Waucoma Forma-
tions, were equivalent to the pre-Niagaran strata he had previously named the
Alexandrian Series (Savage, 1908). His intent was to assign the beds below
the prominent chert zone to the Winston and the chert zone to the Waucoma, as
shown in figure 9, but unfortunately the type Winston, which was in Illinois,
did not include the beds later named Tete des Morts, and the type Waucoma,
which was in Iowa, was equivalent to the Winston, as noted by Scobey (1938).
Ulrich (1924) also recognized the presence of the pre-Niagaran Silurian beds
at Savanna and proposed the name "Burroughs Dolomite" for the strata Savage
had previously assigned to the Alexandrian. The Burroughs included two well-
defined units, and therefore "Burroughs" has not been used.

In 1926, Savage, entirely on faunal evidence, correlated the units in
northwestern Illinois with named units in the Silurian section elsewhere in Illi-
nois and established the nomenclature which has been followed in most details
to the present time. Although the nomenclature was based on faunal correlations,
the units, except for the similar Racine and Port Byron, are well-defined litho-
logic units and are essentially those recognized in the earlier work in Iowa.
Savage's classification was later summarized and clarified by Sutton (1935)
and Weller (1935).

Despite the miscorrelation of several sections, the only significant
change in interpretation of the sequence is the recognition, in the present study,
that the Tete des Morts Dolomite, the most conspicuous unit in the Galena and
Dubuque area, thins out southward and is missing in the section at Savanna,
which is the section that has been most frequently studied.

In 1938, Scobey recognized the Edgewood and Kankakee Formations in
Iowa, thus restricting the term "Hopkinton" (Calvin, 1906), which had been
used for all the Silurian strata below the top of the Pentamerus beds. Because
the Hopkinton consists of the two well-defined units that Savage called Joliet
and Waukesha, the term "Hopkinton" has not been used in Illinois.

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

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NORTHWESTERN ILLINOIS 31

In the National Research Council's Silurian correlation chart (Swartz
et al., 1942), Savage made several changes in his classification without
adequate explanation. He restricted the Joliet Formation by splitting off lower
beds, named Rockdale for a locality in northeastern Illinois, that are not recog-
nized either faunally or lithologically in northwestern Illinois. He introduced
"Cordova" as a replacement for "Waukesha" and referred the unit to a section
which, as noted by Willman (1943), was in the Racine Dolomite and about 16
miles from the nearest strata he had previously called Waukesha. He extended
the Port Byron to include the Racine, and he reversed his previous designation
of "Kankakee (Brassfield)" to "Brassfield (Kankakee)." The Kankakee had long
been correlated with the Brassfield of central Kentucky, but the name "Brass-
field" had not been used in Illinois. Inexplicably, he reintroduced "Winston"
and "Waucoma" for Iowa, although "Edgewood" and "Kankakee" had been ac-
cepted there and the sections on both sides of the Mississippi River are nearly
identical.

Although Willman (1943) followed Savage's classification of 1926, he
noted that the Racine and the Port Byron were not separable lithologically and
that the Joliet was lithologically similar to the Kankakee of northwestern Illinois
Later, the name "Port Byron" was dropped from the classification and "Racine"
was expanded to include all the Silurian strata above the Waukesha (Willman
et al., 1967).

In 1960 Brown and Whitlow subdivided the Edgewood Formation into two
members, applying the name "Tete des Morts" to the massive cliff-forming unit
at the top and "Mosalem" to the underlying argillaceous dolomite, units dif-
ferentiated but not named by Willman and Reynolds (1947).

The revisions in nomenclature of the northwestern Illinois Silurian pro-
posed in this report (fig. 2) are in part based on disagreement with the correla-
tions implied by the previous nomenclature and in part on a change in classifi-
cation policy, previously described.

Mosalem Formation

The strata previously called Edgewood in northwestern Illinois consist
of two distinctive lithologic units that are herein called the Mosalem and Tete
des Morts Formations. The Mosalem Formation, the lower unit, is the argil-
laceous dolomite that Calvin and Bain (1900) called "the transition beds" and
assigned to the Maquoketa Shale and that Brown and Whitlow (1960) named the
Mosalem Member of the Edgewood Formation. It is the unit that Savage (1914)
called Winston for exposures at Winston, at the east end of the Winston tunnel
of the Chicago Great Western Railroad, about 6 miles southeast of Galena.
However, Savage defined the unit to include all the Silurian strata below the
cherty zone, which is the Blanding Formation, and, therefore, included the Tete
des Morts Formation, which underlies the cherty zone in the Winston area.

As the Mosalem and Tete des Morts are distinctly different lithologic
units, they are herein considered to be formations and the name "Edgewood" is
discontinued in northwestern Illinois . The Mosalem Formation lithologically
resembles the Wilhelmi of northeastern Illinois and the Cyrene Member of the
Edgewood of western and southern Illinois, but the overlying Tete des Morts,
previously included in the Edgewood, does not resemble the Edgewood else-

32 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

where in Illinois. Although the name "Edgewood" might be restricted to the
strata below the Tete des Morts, the term "Edgewood" has been so widely-
used in a time sense for the Silurian beds below the Platymerella Zone, which
is not present in northwestern Illinois, that it seems better to accept the local
designation of "Mosalem."

Brown and Whitlow (1960) did not designate a type section for the Mosa-
lem but listed three exposures in Mosalem Township of Dubuque County, Iowa.
The unit is poorly exposed at the first two exposures, but it is well exposed at
the third— the roadcut of U.S. Highways 52 and 67 south of the village of Kings,
described herein as the Kings Section, which can be accepted as the type sec-
tion. In this section its contact with the overlying Tete des Morts is well ex-
posed and the 60 feet exposed is nearly its entire thickness. One of the best
exposures of the Mosalem in northwestern Illinois is in the Royal Princess
Section, 6 miles south of Galena, where the upper 50 feet of the Mosalem is
exposed. A nearby diamond -drill boring encountered approximately 75 feet of
Mosalem overlying the shale of the Maquoketa Group. In some areas in north-
* western Illinois, the Mosalem is as much as 100 feet thick, and Brown and
Whitlow (1960) reported 94 feet in Iowa. The Mosalem thins almost to absence
in some areas, and Savage (1914) reported that it feathers out near Sabula, Iowa.
It is only 7 feet thick in Mississippi Palisades State Park at Savanna. As the
contact with the underlying Maquoketa is rarely exposed, the total thickness
of the Mosalem is not well known, despite the fact that the upper part is very
widely exposed beneath the cliff- forming Tete des Morts Formation. The Mosa-
lem is at least 35 feet thick at a quarry 1 mile south of Pearl City, Stephenson Coun-
ty (NE cor. 17, 26N-6E, Lena Quad.), where neither the top nor the base is ex-
posed. It appears to be 70 to 80 feet thick in roadcuts near Schapville (Schap-
ville Southwest Section), but the exposures are not continuous. South of the
area in which the Tete des Morts Formation occurs, the Mosalem is overlain by
the Blanding Formation and it is generally thinner. It is at least 24 feet thick
in the Lost Mound Section, west of Hanover, but in sections near Savanna, both
north of the anticline and on it, the Mosalem is 7 to 10 feet thick.

The upper 3 to 8 feet of the Mosalem consists of gray, fine-grained,
dense, slightly vesicular, slightly argillaceous dolomite, largely in 1- to 4-inch
beds, which distinguishes it from the massive, more pure Tete des Morts strata
above. On some fresh surfaces this part of the Mosalem appears to be nearly
as massive as the Tete des Morts. It is differentiated by the presence of white
chert nodules that commonly occur in three or four bands, but not in layers as in the
Blanding. It also contains two or three beds, 2 to 3 inches thick, of vuggy,
pure dolomite with fossil debris; these beds were probably coquinas or fossil
calcarenites before they were dolomitized. In addition, a 2- to 4-inch bed of
finely laminated dense dolomite commonly occurs in the upper 1 to 2 feet and
locally elsewhere within the interval. A similar bed occurs also at or immedi-
ately below the base. The base generally is a strongly pitted surface, probably
a corrosion or solution surface, although locally pebbles of the underlying lami-
nated bed occur immediately above the surface (Whitton Northeast Section).
These distinctive beds appear to be nearly continuous from Galena to Savanna but
are less prominent to the northeast near Schapville and Stockton.

The middle zone of the Mosalem, 15 to 20 feet thick, is distinctly argil-
laceous. It is yellow-brown, dense dolomite, massive-appearing on fresh sur-
faces, but with faint, wavy, thin ridges reflecting variations in the argillaceous

NORTHWESTERN ILLINOIS 33

content and imparting a rough surface to deeply weathered exposures. The upper
5 feet commonly contains chalky, white chert nodules, generally scattered but
locally in a well-defined band.

The top of the lower zone of the Mosalem is generally marked by 6 inches
to 1 foot of very argillaceous, shaly weathering, dark gray, generally black-
speckled dolomite or dolomitic shale. Similar beds occur below, separated by
1 to 3 feet of massive, very argillaceous dolomite. The whole zone becomes
more argillaceous downward. This zone is present only where the Mosalem is
more than 20 to 25 feet thick, but it is as much as 75 feet thick in the areas
where the Mosalem is thickest.

Fossils are rare in the Mosalem, except for fossil debris in the thin cal-
carenitic beds, but Savage (1914) reports a fauna of 11 species from the ex-
posures at Winston, and Scobey (1938) lists 41 species from the Edgewood,
many of which probably came from the Tete des Morts, which he included in the
Edgewood .

The Mosalem rests unconformably on shale of the Maquoketa Group, and
a sharp local relief of 2 feet can be seen at the Palisades Park Main Entrance
Section. However, where the Mosalem is thick, the lower shaly unit is not so
readily differentiated from the Maquoketa. At Bellevue, Iowa, the contact
appears to be almost gradational. However, the fact that uppermost Maquoketa
strata, the Neda Formation, consisting largely of oolite, are present only where
the Maquoketa is thick and the Mosalem thin suggests that the thick Mosalem
fills channels cut into the Maquoketa.

The upper surface of the Mosalem Formation appears to be conformable.
Where the overlying Tete des Morts thins out, the uppermost unit of the Mosalem,
which contains the distinctive laminated calcarenitic beds, continues southward
and the Blanding Formation rests directly on the Mosalem.

Tete des Morts Formation

The Tete des Morts Formation was named the upper member of the Edge-
wood Formation by Brown and Whitlow (1960, p. 39). It is the prominent cliff-
forming unit in the Driftless Area of northwestern Illinois (Willman and Reynolds,
1947, units 5-7). For reasons previously discussed, it is considered a formation
in this report.

The Tete des Morts Formation was named for exposures along Tete des
Morts Creek in Jackson and Dubuque Counties, Iowa, but no type section was
designated by Brown and Whitlow. In the only section they specifically mentioned
(in fig. 12), the relations to the overlying and underlying units are not well
exposed, and for that reason the Kings Section, herein designated the type
section for the Mosalem Formation, is also designated the type section for
the Tete des Morts. In the type section, the Tete des Morts is about 24 feet
thick and its relations to the Mosalem below and the Blanding above are well
exposed.

In northwestern Illinois the Tete des Morts Formation occurs only north
of an east-west line approximately through Hanover in southern Jo Daviess
County. In the Mississippi River bluffs it is a conspicuous unit from Rice,
5 miles south of Galena, for about 4 miles to the prominent bluff east of Blanding,
but it is absent less than 3 miles southeast (Whitton Northwest Section, NW
SE NW 18, 26N-2E, Jo Daviess Co., Green Island 7.5-minute Quad.).

