Farmdalian lake deposits and faunas in northern Illinois

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STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

Farmdalian Lake Deposits and
Faunas in Northern Illinois

H.B.Willman
A. Byron Leonard
John C. Frye

ILLINOIS STATE GEOLOGICAL SURVEY

John C. Frye, Chief Urbana, IL 61801

CIRCULAR 467 1971

3 3051

URBANA

Farmdalian Lake Deposits and
Faunas in Northern Illinois

H. B. Willman, A. Byron Leonard, and John C. Frye

ABSTRACT

A recent exposure at Morris, Illinois, reveals that
an extensive lake was present in the upper Illinois Valley
during Farmdalian time. The exposure and the previously
described Wedron Section are the basis for definition of
a new unit, the Peddicord Formation. The formation is a
massive gray silt overlain by pink silt.

The molluscan fauna from the Peddicord Formation
at Morris significantly expands knowledge of Farmdalian
aquatic faunas, previously known only from Wedron.
These faunas indicate the presence of a lake of cool per-
manent water and confirm the fact that the Farmdalian,
although a time of extensive glacial withdrawal, was
characterized by a cool, moist climate.

INTRODUCTION

Recent excavations north of Morris, Illinois, along an erosional escarp-
ment formed by the Chicago Outlet River have furnished significant new informa-
tion on the Wisconsinan stratigraphy and paleontology of the region. The new ex-
posure shows the presence of lake deposits that correlate with similar deposits
previously known only from the vicinity of Wedron (fig. 1) . The stratigraphic se-
quence and a radiocarbon date indicate the deposits are of Farmdalian age. The
two localities are approximately 20 miles apart, and the similarity of the deposits
suggests that an extensive lake existed in this part of the Illinois Valley region
during Farmdalian time. Farmdalian aquatic molluscan faunas previously have
been known only from the Wedron locality (Leonard and Frye, 1960), and the
abundant fauna from the new exposure significantly increases knowledge of Farm-
dalian faunas. At the Morris North Section, described from the new exposure, a
molluscan fauna obtained from the Robein Silt that overlies the lake deposits af-
fords an opportunity for comparison of faunas from two different environments.
The lake deposits in the Wedron and Morris North Sections are included in a new
formation, herein named the Peddicord Formation.

1

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 467

R3E

R4E

R 5E

R6E

R7E

R8E

;-j Marseilles Morainic System
Glacial lake plain

•- ~ Cryder Lake shoreline of Chicago Outlet River
Illinois Valley bottomland

Fig. 1 - Location of Morris North and Wedron Sections.

24,990±280(ISGS-6I)B.P

(bed I)

snail fauna

base of exposure

Fig. 2 - Diagram of Morris North Section.

FARMDALIAN LAKE DEPOSITS AND FAUNAS 3

STRATIGRAPHY

The new roadcut 1 .5 miles north of Morris and 100 yards west of the junc-
tion of highways U. S. 6 and Illinois 47, described below as the Morris North
Section, reveals a condensed section of many of the glacial, glacial-fluvial, and
lacustrine deposits in the Morris region, including five formations of Wisconsinan
age (fig. 2) . The base of the drift is not exposed, but, as Pennsylvanian bedrock
occurs in coal strip mines near by, it probably is not more than 10 or 15 feet be-
low the base of the Morris North Section.

Morris North Section

Morris North Section, in fresh excavations for access road in SE£ NE^ NE£,
Sec. 33, T. 3^ N.f R. 7 E., Grundy County, Illinois (1970, 1971).
Sample numbers preceded by "P" are the numbers used in the
Illinois State Geological Survey collection.

Thickness
(feet)

Pleistocene Series
Wisconsinan Stage

Woodfordian Substage
Henry Formation

8. Sand, gravel, and silt; silt at top; tan, loose; top at

level of Cryder Lake beach ridge of Lake Chicago outlet . . 2.0

Wedron Formation

Yorkville Till Member

7. Till, calcareous, gray, clayey, silty, massive; layer of
pebble concentrate at top derived from erosion of the
till (P-72W 4-.0

Tiskilwa Till Member

6. Till, calcareous, pink-tan, sandy, silty, massive;

boulder pavement at top contains boulders up to 2 feet

in diameter; some of the boulders are weathered and

readily disaggregate, and therefore their possible

origin as a concentrate by an early Lake Chicago outlet

implies subsequent weathering; till pinches out to the

east and boulder pavement rests on underlying sand bed

(P-72^3 till); maximum thickness M- - 0

5. Sand, fine, and silt, bedded, calcareous, tan; present

throughout the exposure (P-724-2); maximum thickness .... 2.0

Morton Loess

4. Silt, calcareous, tan-gray, streaked, vesicular with
thin clay skins in vesicles; occurs only at S¥ part of
exposure and pinches out to the NE (P-7263 from middle
part); maximum thickness 1.0

Farmdalian Substage
Robein Silt

3. Silt, calcareous, yellow-gray, streaked with organic
material, cryoturbated; contains a few fragmentary
mollusk shells (P-7262); pinches out to the NE; maximum
thickness 1.0

ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 467

Thickness
(feet)