34 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

The Tete des Morts Formation is well exposed in literally hundreds of
localities, particularly on the noses of projecting minor ridges of the strongly
dissected Silurian escarpment, which extends north and east from the Missis-
sippi River bluffs to the farthest Silurian outliers in knobs near Stockton and
Scales Mound in Illinois and Platte ville in Wisconsin. It is described in the
Royal Princess, Winston North, Schapville Southwest, and Stockton Southeast
Sections. It is uniformly about 20 feet thick, ranging from 18 to 22 feet. Its
southward termination was not observed. The formation appears to diminish
from a full thickness to absence in about a mile.

The Tete des Morts Formation consists largely of relatively pure, glau-
conitic, gray, fine- to medium-grained, dense to moderately vesicular, massive
dolomite in which corals, particularly Favosites, are common. Chert is per-
sistently present in a thin zone just above the middle. On deeply weathered
surfaces, tight bedding planes show faintly at 2- to 4-inch intervals, and in
places a few stronger bedding planes break the unit into ledges mostly 2 to 4
feet thick. The weathered surface is medium to dark gray and exceptionally
rough, with a sharp irregular relief of 1 to 2 inches, related largely to the un-
even texture. The rough surface distinguishes the formation from other Silurian
units. In many areas large fallen blocks of the Tete des Morts are common on
the debris- and loess-mantled slopes of the Maquoketa and some rest directly
on the underlying Galena Dolomite Group. Although the Tete des Morts contains
many corals, other fossils are generally scarce and poorly preserved.

The Tete des Morts is subdivided into three unnamed members, referred
to as the "lower massive," the "middle cherty," and the "upper massive" units.
The lower massive unit is 12 to 15 feet thick. It locally contains scattered
chert nodules in the lower 1 foot, and a few chert nodules occur at other hori-
zons. It is sparsely glauconitic.

The middle cherty unit is commonly 2 to 3 1/2 feet thick. The dolomite
is slightly argillaceous, dense, and fine grained, and it occurs in well-defined
2- to 4-inch beds. In the middle it contains a persistent 2-inch band of closely
spaced chert nodules, which in places becomes a continuous chert layer with
scattered chert nodules common above and below. The chert band is so com-
monly exposed that it makes an excellent marker for structure mapping (Willman
and Reynolds, 1947). The cherty unit weathers to a smooth surface and makes
a distinct reentrant in most of the cliffs, except in the extreme eastern part of
the outcrop area, where it is not as well bedded and does not form a reentrant
in the cliff exposures.

The upper massive unit, generally 5 to 8 feet thick, differs from the
lower in containing a greater abundance of silicified corals and coarse grains of
glauconite. It also contains dense white chert in thin layers or veins 1/8 to
1/4 inch thick and as much as 1 foot long. Layers of veins of chert are only
locally present, and much less abundant, in the lower massive unit.

The top surface of the upper massive unit is a smooth but deeply pitted
corrosion surface. There is no evidence that it represents more than a diastem.
However, in western Dubuque County, Iowa, well-bedded dolomite, as much
as 20 feet thick, called the "lower quarry beds" by Calvin and Bain (1900) and
assigned to the lower part of the Kankakee by Brown and Whitlow (1960), over-
lies the Tete des Morts. The quarry beds differ lithologically from the under-
lying Tete des Morts and the overlying Kankakee (Blanding in this report) and
merit classification as a formation, but they are very weakly developed in Illinois

NORTHWESTERN ILLINOIS 35

Although the Tete des Morts is not present south of Hanover, in that
area the lower 2 to 3 feet of the Blanding Formation, which is characterized
by many layers of chert, contains only a few chert nodules and it might be a
lateral equivalent of the Tete des Morts or of the "lower quarry beds" of Iowa.
However, it is better bedded than the Tete des Morts and it lacks the glauconite
that characterizes the upper Tete des Morts. In many places where the Tete des
Morts is present, the lower few feet of the Blanding contains only scattered
chert nodules. These beds more likely correlate with the "lower quarry beds,"
but they lack the well-defined, even bedding of the "lower quarry beds."

The Tete des Morts resembles the basal Drummond Member of the Kanka-
kee Dolomite in northeastern Illinois in containing coarse glauconite and in
having an abundance of silicified corals, but it lacks the sand grains and the
fossil Platvmerella manniensis that occur in that region. More importantly, it
occurs below the interval with prominent white chert bands (Blanding Formation),
whereas in northeastern Illinois the Drummond occurs above the interval with
white chert bands (El wood Formation) .

Blanding Formation

The Blanding Formation consists of dolomite with many layers of white
chert, and for many years it was referred to as "the cherty beds." It overlies
the Tete des Morts Formation north of Hanover and the Mosalem south of there,
and it is overlain by the Sweeney Formation. The Blanding Formation is named
herein for the village of Blanding, which is 3 miles south of the type section,
an exposure in the upper part of the Mississippi River bluffs described as the
Royal Princess Section. The unit was named Waucoma by Savage (1914) but
has been called Kankakee since 1926, when Savage correlated it with the Kanka-
kee of northeastern Illinois. As the type section of the Waucoma is Mosalem,
the selection of a new type section to revive the name "Waucoma" seems un-
desirable. As the unit resembles the cherty zone in northeastern Illinois pre-
viously included in the Edgewood, and herein named Elwood, the term "Kankakee"
is not appropriate, and even if it correlates with the Kankakee, it is a different
rock-stratigraphic unit.

The Blanding Formation is 35 to 50 feet thick, and it is present throughout
the outcrop area. In the region north of Hanover the lower part commonly caps
the hills above the cliffs of the Tete des Morts Formation. South of Hanover
the Blanding is well exposed in Mississippi Palisades State Park and along Plum
River and its many tributaries (Lost Mound, Whitton Northeast, Lanark Northwest,
Palisades Park High Face, Palisades Park Main Entrance, and Winston North
Sections) .

The Blanding is eroded along the Savanna Anticline, but south of the
anticline it descends from the top of the Mississippi bluffs to the floodplain
level near the mouth of Otter Creek, 5 miles northeast of Fulton. Farther east,
the formation is exposed at many places in the headwaters of Johnson, Otter,
and Rock Creeks, as, for example, 1 1/2 miles east of Franklin Corners., White-
side County (SE SE SE 2, 22N-5E, Morrison Quad.).

The Blanding Formation consists largely of brownish gray to slightly
pinkish, fine-grained, slightly vesicular, moderately pure dolomite in 2- to
6-inch beds with generally tight bedding planes. It contains layers of white

36 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

chert mostly 1 to 3 inches thick, some as much as 5 inches thick, and commonly
separated by 4 to 6 inches of dolomite. Locally the chert forms 50 percent of
the unit, but 20 to 30 percent is more common. In some areas the lower 3 to 8
feet is slightly argillaceous, dense, and light gray, and it contains chert in
nodules rather than in layers. Silicified corals are common throughout the for-
mation .

The Blanding Formation is similar in lithology to the Elwood Formation
of northeastern Illinois, which has been traced in subsurface westward to Peru,
La Salle County, but not definitely to the nearest outcrop areas, near Fulton
and north of Morrison in northern Whiteside County. The Elwood has previously
been included in the Edgewood and the Blanding in the Kankakee on the basis
of their faunas, and lacking more definite evidence of their equivalence, local
names are used in each outcrop area.

Sweeney Formation

The Sweeney Formation consists of dolomite in thin, wavy beds separated
by green clay partings. It generally contains little chert, except near the middle,
in a zone about 5 feet thick. It conformably overlies the Blanding Formation and
conformably underlies the Marcus Formation throughout the area. The Sweeney
Formation is named herein for Sweeney Islands, which occur in the Mississippi
River near the type section in Mississippi Palisades State Park, north of Savanna,
described as the Palisades Park High Face Section. Part of the type section is
in a cliff and difficult to examine, but the lower part of the formation is accessi-
ble at other places in the bluffs and also in a quarry northeast of Savanna (Camp
Creek Quarry Section) and the upper part is accessible in the Palisades Park Old
Quarry Section.

This unit was differentiated as the Syringopora tenella beds in Dubuque
County, Iowa, by Calvin and Bain (1900) and later correlated with the Joliet
of northeastern Illinois by Savage (1926). As it is lithologically unlike the
Joliet and more closely resembles the major part of the Kankakee Dolomite of
northeastern Illinois, the name "Joliet" is not appropriate.

The Sweeney Formation is 45 to 55 feet thick. In the area north of Han-
over, it and higher Silurian strata are generally eroded, but it is locally present
in the southern part of that area in the Mississippi River bluffs (Royal Princess
Section) and in the highest roadcuts of U.S. Highway 20, 6 miles east of Galena
(Smallpox Creek East Section). From Hanover south it is exposed in many places
in the synclinal area north of the Savanna Anticline. Although eroded along the
anticline in a belt 4 to 5 miles wide (fig. 7), it is present in the bluffs south
of the anticline, gradually lowering until it disappears at floodplain level about
a mile south of Otter Creek, 3 miles northwest of Fulton, Whiteside County.
Its southernmost exposures along the Mississippi Valley are in quarries at Ful-
ton. In the synclinal area north of Savanna, it is exposed eastward as far as
Georgetown, which is north of Lanark, Carroll County; and south of the anti-
cline, it is exposed eastward to Coleta, Whiteside County.

The Sweeney Formation consists largely of light pinkish gray to brownish
gray, vesicular, pure dolomite in tight 1- to 4-inch wavy beds with thin green
clay partings. In 2- to 5-foot intervals, most commonly at the base and about
20 feet above the base, it becomes very thin bedded and contains strong green

NORTHWESTERN ILLINOIS 37

shaly partings. In the high bluff exposures, particularly in Palisades Park, it
appears to be massive, but weathering brings out the characteristic thin bedding
that differentiates it from the overlying very massive Marcus Formation.

In some exposures the formation is entirely without chert, but a few
nodules of chert may be found at scattered positions in many of them. Chert
is most abundant in a zone 3 to 5 feet thick, rarely as much as 15 feet thick,
the base of which is 15 to 20 feet above the base of the formation. The cherty
zone is exposed in the Smallpox Creek East, Lost Mound, Palisades Park North,
and Camp Creek Quarry Sections. In this zone the chert locally occurs— with
notable variations in quantity— in layers, lenses, and nodules.

The formation is generally fossiliferous, with silicified corals common
to abundant in all exposures . Pentamerus oblonqus and Microcardinalia occur
in a zone that is 15 to 25 feet above the base. Microcardinalia generally
occurs in only one or two beds in that interval, commonly in a chert band. Al-
though Microcardinalia , formerly Stricklandia, was reported by Savage to occur
at the top of the Blanding (then the Kankakee Formation), it was found in this
study, as previously noted, only in the Sweeney Formation. It seems likely
that Savage, noting the association with chert, assumed the fossils were in his
Kankakee Formation, because there is no indication that he was aware of a
cherty zone in the Sweeney Formation. Microcardinalia was found in (1) the
Camp Creek Quarry Section; (2) a quarry 1 1/2 miles southwest of there on the
north side of Plum River (NW NE SE 35, 25N-3E, Savanna Quad.), where the
fossils occur in a cherty zone 3 feet thick, 32 feet below the Marcus Formation;
(3) at Bob Upton's Cave in Palisades State Park (NW NW SE SE 33, 25N-3E, Savan-
na 7.5-minute Quad.), where the fossils are present but scarce in a thin-bedded
cherty zone 1 foot above the cave floor, 3 feet above the lower, more massive
zone, and about 20 feet above the Blanding Formation; and (4) in a quarry 1 mile
east of Hanover (NW NE SE 10, 26N-2E, Hanover 7.5-minute Quad.), where the
fossils occur largely at the base of a cherty zone 4 feet thick and 15 feet above
the top of the Blanding .

The Sweeney Formation resembles the Kankakee Formation of north-
eastern Illinois in its thin wavy bedding with green shale partings and the
abundance of silicified corals. The presence of Microcardinalia with Pentamerus
in the middle part of the Sweeney and in the Plaines Member of the Kankakee
of northeastern Illinois further suggests this correlation and favors classifica-
tion of the Sweeney as Alexandrian. Because the Marcus, which overlies the
Sweeney in northwestern Illinois, in no way resembles the Joliet, which over-
lies the Kankakee in northeastern Illinois, and because the correlation of the
basal contacts of the Sweeney and Kankakee also is uncertain, separate nomen-
clature in the two outcrop areas is preferred .

Marcus Formation

The Marcus Formation consists of about 40 feet of massive pure dolomite,
the lower 5 to 15 feet of which is generally a nearly solid mass of shells of the
fossil Pentamerus oblonqus. It conformably overlies the Sweeney Formation and
underlies the Racine Formation throughout the area. It is herein named for the
village of Marcus, which is 4 miles northwest of the type section in an abandoned
quarry in the south part of Mississippi Palisades State Park in Carroll County
(Palisades Park Old Quarry Section).

38 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

Although previously identified as "the Pentamerus beds" (fig. 9), the
unit was named Waukesha by Savage (1926), who later (1942) proposed the local
name "Cordova" for the unit. Savage was under the misapprehension that the
beds in the quarry at Cordova, which are Racine, belong to this unit. As the
name "Cordova" was not appropriate (Willman, 1943), "Waukesha" continued
to be used for the unit. However, the unit differs strongly from the less pure,
well-bedded strata of the type Waukesha at Waukesha, Wisconsin, and from
the restricted Waukesha of northeastern Illinois, which is renamed Sugar Run
in this report. It is stratigraphically at the position of the Joliet Formation of
northeastern Illinois, which it also does not resemble.

The Marcus Formation has been eroded from the top of the Silurian section
in northwestern Illinois north of Carroll County. It appears at the top of the
Mississippi River bluffs northeast of Marcus and about 4 miles north of Savanna.
It descends southward to the type section, which is close to the synclinal axis
north of the Savanna Anticline, where the base of the formation is about 80 feet
above the level of the river. It is sharply truncated a quarter of a mile south of
there on the steep slope of the Savanna Anticline. It is absent on the anticline
and southward to just north of the Carroll- White side county line. It is ex-
posed in the bluffs southward from there to about 3 miles east of Fulton, where
it is exposed in several quarries (SW NE NW 19, 22N-4E, and SE SE NE 24,
22N-3E, Clinton Quad.). It is also exposed in quarries on the north side of
Fulton, but south of Fulton it is overlain by the Racine Formation and is below
floodplain level.

Along the synclinal axis north of the anticline, the Marcus extends east-
ward in a narrow belt as far as Shannon in northeastern Carroll County, and in
that area it is well exposed in a small quarry along Illinois Highway 78, 2 miles
north of Mt. Carroll (SE NW NW 25, 25N-4E, Mt. Carroll Quad.), where small
pentamerids crowd the lower 3 feet and very large specimens occur above.
From the bluffs at Fulton, the Marcus Formation extends eastward to about 5
miles northeast of Morrison, and it is exposed near Us tick and in quarries at
White Pigeon and Malvern.

The Marcus Formation is a high vesicular, very massive, brown, pure
dolomite. It commonly occurs in ledges 5 to 15 feet thick without noticeable
bedding. Because of the scarcity of exposures showing both top and bottom,
variations in its thickness are not well known, but it appears to generally
be within 5 feet of 40 feet thick. It forms prominent cliffs in valley bluffs, but
in areas where dissection is not deep, it commonly weathers to a dolomite sand,
in places 10 to 15 feet thick. The basal contact with the Sweeney Formation
is generally distinguished by a sharp reentrant. The abundance of Pentamerus
and the massive character of the Marcus generally distinguish it from the Sweeney,
which has thin but weak bedding, green clay partings, and abundant corals.
However, some pentamerids occur in the Sweeney and corals occur in the Marcus.
The contact with the overlying Racine is more difficult, as the latter also is
commonly a very pure dolomite. However, the well-bedded character, the vari-
ations from dense to highly vuggy texture, and the gray color of the Racine gen-
erally distinguish it from the Marcus.

The Marcus Formation is lithologically unlike any other Silurian unit in
Illinois. It is equivalent to the upper part of the Hopkinton in Iowa, and the
abundance of Pentamerus suggests its correlation with the Schoolcraft in Wis-
consin and Michigan, the Fossil Hill in Ontario, and the Reynales in New York.

NORTHWESTERN ILLINOIS 39

Racine Formation

In northwestern Illinois, the Racine Formation consists largely of gray,
vuggy, pure dolomite about 300 feet thick. It includes all the Silurian dolomite
above the Marcus Formation, which it overlies conformably. It is the reef-
bearing part of the Silurian section and corresponds to the "upper coralline beds"
of early reports (fig. 6).

Savage (1926) introduced the name "Racine" for the lower part and "Port
Byron" for the upper part, the differentiation based entirely on faunas— the Racine
correlating with the Lockport and the Port Byron with the Guelph in the New York
type section. Savage (1942) later suggested that the entire section be called
Port Byron in Illinois and Gower (Norton, 1899) in Iowa, although the units are
equivalent, especially as Gower has been redefined to include both reef and
interreef rocks (Hinman, 1968). As the unit is similar lithologically to the type
Racine (Hall, 1861) at Racine, Wisconsin, the name "Port Byron" was dropped
in northeastern Illinois (Willman, 1942) and in northwestern Illinois (Willman
et al., 1967) and "Racine" used for the entire interval.

North of the Savanna Anticline, the Racine Formation occurs only in a
narrow belt along the synclinal axis. The basal 3 0 feet is exposed at the top
of the Mississippi River bluffs in Mississippi Palisades State Park (Palisades
Park Old Quarry Section) and also along Plum River, 2 miles east of the park
(NW NW NE 2, 24N-3E, Savanna Quad.).

South of the anticline, the Racine is present only south of an east-west
line through Fulton, in an area extending eastward from the Mississippi River
bluffs to Morrison, Fenton, and Hillsdale. In this area it thickens southwesterly
to about 300 feet at Port Byron, south of which it is overlain unconformably by
Middle Devonian strata. The lower part of the Racine is exposed in a small
quarry 2 miles southeast of Fulton, Whiteside County (SW SW NW 36, 22N-3E,
Clinton Quad.) and at Morrison (NE SW SE 7, 21N-5E, Morrison Quad.) . The
middle part is exposed at Albany (SE NE SE 24, 21N-2E, Clinton Quad.), near
Fenton (SE SE SE 32, 21N-4E, Clinton Quad.), and 4 miles northwest of Erie
(SW SE 22, 20N-3E, Erie Quad.). The upper part is exposed in quarries at Cor-
dova, Rock Island County (NE NW SW 31, 20N-2E, and NW SE 1 , 19N-1E,
Cordova 7. 5 -minute Quad.), and in numerous outcrops in the bluffs along the
south side of Meredosia Slough, including a quarry 3 miles north of Hillsdale
(NE SE SE 6, 19N-3E, Erie Quad.). The highest part of the Racine is exposed
in a quarry on the south side of Port Byron, Rock Island County (NW NW SE 25,
19N-1E, Port Byron 7.5-minute Quad.), in a quarry 2.5 miles southwest of
Joslin, Rock Island County (SW 16, 18N-2E, Port Byron 7.5-minute Quad.),
and in a quarry south of Cleveland, Henry County (S 1/2 31, 18N-2E, Port
Byron Quad.). The Racine Formation is overlain by Middle Devonian limestone
in the latter exposure (Edmund and Anderson, 1967).

The Racine Formation is largely a medium gray, fine- to medium-grained
dolomite with textures that vary from dense to vesicular to vuggy. It weathers
gray. In most outcrops it is well bedded, the beds varying from 2 inches to 4
feet thick. In some exposures the dolomite occurs in massive ledges 10 to 15
feet thick, which may be the cores of reefs. However, steeply dipping reef-
flank beds are only locally present, as in the quarry on the south side of Port
Byron, and definite reef structures appear to be less common than in the Gower
in Iowa (Hinman, 1968). The flat -lying interreef beds generally have the same

40 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

pure dolomite as the reefs and, like the reefs, almost entirely lack chert. Ar-
gillaceous interreef rocks of the types associated with reefs in northeastern
Illinois are exposed at only a few localities, generally interbedded with reef-
type dolomite, as near Albany (SE SE NE 25, 21N-2E, Clinton 7.5-minute Quad.),
but as these rocks are more readily weathered than the reef-type dolomite,
they may be more common than indicated by the outcrops .

The Racine Formation in northwestern Illinois is essentially equivalent
to the Racine in northeastern Illinois; but because of differences in the under-
lying formations, there is no assurance that the basal contact is the same nor
that the upper surface, truncated by Middle Devonian strata in both areas, is
the same. Savage (1926) identified upper Niagaran Guelph fossils in the youngest
Racine in both areas. The Racine is probably equivalent to the lower and middle
parts of the Moccasin Springs Formation in central and southern Illinois.

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ROCK STRATIGRAPHY OF THE SILURIAN SYSTEM 41

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42 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

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ROCK STRATIGRAPHY OF THE SILURIAN SYSTEM 43

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44 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 479

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GEOLOGIC SECTIONS

NORTHEASTERN ILLINOIS

1. COWAN'S QUARRY SECTION

Exposure in bluff and small abandoned
quarry, north side of Kankakee River, 3h miles
southeast of Ritchey, Will Co. (SE SW NW 26,
32N-10E, Herscher Quad.)- Type section of the
Kankakee Formation.

Dolomite, medium-grained, vesicular, massive,

lenticular ; 6" to

Sugar Run Formation

Dolomite, argillaceous, very fine grained,
dense; in 2-8" beds; bedding planes smooth;
weathers reddish brown; base concealed at
water level

Silurian System

Joliet Formation

Brandon Bridge Member

Limestone, dolomitic, argillaceous, greenish
gray, fine-grained, dense; in beds h-3" thick;
contains thin green shale partings 4 '

Limestone, dolomitic; like above but locally
red; contains many shale partings and forms

a shaly reentrant on weathered surfaces 1 '

Kankakee Formation (17 '9")
Plaines Member

Limestone, dolomitic, very light gray, massive,
medium-grained, dense; contains many poorly
preserved fossil fragments; 3" fossil coquina
10" above base; pitted smooth surface on top;

contains Pentamerus and Microcardinalia 2 ' 7"

Troutman Member

Dolomite, argillaceous, gray, fine-grained,

dense; in beds h-2" thick 1' 7"

Dolomite, slightly argillaceous, gray, fine- to
medium-grained, dense; in beds 1-2" thick;
contains thin green clay partings 6 ' 3"

Clay, gray; in lenses, making strong reentrant;

0 to 2"

Dolomite, similar to above, but weathers brown;

2" shaly reentrant at base 1 ' 2"

Dolomite, similar to above, but thicker bedded
and contains thicker green shale partings;

1*8" to 2' 8"

Offerman Member

Dolomite, argillaceous, fine-grained, dense;

contains streaks of finely vesicular, crinoidal

dolomite; prominent bedding plane at base;

6" to 1 '

Dolomite, argillaceous, fine-grained, dense to
vesicular; in beds 1-3" thick; contains 1"

laminated beds at top and 4" below top 1 '

Drummond Member

Dolomite, weathered reddish brown, medium-
grained, glauconitic, massive, vesicular;
contains large vugs; small pentamerids com-
mon; Platymerella present 1' 4"

Ordovioian System
Maquoketa Group
Neda Oolite
Oolite, reddish brown; base concealed 1 * 6"

2. EGGLESTON SECTION

Abandoned quarry on east side of Wiley
Creek and exposure in ditch along north side of
road, h mile west of Eggleston School, 3 miles
northwest of Kankakee, Kankakee Co. (SW SE NE
27, 31N-11E, Kankakee Quad.).

Silurian System

Racine Formation (25' 8")
Dolomite, brown, highly vesicular, massive, fos-
siliferous; contains many crinoid stems; upper
5' has many large vugs; exposed largely in

road ditch, lower 3' at top of quarry 19'

Dolomite, very argillaceous, in part shaly,
light brownish gray, weathers buff, thin-
bedded; contains wavy, thin, dark gray,

lenticular clay partings 1 '

Dolomite, argillaceous; in beds 1-s-2" thick 1' 6"

Dolomite, like 1' bed above 8"

Dolomite, like 1'6" bed above; 6" to 1' 6"

3. ELMHURST QUARRY SECTION

Quarry of Elmhurst-Chicago Stone Com-
pany in Elmhurst, Du Page Co. (SW NW 2, 39N-11E,
Elmhurst 7.5' Quad.).

Devonian System

Joints and pockets filled with a brown
shale containing Devonian fossils in the upper
part of the Silurian rocks in this quarry were
described by Weller (1899) and Alden (1902).
None have been found in recent years.

Silurian System

Racine Formation
Dolomite, pure, medium-grained, highly porous,
medium to dark gray, massive to faintly bed-
ded; contains many large crinoid stems; the
base of this unit, which is a reef, rises
rapidly to the east so that the unit thins to
40 feet; as it thins, it grades to dense,
argillaceous, cherty interreef rock, except
for the uppermost 2-5 ' , which continues over

most of the area quarried; 40' to 60'

Dolomite, silty, argillaceous, very fine

grained, dense, light brownish gray to green-
ish gray; in beds 3-6" thick; contains many
thin, wavy, green clay partings; this unit
has the same lithology as the overlying beds
that grade laterally to the reef; base of
Racine is 80-90' below top of quarry; 25' to.. 45'
Sugar Run Formation
Dolomite, light brownish gray; argillaceous and
silty at top but grades to only slightly im-
pure at base; similar to above but more mas-
sive and contains little chert 10'

Joliet Formation (81 '3")
Romeo Member
Dolomite, pure, light gray, in part mottled med-
ium to dark gray, vesicular; in 3-10" beds

with tight, stylolitic, bedding planes 34'

Markgraf Member
Dolomite, slightly argillaceous at top to silty

at base; contains scattered chert nodules 28' 3"

Brandon Bridge Member
Dolomite, argillaceous, dense, fine-grained,
gray at top; red silty beds at base; contains

green shale partings 19 '

Kankakee Formation (46 '6")
Plaines Member
Dolomite, pure, white, massive; smooth surface

with pits filled with green clay on top 2'

Troutman Member
Dolomite, pinkish gray, slightly vesicular; in
thin beds with green shale partings; thin
lenses of nearly white clay along prominent

reentrant 13'4" above base 29' 4"

Offerman Member
Dolomite, similar to above, but dense and more

argillaceous 9 ' 2"

Drummond Member

Dolomite, pure, gray, vesicular, massive 6'

Covered to quarry floor, which is on the top of
the Ordovician Brainard Shale 3'

4. HILLSIDE QUARRY SECTION

Quarry of Consumers Company in Hill-
side, Cook Co. (SE NE 17, 39N-12E, Hinsdale
Quad.). Summary of section.

Silurian System

Racine Formation
Dolomite, pure, light gray, massive, vesicular;

weathered brown 15 '

Dolomite, cherty, slightly argillaceous and

silty, brownish to greenish gray 80'

Sugar Run Formation
Dolomite, silty, greenish gray; contains dark
green, wavy, argillaceous partings; contains
lenses of chert; lower 6' transitional to

below 29

Joliet Formation (71')
Romeo Member
Dolomite, pure, gray, partly mottled pink; in

6-12" beds 26'

Markgraf Member
Dolomite, slightly argillaceous, light brownish
gray, dense; contains scattered chert nodules. 20'
Brandon Bridge Member
Dolomite, argillaceous, greenish gray, locally

pinkish; contains strong green shale partings. 25'
Kankakee Formation (32 '5")
Plaines Member

Dolomite, pure, white, massive; pitted smooth

surface on top 2 ' 5"

Troutman Member
Dolomite, greenish and pinkish gray; in thin

beds with green shale partings 17 ' 3"

Offerman Member
Dolomite, slightly argillaceous, dense, thin-
bedded 8' 9"

Drummond Member
Dolomite, pure, gray, vesicular; base concealed
at water level in sump 4 '

5. JOLIET— LINCOLN QUARRY SECTION

Quarry of the Lincoln Stone Company
south of Brandon Bridge, southwest of Joliet,
Will Co. (S^ SE 20 and N*2 NE 29, 35N-10E, El-
wood 7.5' Quad.). Type section of the Brandon
Bridge Member of the Joliet Formation.

Silurian System

Joliet Formation (54' 7")
Romeo Member

FORMATIONS

Racine

Sugar Run

Joliet

NORTHEASTERN ILLINOIS

5 6

Kankakee

Elwood

Wilhelmi

NORTHWE STE RN

ILLINOIS

Racine

Marcus

Sweeney

25 26

24-

27"

31.

Blanding

Tete des Morts

30-

Mosalem

*Type section.

Fig. 10 - Index to stratigraphic units described in the geologic sections

Dolomite, pure, light gray, fine-grained, vesic-
ular; in beds 2-6" thick 3' 2"

Markgraf Member

Dolomite, slightly argillaceous, light gray,
slightly vesicular; in beds 2-4" thick;
slightly shaly zones occur 1'9" below top and
6" above base; chert nodules scattered
throughout and in six bands 11 ' 8"

Dolomite, silty, light gray, dense, very fine
grained; many wavy argillaceous partings show
on weathered surface; in beds 2-8" thick;
contains scattered chert nodules, zones
crowded with chert nodules 1'4"-1'10" and
2»io"-3'4" below top, and prominent bifurcat-
ing band of chert 4'4"-4,8" below top 6'

Dolomite, as above, but very silty; not cherty
as in the National and Markgraf quarries;
prominent bedding plane at base 2' 1"

Dolomite, as above, but massive; weathers to

4"-l'6" beds 6' 8"

Brandon Bridge Member

Dolomite, argillaceous and silty, gray, mottled
pink and green; contains thin lenses of green
clay ; shaly near base 6 ' 10"

Dolomite, as above, but very argillaceous and

contains many strong green shale partings.... 2' 8"

Shale, dolomitic, black and green interbedded
and mottled or in places all green; grades to
argillaceous finely laminated dolomite; 0 to. 1'

Dolomite, pink to red, fine- to medium-grained,
dense to slightly vesicular; in beds 1-2"
thick separated by beds of green shale or
green and pink mottled argillaceous dolomite. 14' 6"
Kankakee Formation (49' 9")
Plaines Member

Dolomite, very light gray, medium-grained, ve-
sicular; in one ledge; pitted smooth surface
on top; Pentamerus oblongus abundant in local

pockets 2' 2"

Troutman Member

Dolomite, as above, but has thin green shale

partings 9"

Dolomite, light gray, slightly pinkish, fine-
to medium-grained, dense to finely vesicular;
in beds 1-2" thick separated by thin green
clay partings; Microcardinalia abundant in 6"

beds 3 ' 10" below top 26 '

Offerman Member

Dolomite, gray, dense, fine-grained, slightly

argillaceous , thin-bedded 11 ' 8"

Drummond Member

Dolomite, pure, gray, vesicular, massive, glau-
conitic; lower 2' cherty and contains Platy-

merella 9 ' 2"

Elwood Formation

Dolomite, dense, brownish gray, very cherty;
contains Platymerella in upper 2'; base at
quarry floor 12 ' 4"

The lower 40' of this section was exposed in a
lower bench now filled with cinders.

6. JOLIET— MARKGRAF QUARRY SECTION

Abandoned Markgraf quarry on north side
of Des Plaines River Valley, in southwest part
of Joliet, Will Co. (SW SW 16, 35N-10E, Joliet
7.5' Quad.). Type section of the Markgraf Mem-
ber of the Joliet Formation.

Silurian System

Joliet Formation (64' 7")
Romeo Member
Dolomite, pure, gray, fine-grained, vesicular;
in beds 2-4" thick; contains a few silicified

fossils 7' 1"

Dolomite, as above, but contains chert nodules

largely in five bands 4 ' 5"

Dolomite, as above, but without chert; prominent

wavy bedding plane at base 5 ' 6"

Markgraf Member
Dolomite, slightly argillaceous, light brownish
gray, fine-grained, slightly vesicular; in
beds 2-8" thick; contains slightly shaly zone

1'7" from top; chert nodules in five discon-
tinuous bands 6 ' 2"

Dolomite, slightly more argillaceous than

above; contains prominent band of large white

chert nodules in the middle and a lenticular

bed of chert at the base 2 ' 11"

Dolomite, silty and argillaceous, light gray,
very fine grained, dense; in beds 1/4-8"
thick; contains light green wavy clay part-
ings and thin shaly zones 9" and 4' below
top; small chert nodules are closely spaced
in zones 9-12", 2'6"-2'll", and 3'll"-4'4"
below top 4 * 4"

Dolomite, as above, but more silty; contains
scattered nodules of chert and a persistent
band of chert nodules below top 4 ' 5"

Dolomite, silty, very light gray, dense, very
fine grained; massive on fresh surfaces but

weathers into three ledges 4' 3"

Brandon Bridge Member

Dolomite, argillaceous and silty, gray, mottled
light pink and green; contains many wavy
argillaceous partings; 2" shaly zone 1'8"
below top 6 ' 9"

Dolomite, as above, but less shaly and has

smoother weathered surface 2 ' 10"

Dolomite, as above, but very shaly; grades

westward to a 2-4" shale bed 5"

Dolomite, mottled and interbedded red, green,
and gray; the red dolomite is mostly fine to
medium grained and relatively pure; the green
dolomite and most of the gray are argillace-
ous and silty; contains many green shale

partings 15' 6"

Kankakee Formation
Plaines Member

Dolomite, pure, white, vesicular, massive; con-
tains Microcardinalia ; top surface is smooth

and has deep pits filled with green clay 1' 6"

Troutman, Offerman, and Drummond
Members

Dolomite, light pinkish or greenish gray, fine-
grained; low to medium vesicular ity; in beds
mostly 2-3" thick separated by thin green
clay partings; bedding planes very wavy; con-
tains chert in lenses near base 38 '

The lower 45' of this section is now covered by
water and fill.

7. JOLIET— NATIONAL QUARRY SECTION

Quarry of National Stone Company on
south side of Joliet, Will Co. (NE SE 21, 35N-
10E, Joliet 7.5' Quad.). Type section of the
Joliet and Sugar Run Formations and the Romeo
Member of the Joliet Formation.

Silurian System

Sugar Run Formation

Dolomite, silty, argillaceous, gray, brown

weathering, very fine grained, dense; in beds
mostly 4"-l' thick; bedding planes smooth
with thin shaly partings; generally non-
cherty, but bands of white chert occur about
8' below top near a clay pocket in south
face; insoluble residue increases upward from
10%-20% 18 '

Dolomite, slightly argillaceous and silty, very
fine grained, dense, light gray weathering at
top; grades downward to fine-grained, low to
medium vesicular, gray weathering dolomite at
base; insoluble residue 3%-8%; transition

zone to Joliet Formation 8' 4"

Joliet Formation (67'10")
Romeo Member

Dolomite, pure, light gray, fine-grained, ve-"
sicular (1" vugs common), fossilif erous; in
beds 6"-2' thick; bedding planes tight,
rough, in part stylolitic; locally contains
thin green clay partings; l%-2% insoluble
residue 6

Dolomite, as above, but very cherty; contains

silicified fossils and chert in 6-15 discon-
tinuous bands of closely spaced nodules and
lenses 9' 3"

Dolomite, as above, but massive and locally

has light pink mottling 4' 11"

Markgraf Member

Dolomite, very light gray, very fine grained,
dense to slightly vesicular; slightly less
pure than overlying beds; faint, very thin
argillaceous streaks show on weathered sur-
faces; in beds 2-10" thick; chert nodules
scattered throughout, but most abundant in
seven discontinuous bands; 3%-4% insoluble
residue 11' 4"

Dolomite, silty, light gray, very fine grained,
dense; contains green argillaceous lenses,
silicified fossils, and scattered chert
nodules mostly in six bands; 9%-12% insol-
uble residue 8' 2"

Dolomite, silty, light gray, locally pink mot-
tled, massive, noncherty; 10% insoluble resi-
due 5 '

Brandon Bridge Member

Dolomite, silty, argillaceous, gray, pink and
green mottled, 2" gray laminated bed 11"
below top; thin shaly partings prominent on
weathered surface; 10%- 15% insoluble residue. 3'

Dolomite, as above, but very silty; weathered

surface smooth 3' 2"

Dolomite, as above, but darker colored; con-
tains many green shale partings; base con-
cealed 3' 8"

To quarry floor 6 '

Shale, green; 0 to 4"

Summary of section in lowest bench

Dolomite, argillaceous, gray, red, green, thin-
bedded 13'

Kankakee Formation (38' 7")
Plaines Member

Dolomite, pure, white, massive; pitted smooth

surface on top 3' 4"

Troutman Member

Dolomite, greenish and pinkish gray, vesicular;
in thin wavy beds separated by green clay

partings 18' 6"

Offerman Member

Dolomite, similar to above, but slightly argil-
laceous 5 ' 9"

Drummond Member

Dolomite, gray, vesicular, massive, glauconi-
tic; contains corals and a few rounded quartz

sand grains 11 '

Elwood Formation

Dolomite, brownish gray, cherty; Platymerella
manniensis in chert; base concealed at quarry
floor 1'

8. KANKAKEE RIVER CAMPGROUND SECTION

Outcrop, north side of Kankakee River,
3/4 mile west of Warner Bridge, at campground
of Kankakee River State Park, 5 miles southeast
of Ritchey, Will Co. (NE NW SW 36 , 32N-10E,
Herscher Quad.).

Silurian System

Kankakee Formation (19 '8")
Troutman Member
Dolomite, light brownish gray, fine-grained,
dense to slightly vesicular; in 4-8" beds;

prominent bedding plane at base 5'

Dolomite, similar to above but slightly more
argillaceous; in 1-3" beds with thin wavy
partings of green clay 5' 6"

Offerman Member
Dolomite, argillaceous, yellowish brown, very

fine grained, dense; in 3-8" beds 3' 10"

Drummond Member
Dolomite, brownish gray to brown, medium-
grained, massive, dense to highly vesicular
(contains large vugs) ; upper 2" locally
laminated; large lenses of chert along base;

r to r 8"

Dolomite, similar to above, but well-bedded
and uniformly highly vesicular; contains
dolomite lenses consisting largely of casts
of a small pentamerid 2 ' 3"

Dolomite, reddish brown, highly vesicular; in

single bed; 7" to 1' 4"

Conglomerate, light gray to black nodules or
pebbles as large as 1" in diameter; possibly

algal; V to 1"

Wilhelmi Formation (10%")

Dolomite, very argillaceous and shaly; contains

scattered oolites : %"

Dolomite, brown, dense, hard, laminated; 2" to. 3"

Dolomite, very argillaceous, soft; contains

pockets of dark brown oolites 4"

Dolomite , laminated ; 2" to 3"

Ordovioian System
Maquoketa Group
Neda Oolite

Oolite, reddish brown, locally conglomeratic;
in thin to 6" thick beds ; some beds have
dark green clay matrix; oolites hematitic and
flattened; a 6-10" hard ledge 6" below top
is dark red and has concretionary structure.. 3' 2"

Shale, purple and green; upper 2-6" is oolitic. 8"

Shale, purple, partly green mottled; base con-
cealed 6"

9. LEHIGH QUARRY SECTION

Quarry of Lehigh Stone Company at Le-
high, 8 miles west of Kankakee, Kankakee Co.
(NE 7, 30N-11E, Herscher and Kankakee Quads.).

Silurian System

Raoine Formation (76 '1%")

Dolomite, light gray, fine-grained, dense,
slightly to moderately argillaceous; mostly
in 1-6" even beds 13' 2"

Dolomite, as above, but in 2-4" wavy beds,

which give a nodular appearance; strong shaly
reentrant at base 6'

Dolomite, gray, slightly argillaceous, dense;
in 6"-l' even beds with shaly partings in
lower part 6 ' 4"

Dolomite, light gray, dense, laminated; 1" to..

Dolomite, light gray, mottled pink, slightly
argillaceous; in 3"-l'6" beds; large vugs at
top 7' 2"

Shale, green; makes persistent reentrant h"

Dolomite, light gray, mottled pink, slightly
argillaceous; in 3"-l' beds with strong green
clay partings 8'

Dolomite, as above, but interbedded with green

shale; 1" to 3"

Dolomite, light gray, mottled pink, argilla-
ceous; contains closely spaced dark gray
shaly zones; in 8"-2' beds with clay part-
ings 35 '

Sugar Run Formation

Dolomite, white, slightly argillaceous, slightly
vesicular; in 8"-2' beds, most with smooth
bedding planes, but some bedding planes are

stylolitic 20 '

Joliet Formation
Romeo Member

Dolomite, white, pure, fine-grained, moderately
vesicular with a few large vugs ; in 4-8" beds
with stylolitic bedding surfaces; base con-
cealed at quarry floor 10'

Drilling in the quarry floor showed 35' of dolo-
mite to the shale of the Maquoketa Group, and
samples from a well near the crushing plant
indicate that the interval consists of 12' of
Joliet, 18' of Kankakee, and 5' of Millsdale,
thicknesses comparable to those in exposures
along the Kankakee River, 8 miles north.

10. LEMONT BLUFF SECTION

Abandoned quarry and ravine above it
in the south bluff of the Des Plaines Valley on

the east side of Lemont, Cook Co.
37N-11E; Sag Bridge 7.5' Quad.).

(W?s 21,

Dolomite, dark red, argillaceous;
below water level

excavated from

Silurian System

Racine Formation (59')

Dolomite, pure, gray, light brown weathering,
medium-grained, vesicular; massive where
fresh but weathers to 1-3" beds; contains
thin lenticular films of green clay on bed-
ding planes and scattered chert nodules in
lower 3' ; prominent shaly bedding plane at
base 17

Dolomite, similar to above but better bedded;
contains chert in lower 6"; prominent shaly
bedding plane at base 2

Dolomite, mostly argillaceous, silty, fine-
grained, dense; contains a few beds that are
pure and vesicular; chert abundant. in scat-
tered nodules and bands of nodules; in beds
mostly *s-4" thick

Section above measured in ravine above west
end of quarry.

Dolomite, similar to above but thinner bedded
and contains a large concentration of chert
nodules, which gives it a conglomeratic
appearance 12

Dolomite, silty, argillaceous, fine-grained;
contains scattered chert nodules; in 4-8"
beds 6

Dolomite, very argillaceous, fine-grained; con-
tains lenses of green shale on rough-sur-
faced bedding planes, several 3-6" beds of
medium- grained relatively pure dolomite, and
much chert in scattered nodules and beds....

Dolomite, argillaceous, fine-grained, dense;
contains green clay lenses that give a mot-
tled appearance

Dolomite, slightly argillaceous, fine-grained;
crowded with chert nodules 1

Dolomite, like 10" bed above

Dolomite , pure , medium-grained

Dolomite , like 10" bed above

Chert, laminated, persistent layer

Sugar Run Formation (11 '3")

Dolomite, light gray, silty, argillaceous,
dense; in beds 4-8" thick; contains chert
lenses in upper part 3

Dolomite, as above, but not cherty; in 6"-l'

beds 4

Dolomite, as above, but single massive ledge;

base concealed at water level 3

11. LEMONT WEST QUARRY SECTION

Quarry of Lemont Stone Co. (R. P. Don-
ohoe Co., Inc.) excavated for a boat slip, 1
mile west of Lemont, Cook Co. (SW SW 19, 37N-
11E, Romeoville 7.5' Quad.).

Silurian System

Joliet Formation (51')
Romeo Member

Dolomite, pure, light gray, slightly vesicular;
in 4-8" beds 3'

Dolomite, pure, light gray, vesicular; in 1-2'
beds; contains 6" band of white chert nodules
at base 12 '

Dolomite, pure, light and medium gray, vesicu-
lar , massive 5 '

Markgraf Member

Dolomite, slightly argillaceous, light gray,
slightly vesicular; contains chert nodules
in bands at base and 1' and 2' above base
and scattered above; in 1-2' beds 11'

Dolomite, argillaceous, light yellowish gray,
dense; contains a few chert nodules; in 1-2"

beds 7'

Brandon Bridge Member

Dolomite, light gray, slightly pinkish, fine-
grained, slightly vesicular; contains a few
thin green clay partings 3 *

Dolomite, light pink with greenish shale

lenses ; base concealed at water level 10'

12. NORTH AURORA QUARRY SECTION

Conco Western Stone Company quarry on
the east side of the Fox River Valley, south of
North Aurora, Kane Co. (SE SW 3, 38N-8E,
Aurora North 7.5' Quad.).

Silurian System
6" Kankakee Formation

Dolomite, pure, fine-grained, vesicular; in
1-3" wavy beds with green shale partings ;

2" weathered brown ' 13'

Drummond Member
Dolomite, pure, gray; in 1-3' beds; contains

large vugs; strong shaly partings at base.... 8'
Elwood Formation
8' 6" Dolomite, slightly argillaceous, very fine

grained, dense to slightly vesicular; in beds
mostly 8"-l' thick; greenish clay streaking;
contains white chert in layers and bands of
nodules as much as 4" thick; base concealed
at quarry f loor 20 '

13. PLAINES EAST SECTION

6' 3" Abandoned quarry at Plaines Station

(railroad switching area) , south side of the Des
Plaines River, lh miles southwest of Brandon
Bridge, Will Co. (SW NW SE 30 , 35N-10E, Channa-
hon 7.5' Quad.). Type section of Plaines Member

8' 6" and (with nearby Plaines West) the Troutman
Member of the Kankakee Formation.

10" Silurian System

Joliet Formation
9" Brandon Bridge Member

6" Dolomite, pink and gray, medium-grained, dense;
1" contains green clay partings; exposed at east

9" end of quarry 2' 6"

2" Kankakee Formation (22 '9")
Plaines Member
Dolomite, very light gray, fine-grained, vesicu-
lar, massive, f ossilif erous ; upper surface is
4" smooth but contains pits filled with green

clay 2 ' 2"

7" Troutman Member

Dolomite, light brownish and pinkish gray, fine-
4" to medium-grained; in 1-4" wavy beds; con-
tains thin green shale partings 19 '

Clay, very light gray; in pockets on wavy bed-
ding plane ; makes reentrant 1"

Dolomite, light gray, fine-grained, dense,
slightly argillaceous; base concealed at
quarry floor 1' 6"

14. PLAINES WEST SECTION

Railroad cut southwest of Plaines
Station, south side of the Des Plaines River,
1 3/4 miles southwest of Brandon Bridge, Will
Co. (NW SE SW 30, 35N-10E, Channahon 7.5'
Quad.), h mile southeast of Plaines East sec-
tion. Type section of the Troutman (lower
part), Offerman, and Drummond Members of the
Kankakee Formation.

Silurian System

Kankakee Formation (22' 10")
Troutman Member

Dolomite, brownish gray, buff weathering, fine-
to medium-grained, dense; in 1-4" wavy beds
with green shale partings 3' 6"

Clay, very light gray, locally white and flaky;
in pockets along a rough-surfaced bedding
plane *■

Dolomite, slightly argillaceous, light brownish
gray, mostly fine-grained; glauconite gen-
erally rare but present in a few beds ; in
1-4" beds with thin film of green clay on
bedding surfaces, except for the upper 1'
which is massive; contains 1-2" beds with
fine laminations on weathered surfaces 8' 3"

Offerman Member
Dolomite, argillaceous, very light gray, dense;
in 1-2" beds that weather lighter and smoother
than above „ 2' 6"

Drummond Member

Dolomite, glauconitic, light gray, medium-
grained, dense to vesicular; vugs as large as
6" in diameter common; massive, but weak beds
2-8" thick shown on weathered surfaces; con-
tains large chert nodules in lower 1' and
rarely in middle, a few rounded grains of
quartz sand, and large corals and stromato-
porids, mostly silicified; Platymerella is

locally abundant in lower 6" 8' 6"

Elwood Formation

Dolomite, argillaceous, light brownish gray,
very fine grained, dense to slightly vesicu-
lar; in beds %-4" thick; some beds weather
slightly shaly; contains white chert in
nodules and layers as much as 6" thick;
glauconite present but rare; corals common
in chert near top; base concealed. 10' 6"

15. ROCK CREEK CANYON SECTION

Outcrops on the west side of Rock Creek
and south of the bridge of Illinois Highway 102,
in Kankakee River State Park, 6 miles northwest
of Bourbonnais, Kankakee Co. (SW SE SW 32,
32N-11E, Kankakee Quad.).

Silurian System

Sugar Run Formation (12' 6")

Dolomite, argillaceous, silty, yellowish brown,

very fine grained, dense; in 4"-l' beds 3' 8"

Dolomite, as above, but weathers shaly k.. 7"

Dolomite, as above, but less argillaceous and

silty; a few beds slightly vesicular 4' 6"

Dolomite, brownish gray to brown, fine-grained,
slightly vesicular; in 3"-l' beds; contains
a few slightly argillaceous beds; prominent

bedding plane at base 3' 9"

Joliet Formation (40*1")
Romeo Member

Doiomite, pure, light gray, fine-grained, vesic-
ular, vuggy; in 2-8" beds 5' 8"

Dolomite, as above but light brownish gray,

less vuggy, and more massive 4' 9"

Dolomite, light brownish gray; less vesicular
and slightly less pure than above; chert

nodules 1 ' below top 7 ' 6"

Markgraf Member

Dolomite, slightly argillaceous, light brownish
gray, very fine grained, dense to slightly
vesicular; in weak 8"-l,6" beds 7' 6"

Dolomite, slightly argillaceous, light brownish
gray, very fine grained, dense to slightly
vesicular; in weak 8"-l'6" beds 7' 6"

Dolomite, slightly argillaceous, very light
gray, very fine grained, dense to slightly
vesicular; in 4"-l' beds; faint light green
and light pink mottling in lower 1' , promi-
nent bedding plane at base 4' 9"

Brandon Bridge Member

Dolomite, light greenish gray, very fine

grained, dense; contains a few coarse-grained,
pinkish lenses 6"

Dolomite, as above, but more shaly and con-
tains bright green shale partings 7"

Dolomite, gray, green and pink mottled, very
fine grained, dense; in 2-8" beds with green
shale partings; base concealed at low water
level 1' 4"

The section is cut by a fault at the south side
of the bridge; the north side of the fault is
down about 6 feet.

16. SCHWEIZER NORTH SECTION

Railroad cut at culvert-marker 43A
and outcrops along a ravine extending southeast
from the railroad, on the southeast side of
the Des Plaines Valley, 3 miles southwest of

Brandon Bridge, Will Co. (NW NW SE 36, 35N-9E,
Channahon 7.5' Quad.). Type section of Elwood
Formation and Birds Member of Wllhelmi Forma-
tion.

Silurian System

Kankakee Formation

Troutman and Offerman Members

Dolomite, light brownish gray, fine-grained,
dense to slightly vesicular; in wavy 2-6" beds
with thin green clay partings ; lower beds more

argillaceous than higher; partly covered 18'

Drummond Member

Dolomite, light brownish gray, medium-grained,
dense to vesicular, vuggy, strongly glauco-
nitic, massive; contains silicified corals.... 5' 6"

Covered 4 '

Elwood Formation

Dolomite, argillaceous, yellow-buff, very fine
grained, dense, in 1-4" beds; contains chert
in scattered nodules and layers, especially
abundant in higher beds; only a few scattered
nodules of soft chert in lower 3 feet; partly

covered 27'

Wilhelmi Formation (13' 10")
Birds Member

Dolomite, slightly argillaceous, light gray,
dense; alternating thin beds fine- and
medium-grained, giving a faintly laminated
appearance on weathered surfaces 3"

Dolomite, slightly argillaceous, brownish gray,

dense to slightly vesicular 3' 9"

Dolomite, laminated bed like 3" bed above 3"

Dolomite, fine-grained and slightly vesicular
at top; argillaceous, very fine grained, and
faintly laminated at base; in 1-2" beds.. 2'

Dolomite, laminated bed 4"

Dolomite, argillaceous, very fine grained,

slightly vesicular 2' 6"

Dolomite, laminated bed. 3"

Dolomite, argillaceous, brownish gray, very
fine grained, dense; in 2-6" beds; base
concealed 4 ' 6 "

17. SCHWEIZER WEST SECTION

Railroad cut along lower tracks on
the southeast side of the Des Plaines Valley,
2 miles north of Millsdale, Will Co. (SE SW
SE 35, 35N-9E, Channahon 7.5' Quad.). Type
section of the Wilhelmi Formation and the Birds
and Schweizer Members of the Wilhelmi Formation.

Silurian System

Wilhelmi Formation (31 '9")
Birds Member

Dolomite, slightly argillaceous and silty,

gray, weathers yellowish brown; in 2-6" beds;
a few beds are more pure, slightly vesicular,
fossiliferous ; fragments of laminated dolo-
mite like that in the lower part of the Birds
in the Schweizer North Section occur at the

top 5'

Schweizer Member

Dolomite, highly argillaceous, silty, gray,
brown weathering, very fine grained, dense;
interbedded with dolomitic shale; massive on
fresh surfaces but weathers shaly; fossils
common in a few of the more dolomitic beds 14' 6"

Shale, dolomitic, medium to dark gray, brown

weathering 5 ' 6"

Shale, as above, but contains 1-2" beds of very

argillaceous dolomite 3' 3"

Dolomite, very argillaceous and silty; in 1-2"

beds 8"

Dolomite, very argillaceous and silty, finely

laminated; upper 4" shaly 2' 4"

Dolomite, conglomeratic 6"

Ordoviaian System
Maquoketa Group
Brainard Shale
Shale, green; partly covered 15'

Fort Atkinson Limestone
Limestone, argillaceous, fine-grained, and
limestone, pure, coarse-grained, fossilifer-
ous; contains shaly partings; broken and
faulted at contact with Brainard Shale above;
base concealed 8 '

18. SOUTH ELGIN SECTION

Quarry of Fox River Stone Company
north of Fox River, 1 mile southwest of South
Elgin, Kane Co. (NE NW SW 3, 40N-8E, Geneva
7.5' Quad.), and outcrop h mile south along
the ravine south of Silver Glen School on the
west side of the Fox River (NE NE NE 9).

Silurian System

Kankakee Formation (25' 3")

Dolomite, gray and pinkish gray, fine- and
medium-grained; in 1-2" wavy beds with thin
green clay partings; contains a few scattered
chert nodules and silicif ied corals 9 ' 6"

Clay, light gray to white; makes prominent re-
entrant ; 0 to 1"

Dolomite, as above, but in thicker beds; strong
green shale partings at base 1 '

Dolomite, as above; contains white chert nod-
ules in 2" band 3" above base; strong shaly
partings at base 1 ' 8"

Dolomite, as above but thinner bedded and more
shaly; 2-4" layers of white chert 1'6" below
top; base concealed in quarry, but shale of
the Maquoketa Group was encountered in sump
only a few feet lower 9 '

The above unit with the chert layer near the
top forms the upper 9' of the exposure at
Silver Glen School, where it overlies dolo-
mite, as above but containing several 6-8"
beds and a few thin laminated beds 4'

Ordovioian System
Maquoketa Group
Brainard Shale
Shale, greenish gray; contains beds of fossili-
ferous limestone up to 6" thick; base con-
cealed 10 '

19. Thornton Quarry Section

Exposures in south part of the south
quarry of the Material Service Corporation
(General Dynamics Corp.) at Thornton, Cook Co.
(N% SE 33, 36N-14E, Calumet City 7.5' Quad.).

The north quarry and the north part of
the south quarry are located in a large Racine
reef consisting of light to medium, dark gray
mottled, medium-grained, highly vesicular, fos-
siliferous, pure dolomite that in places con-
tains asphaltum in the small pores and larger
vugs. At a depth of about 150' the quarry is
still in reef rock. Reef-flank beds dip steep-
ly away from the more massive central part of
the reef. The south quarry, separated from the
north by only a roadway, exposes reef-flank
beds in its north face, the beds dipping south
at 30-40 degrees. Farther south, the beds
flatten rapidly and contain tongues of impure
interreef deposits. Small fore-reefs occur in
the interreef deposits near the reef flank.
Only interreef deposits are exposed in the
south face. The interreef deposits are strati-
graphically higher than the reef to the north,
but they are probably contemporaneous with
higher parts of the reef, which have been erod-
ed. The section of interreef deposits exposed
in the south face follows:

Silurian System

Raoine Formation (48' 9")
Dolomite, argillaceous, brownish gray, fine-
grained, slightly vesicular; in 4"-l' beds... 5'
Dolomite, pure, gray, medium-grained, massive;

crowded with horn corals; many fossils silic-
ified; 0 to 1* 8"

Dolomite, argillaceous; contains lenticular

shale partings ; 0 to 1 ' 6"

Dolomite, shaly, light gray 8"

Dolomite, fine-grained, slightly vesicular;
contains thin shale partings near top and
chert nodules in lower part 1 ' 4"

Dolomite, argillaceous; contains lenticular

shale partings; very shaly at base 10"

Dolomite; beds are slightly vesicular and rela-
tively pure in center but grade to dense and
argillaceous at bedding planes; in beds 2"-l'
thick; silicified corals common 5'

Dolomite, cherty, fossilif erous; corals common;
transitional from well-bedded dolomite above
to nodular shaly dolomite below 2' 3"

Dolomite, argillaceous, very fine grained, con-
tains lenticular partings of green shale at
about h" intervals which give a nodular
appearance when weathered; grades laterally
to shale containing dolomite nodules; con-
tains chert in upper 2' 9' 6"

Dolomite, gray, medium-grained, vesicular; con-
tains asphaltum; weathers brown 6"

Dolomite, like 9' 6" unit above; locally grades

to nodules in shale 4 ' 3"

Shale , green , h" to 1"

Dolomite, highly varied; at places a massive
reef-type dolomite with large vugs, many con-
taining asphaltum; contains masses of brown
weathering, slightly argillaceous dolomite
and large silicified corals; changes lateral-
ly to a dolomite in which the argillaceous
rock predominates and the reef rock occurs as
boulders and lenses, forming a breccia or a
conglomerate of reef detritus; upper 6" is a
distinctive bed of fine- to medium-grained,
brown weathering, uniformly vesicular
dolomite 8 ' 6"

Dolomite, greenish gray, very argillaceous;
gradational to beds above; prominent shaly
bedding plane at base; 0 to 8"

Dolomite, greenish gray, very argillaceous,
very fine grained, dense; contains thin shaly
partings at 1-2" intervals; base concealed at
quarry floor 7 '

NORTHWESTERN ILLINOIS

20. CAMP CREEK QUARRY SECTION

Quarry near mouth of Camp Creek, 2.5
miles northeast of Savanna, Carroll Co. (SE NE
SE 25, 25N-3E, Savanna Quad.).

Silurian System

Sweeney Formation (40 ')

Dolomite, brown, fine-grained, vuggy; in 1-2'
beds with weak green clay partings; contains
many corals 10 '

Dolomite, pinkish gray, fine-grained, vesicu-
lar; in 2-3" wavy beds with green clay part-
ings; contains silicified corals; Micro-
cardinalia in lower 3 ' 8 '

Dolomite, as above, but contains fewer green
clay partings; contains chert nodules and
lenses in upper 5 ' ; Microcardinalia in upper
1' 21'

Dolomite, as above, but contains many strong

green clay partings 1 '

Blanding Formation

Dolomite, gray, fine-grained, dense to finely
vesicular; in 4-6" tight beds; contains sev-
eral 2-6" layers of dense white chert; base
concealed at quarry floor 5 '

21. FULTON QUARRY SECTION

Quarry on north side of Fulton, White-
side Co. (cen. NE SW 21, 22N-3E, Clinton NW
7.5' Quad.).

Silurian System

Marcus Formation

Dolomite, buff, very porous, soft, massive; in

5-10 ' ledges ; Pentamerus near base 25 '

Sweeney Formation (22')

Dolomite, mottled light and dark buff; less
porous than above; contains few corals; mas-
sive except for traces of 2-4" beds with
green clay partings 8 '

Dolomite, brownish gray; contains many large
vugs; corals abundant; massive, with traces
of thin bedding 9 '

Dolomite, brownish gray, gray weathering; con-
tains many corals; beds mostly 2-3" thick
with green shale partings; base concealed at
quarry floor 5 '

22. KING SECTION (IOWA)

Roadcut of U.S. Highways 52 and 67
about 1/3 mile long, extending south from the
village of King, Dubuque County, Iowa (E% SE
27, 88N-3E, Menominee 7.5' Quad.). Type sec-
tions of Tete des Morts and Mosalem Formations.

Silurian System

Sweeney Formation (Lower Hopkinton in
Iowa)
Dolomite, gray and pinkish gray, vesicular,
tight 2-4" wavy beds with green shale part-
ings; contains silicified corals 10'

Blanding Formation (Kankakee in Iowa)
(58')
Dolomite, gray, fine-grained, slightly vesicu-
lar; in 2-6" beds containing white chert nod-
ules; interbedded with 2-4" layers of white
chert; about 30% chert; contains silicified

corals 52'

Dolomite, brownish gray, very fine grained,
dense, slightly argillaceous; in smooth-sur-
faced 4-8" beds; contains a few scattered
chert nodules; "lower quarry beds" of Calvin
and Bain (1900) 6'

Tete des Morts Formation (member of
Edgewood Formation in Iowa) (24 '2")

Dolomite, pure, gray, massive; contains glauco-
nite, silicified corals, and thin lenses of
white chert „ 6 '

Dolomite, as above, but contains chert nodules. 1' 2"

Chert; nearly continuous bed of chert nodules.. 2"

Dolomite , as above 10"

Dolomite, as above, massive, not cherty; strong

bedding reentrant at base 16'

Mosalem Formation (member of Edgewood
Formation in Iowa) (61 '6")

Dolomite, similar to above, but has weak bed-
ding 1' 6"

Dolomite, gray, slightly vesicular; contains
thin argillaceous streaks and laminations;
bedding stronger than above; strong bedding
reentrant at base 1 ' 6"

Dolomite, slightly argillaceous; in 2-6" beds;

contains scattered chert nodules 6 ' 6"

Dolomite, argillaceous, dense; in 2-6" beds;

contains many bands of chert nodules 16'

Dolomite, very argillaceous; interbedded with
shaly dolomite; in 6"-l' beds; base con-
cealed 36 '

23. LANARK NORTHWEST SECTION

Quarry 4 miles northwest of Lanark,
Carroll Co. (SW SW SW 14, 25N-5E, Mt. Carroll
Quad . ) .

Silurian System

Blanding Formation (31')

Dolomite, gray, slightly pinkish, fine-grained;
dense but has vesicular areas; in 3-5" wavy
beds with green partings; contains many 2-5"
layers and lenses of white chert (25% chert). 20'

Dolomite, as above; contains only a few discon-
tinuous chert layers 7 '

Dolomite, as above; contains one chert layer at

base and chert nodules locally at top 4'

Mosalem Formation

Dolomite, argillaceous, dense, brown; base con-
cealed at quarry floor 2 '

24. LOST MOUND SECTION

Quarry at top of Mississippi River
bluff, 1.3 miles southeast of Whitton and .7
mile northwest of Lost Mound, Jo Daviess Co.
(cen. NW SW 28, 26N-2E, Green Island 7.5'
Quad . ) .

Silurian System

Sweeney Formation

Dolomite, gray, dense to finely vesicular, mas-
sive except for tight 1-3" wavy bedding
planes with thin green clay partings; con-
tains chert lenses and nodules in a 3' zone

20' above the base; corals common 25'

Blanding Formation (41')

Dolomite, brownish gray, mostly dense; in 2-6"
beds; contains 1-3" chert layers at about 6"
intervals, except in the lower 2 '6", where
there is little chert; good bedding reentrant
at base 35 '

Dolomite, brownish gray, dense; in 1-6" wavy
beds; greenish bedding surfaces; lenses of
chert 1' below top; smooth sharp bedding

plane at base 6'

Mosalem Formation (30 '1")

Dolomite, dense, laminated 2"

Dolomite, gray; in 1-3" beds; contains lenses
of chert locally 1' below top; 6" vuggy bed
3' below top; 0-1" calcarenitic porous dolo-
mite at base 5' 3"

Dolomite, pure, red-brown, vuggy; contains horn

corals; 6" to 8'

Dolomite, argillaceous, medium to dark gray,
gray weathering; h-1" beds; interbedded
with purer, very fine grained, dense, brown-
weathering dolomite in lenticular 1-2" beds
and lenses; contains a few thin chert lenses;
massive face except for strong reentrant 3'
below top 15 '

Dolomite, as above, but more argillaceous;

makes reentrant 1 '

Shale, dolomitic, dark brown, thinly laminated. 6'

Dolomite, brown, massive; slightly argillaceous
but has strong argillaceous streaking in mid-
dle and at base; contains fine crinoidal
debris in upper 3" 1 ' 6'

Dolomite, very argillaceous, weathers shaly;

smooth surface on top 3 *

Shale, dolomitic, black specked 1*

Covered to quarry floor (Ordovician Brainard
Shale is exposed in gully 12' below quarry
floor) 2 '

25. PALISADES PARK HIGH-FACE SECTION

Bluff of Mississippi River in Missis-
sippi Palisades State Park at parking area, .95
miles north of Savanna-Sabula bridge, Carroll
Co. (SW SE NE 33, 25N-3E, Savanna 7.5' Quad.).
Type section of Sweeney Formation.

Silurian System

Marcus and Raaine Formations

Dolomite; upper part inaccessible; lower part
is buff, vesicular, massive, and Pentamerus

is abundant in lower 5 ' 80 '

Sweeney Formation

Dolomite, gray and pinkish gray, fine- to medi-
um-grained; dense and irregular vesicular
areas; in wavy, tight beds, mostly 2-4"
thick, with green clay partings, except in
upper 10', which has 1-5' beds with very weak

partings 55 '

Blanding Formation

Dolomite, light gray to light tan, mostly fine-
grained, dense; in 2-6" beds; contains many
bands of white chert and beds with chert nod-
ules (20% chert); base concealed about 5'
above level of railroad 30 '

26. PALISADES PARK MAIN ENTRANCE SECTION

Mississippi River bluff, above large
blocks containing Pentamerus , .2 mile north of
main entrance to Mississippi Palisades State
Park, Carroll Co. (SW NE SE 28, 25N-3E, Black-
hawk 7.5' Quad.).

Silurian System

Racine and Marcus Formations

Dolomite, massive; mostly inaccessible for de-
tailed study; Pentamerus abundant at base.... 82'
Sweeney Formation

Dolomite, gray, fine-grained; massive-appearing
but has weak, thin, wavy bedding with green
clay partings; contains many silicified

corals 45 '

Blanding Formation

Dolomite, light brownish gray, fine- to medium-
grained, slightly argillaceous, dense to
finely vesicular; in 2-4" beds; contains many
2-6" layers, lenses, and nodules of white
chert; corals common 28'

Dolomite, as above but thicker bedded and con-
tains chert only in scattered nodules in up-
per 2 ' and in band 3 ' below top 6 '

Mo sal em Formation

Strongly pitted, iron-stained corrosion sur-
face.

Dolomite, argillaceous, brown, mottled greenish
gray; massive except for a few tight wavy
bedding planes and weak laminations; upper 1'
relatively pure and vuggy, with fucoidal

mottling; lower 3" is fossilif erous, vuggy,
calcarenitic dolomite; strong bedding reen-
trant at base 7 ' 3"

Dolomite, brown, fine-grained; massive ledge
but strongly laminated; upper 1' irregularly
vuggy; upper 3" contains many casts of fos-
sils; 0-1" lens of white chert at base; lo-
cally thins sharply to 6" 2 '

Dolomite, argillaceous, light brown and green,
laminated; thins out at margin of channel cut
into underlying shale 8"

Unconformity

Ordovician System
Maquoketa Group

Brainard Shale
Dolomite, very argillaceous, soft, light green-
ish gray ; 1 ' 8" to 3 '

Shale, dolomitic, greenish gray 2"

Dolomite, very argillaceous as above; base con-
cealed 3"

27. PALISADES PARK NORTH SECTION

Mississippi River bluff .1 mile north-
west of intersection of Illinois Highway 84
with roads to campground and boat ramp, Missis-
sippi Palisades State Park, Carroll Co. (NW NE
SW 21, 25N-3E, Blackhawk 7.5' Quad.).

Silurian System

Marcus Formation

Dolomite, brown, highly vesicular, massive;
contains a few chert nodules in lower 5';
Pentamerus very abundant in lower 3', which

has a prominent bedding break at top 35'

Sweeney Formation (54')

Dolomite, gray, thin-bedded; contains chert
nodules in upper 3', wavy green clay part-
ings, and many corals, some silicified 22'

Dolomite; somewhat thinner bedded than above
and below; contains several thin beds of
white chert; Pentamerus is especially common
in the bluff about .3 mile north 3'

Dolomite, as above the 3' cherty zone but with-
out chert 29

Blanding Formation

Dolomite, similar to above but contains beds,
lenses, and nodules of white chert; base con-
cealed about 40 feet above the highway 4'

28. PALISADES PARK OLD QUARRY SECTION

Abandoned and largely overgrown quarry
in Mississippi Palisades State Park, just north
of the south boundary, .5 mile north of Savan-
na-Sabula bridge, Carroll Co. (SW SE SE 33.
25N-3E, Savanna 7.5' Quad.). Type section of
Marcus Formation.

Silurian System

Racine Formation

Dolomite, pure, light buff to gray, medium-
grained, dense to highly vesicular; some beds
have large vugs; in 6"-2' beds; some beds

brecciated 25' 2"

Marcus Formation

Dolomite, buff, uniformly highly vesicular,
massive; Pentamerus abundant in lower 6';
corals common in lower 2', which is transi-
tional to the Sweeney Formation below; top is

12 ' above upper bench 40 ' 4"

Sweeney Formation

Dolomite, light brownish to pinkish gray, most-
ly fine-grained, dense to slightly vesicular;
in wavy 2-6" beds with green clay partings;
silicified corals abundant; base concealed. .. 23' 10"

29. ROYAL PRINCESS SECTION

Upper part of Mississippi River bluff
.5 mile northwest of Royal Princess Mine and

2.8 miles north of Blanding, Jo Daviess Co. (SE
SW SE 21, 27N-1E, Bellevue 7.5' Quad.). Type
section of the Blanding Formation.

Silurian System

Sweeney Formation

Dolomite, gray; appears massive but has weak

wavy 1-4" beds; contains silicified corals... 23'
Blanding Formation (51 ' 6")

Dolomite, light brownish gray, very slightly
argillaceous, dense to finely vesicular; in
beds largely 6-10" thick; contains white
chert in 1-8" layers and in scattered nod-
ules; in part nearly 50% chert 48'

Dolomite, as above but without chert 3' 6"

Tete des Morts Formation (20 '11")

Smooth surface

Dolomite, gray, fine- to medium-grained, glau-
conitic, massive; contains silicified corals
and thin streaks of white chert; "upper mas-
sive unit" 5' 7"

Dolomite, similar to above but slightly denser
and in weak 1-3" beds; contains a few chert
nodules ; "middle cherty unit" 3 '

Dolomite, gray, fine- to medium-grained, mas-
sive; contains a few chert nodules 1' above

base ; "lower massive unit" 12 ' 4"

Mosalem Formation (50' 7")

Dolomite, similar to above, but in 1-4" beds
with weak shaly partings; contains chert nod-
ules in bands at top and 1' and 1'6" below
top; strong shaly bedding break at base 8' 3"

Dolomite, as above, but faintly laminated; up-
per surface deeply pitted 3"

Dolomite, argillaceous, dense, light yellow-
brown; in 2"-l' beds with very thin shaly
partings; contains chert nodules in bands at
3', 5', and 5 '8" below top; 3" laminated bed
2' 6" below top; 3" pure bed with fossil
debris at base 5 ' 10"

Dolomite, similar to above, but not cherty;

weathers to 1-2" beds, lower 6" shaly 6'

Dolomite, pure, calcarenitic, vesicular; con-
tains fossil debris 3"

Dolomite, argillaceous at top grading to very
argillaceous at bottom, gray, dense; in h-3"
beds with strong shaly partings; contains a
few white chert nodules in upper 4' and one
band of nodules 2' 3" below top; largely non-
f ossilif erous ; base concealed 30 '

Diamond-drill core about 2 miles northeast
shows 51' of cherty argillaceous dolomite
overlying 21' of noncherty, very argillaceous
dolomite overlying shale of the Maquoketa
Group in the interval below the 3" calcareni-
tic dolomite.

30. SCHAPVILLE SOUTHWEST SECTION

Roadcuts 1.5 miles southwest of Schap-
ville, Jo Daviess Co. (NE NE 1, 27N-2E, and SE
SE 36, 28N-2E, Elizabeth 7.5* Quad.).

Silurian System

Tete des Morts Formation

Dolomite, pure, brownish gray, vuggy, massive.. 18'
Mosalem Formation (80')

Dolomite, slightly argillaceous, brownish gray,
dense to slightly vesicular; in 6"-l'6" beds
with strong shaly partings; contains bands of
chert nodules in upper 3'; a few 1-2" beds of
brown, medium-grained, calcarenitic, pure
dolomite 15'

Dolomite, brown, dense, argillaceous; in thin-
ner beds and with stronger shale partings
than above 10 '

Covered to exposures in roadcut in hill to

north, 30' to 40'

Dolomite, very argillaceous, brown, nonfossili-
ferous; in weak, thin, wavy beds 15'

Ordovioian System
Maquoketa Group
Brainard Shale

Shale, green; 6" to 1'

Limestone, coarse, calcarenitic, brown; fossil
debris abundant on upper surface, which has
mega-ripples ; 2" to

Shale, green; contains 1-2" beds of very argil-
laceous dolomite at 1-3' intervals and a few
thin beds of very f ossilif erous limestone;
base concealed 40 '

31. SMALLPOX CREEK EAST SECTION

Roadcut of U.S. Highway 20 at top of
hill east of Smallpox Creek, 6 miles east of
Galena, Jo Daviess Co. (NE NE NW 6, 27N-2E,
Hanover 7.5' Quad . ) .

Silurian System

Sweeney Formation (33 '9")

Dolomite, slightly argillaceous, brown, dense to
slightly vesicular; in 2-6" beds; contains
several bands of chert nodules 4 '

Dolomite, pure, brown, vesicular; in one bed;

Pentamerus common ; 8" to 1 '

Dolomite, gray, slightly vesicular; in weak 2-6"
beds; contains several bands of chert nodules. 2'

Dolomite, vesicular; Pentamerus common 1'

Dolomite, slightly argillaceous, dense; in 2-8"
beds; contains a few chert nodules; Pentamerus
in upper 1 ' 3 '

Dolomite, argillaceous, dense; contains green
shale partings to as much as V

Dolomite, argillaceous, dense; contains 2-4"
bands of chert nodules; dense smooth-surfaced
beds at top and bottom 3 '

Shaly reentrant with lenses of chert

Dolomite, gray to pinkish gray, fine-grained,
massive but has thin green shaly partings and
weathers thin bedded; contains many corals 18'

Covered ' 5 '

Blanding Formation

Dolomite, very cherty; base concealed 5'

32. STOCKTON SOUTHEAST SECTION

Quarry in north end of ridge, south
side of road, 2 miles southeast of Stockton, Jo
Daviess Co. (SW SE SW 17, 27N-5E, Lena Quad.).

Silurian System

Blanding Dolomite

Dolomite, slightly argillaceous, light brownish
gray; in 2-6" beds; contains 2-4" beds of
white chert, except for the lower 2 '6", which

is without chert 7 '

Tete des Morts Formation

Dolomite, light gray to brown, fine-grained;
varied low to medium vesicular ity; massive,
except on weathered surfaces, where faint 2-4"
beds show; top is pitted smooth surface; glau-
conite and thin white chert lenses in upper
ledge; strong shaly reentrant with thin lenses

of white clay at base 13'

Mosalem Formation

Dolomite, slightly argillaceous, yellow-brown,

slightly vesicular , massive 4 '

Dolomite, argillaceous, yellow-brown, very fine
grained; massive-appearing but has thin shaly
partings at *s-l" intervals; base concealed at
road level 18 '

33. WHITTON NORTHEAST SECTION

North end of long cliff in Mississippi
River bluffs, 1/3 mile northeast of Whitton, Jo
Daviess Co. (NW NW SE 20, 26N-2E, Green Island
7.5' Quad.).

10'

Silurian System

Sweeney Formation

Dolomite, dense, fine-grained; in 1-6" tight,
wavy beds with thin green clay partings; mas-
sive-appearing ledge; contains many silici-

f ied corals 15 '

Blanding Formation

Dolomite, dense to finely vesicular, light
brownish gray; contains chert in nodules and
in layers as much as 8" thick (about 25%
chert) 35 '

Dolomite, shaly; forms a strong reentrant

Mosalem Formation (9'1")

Dolomite, slightly argillaceous, gray, thin-
bedded; contains thin shaly partings

Dolomite, pure, calcarenitic, brown; contains
fossil debris and lenses of chert; 0 to

Dolomite, as above, but thin-bedded

Dolomite, as above, but not cherty

Chert , laminated ; 0 to

Dolomite, as above; strong shaly parting with
pebbles of laminated chert and silicified
corals at base I1

Dolomite, argillaceous, yellow-brown; contains
shaly partings at ^-3" intervals; many spher-
ical pits in basal bed 3 '

Dolomite, argillaceous, brown, massive but has

many thin shaly partings; base concealed 2'

34. WINSTON NORTH SECTION

Quarry 1 mile north of east end of the
Winston tunnel and 2.5 miles northwest of Rod-
den, Jo Daviess Co. (SE SW NE 11, 27N-1E, Han-
over 7.5' Quad . ) .

10"

Silurian System

Blanding Formation

Dolomite, very cherty, weathered 5'

Dolomite, light gray, dense; in 2-6" beds; no

chert 8 *

Tete des Morts Formation (22 '2")

Dolomite, brownish gray, vuggy, glauconitic,
massive; locally contains a few chert nodules
1' above base; "upper massive unit" 7'

Dolomite, light gray, argillaceous; contains
persistent band of chert nodules; "middle
cherty unit" 1 '

Dolomite, brownish gray, massive, vuggy; con-
tains chert nodules in five bands in middle
of face but none at sides; "lower massive
unit" 4 '

Dolomite, as above but no chert 10'

Strong bedding reentrant with lenses of chert.. 21
Mosalem Formation (14' 6")

Dolomite, brown, slightly argillaceous; less
vuggy than above; massive on fresh face; con-
tains bands of chert nodules 1 ' , 2 ' , and 3 '
below top 6 '

Dolomite, brown, argillaceous, dense; contains
distinct, wavy red-brown shale partings but
is massive on quarry face; chert nodules at
top and in middle 3 ' 61

Dolomite, well-bedded; in 4-8" beds; contains
chert in nodules and in a persistent band
1'6" below top; a few 1-2" brown, pure, cal-
carenitic, and weakly laminated beds; base
concealed at quarry floor 5 '

Illinois State Geological Survey Circular 479

55 p., 10 figs., 37 geol. sees., 2800 cop., 1973

Urbana, Illinois 61801

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25

CIRCULAR 479

ILLINOIS STATE GEOLOGICAL SURVEY

URBANA, IL 61801

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Full text of "Rock stratigraphy of the Silurian system in northeastern and northwestern Illinois"

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