Peddicord Formation

2. Silt, clayey, calcareous, massive, pink-tan (P-7261

upper, P-7241 middle); contains abundant shells 'of mol-
lusks in bottom 1 foot; basal part contains fine to very
fine sand; overlies gray silt and clay with a sharp con-
tact; confined to the SW part of the exposure; truncated
to the NE by the overlying sand bed (unit 5 of this sec-
tion) which there rests directly on the gray silt and
clay below (unit 1); maximum thickness 5.0

1. Silt, clayey, calcareous, gray, massive; a few small peb-
bles sparsely dispersed throughout; contains some frag-
ments of wood, one piece of which from the middle of the
unit was dated at 24,990 ± 280 (ISGS-6D radiocarbon years
B.P.; in the area where this unit is overlain by unit 2,
fossil mollusk shells occur in the top few inches (P-7240
from middle part); maximum thickness exposed 6.0

Total thickness 25.0

Peddicord Formation

The basal gray silt and the overlying pink silt in the Morris North Section
(beds 1 and 2) are almost identical in physical characteristics and stratigraphic
position to beds 1 and 2 in the Wedron Section (Farmdale Silt of Leonard and Frye,
1960, p. 32; Robein Silt of Willman and Frye, 1970). They differ in that more
wood fragments are found in the Wedron Section and a molluscan fauna is present
in both the top of bed 1 and the base of bed 2 at Morris, whereas it occurs only in
bed 1 at Wedron .

Because of their similarity in lithology, the two exposures are included
in a new formation, herein named the Peddicord Formation after the Peddicord
School shown on the Ottawa 15-minute quadrangle map and located 1 mile east
of the Wedron Section. The Wedron Section (Willman and Frye, 19 70) is desig-
nated the type section. As the deposits are lacustrine, the lake in which they
were deposited will be referred to as Lake Peddicord.

The top of the Peddicord Formation is approximately 535 feet above sea
level in both the Morris and Wedron exposures. The formation was deposited in
a lake that occupied the Ticona Valley (Willman, 1940; Willman and Payne, 1942),
a deep bedrock valley that in early Wisconsinan time was only partially filled
with Illinoian and older glacial deposits. As the Ticona Valley and its major
tributaries are buried by Woodfordian drift and are crossed at sharp angles by
the much younger Illinois Valley, there are few exposures of the Peddicord For-
mation. Much of the area covered by Lake Peddicord is deeply buried by the
drift of the Marseilles Moraine (fig. 1) .

At Wedron the Peddicord Formation is exposed along the Fox Valley where
the valley crosses a tributary of the buried Ticona Valley (Willman and Payne,
1942, p. 150). The principal exposure is in the overburden at the silica sand
pit of the Wedron Silica Company. The formation is largely confined to the south-
ern part of the exposure. The bedrock valley was partly re- excavated during late
Sangamonian or early Wisconsinan time, and the lacustrine beds occur in a steep-
sided channel that cuts into the St. Peter Sandstone.

At Morris the Peddicord Formation is exposed by a shallow roadcut and the
adjacent excavation in the sloping face of the Cryder Lake shoreline. The shore-

FARMDALIAN LAKE DEPOSITS AND FAUNAS 5

line is an erosional escarpment about 25 feet high that was formed by the en-
trenchment of the Chicago Outlet River into the glacial deposits of the Morris
Basin. Although deeply entrenched in bedrock both above and below the Morris
Basin, the river eroded a wide bench in the glacial deposits of the basin and
filled a lake-like expanse about 6 miles wide (Culver, 1922; Willman, 1971).
Erosion of the escarpment is correlated with the deepening, by about 20 feet, of
the Lake Chicago outlet at Chicago down to the Calumet shoreline. The escarp-
ment, therefore, is believed to have been formed between 12, 000 and 13, 000
years before the present (B.P.).

Lacustrine sediments below the distinctive pink till of the Bloomington
Morainic System (classified as the Tiskilwa Member of the Wedron Formation) are
exposed at many places along the Illinois Valley from Morris to La Salle. These
deposits, collectively, were formerly called "Kickapoo beds" from exposures along
North Kickapoo Creek east of Marseilles (Sauer, 1916) . Later they were referred
to as "Lake Kickapoo deposits" and were included in the Shelbyville Drift (Will-
man and Payne, 1942) . Lake Kickapoo was interpreted as having been formed in
the Ticona Valley after the Shelbyville Moraine dammed the valley at Peoria. It
therefore preceded Lake Illinois (Leighton, 1928, and in Fisher, 1925; Willman
and Payne, 1942), which was formed when the Illinois Valley was dammed by the
Bloomington Moraine (Leighton, 1928) or by the younger Dover Moraine (Willman
and Frye, 1970). Lake Illinois is represented by numerous gravel deltas overly-
ing the Bloomington and younger drifts.

In most exposures the "Lake Kickapoo deposits" consist of silty clay in
beds 1 to 2 inches thick that are separated by thin laminae of coarse silt or fine
sand. However, it was noted that at Wedron the lake sediments included in the
"Lake Kickapoo deposits" differ in being massive and in containing large frag-
ments of wood (Willman and Payne, 1942, fig. 93).

Restudy of the Wisconsinan part of the Wedron Section (Leonard and Frye,
1960, p. 31, 32) revealed a gray till below the pink till of the Bloomington Drift
and above the lake silts, which suggested that the silts were older than Shelby-
ville Drift. This was confirmed by radiocarbon dates of the wood, 24, 000 ±700
(W-79) years B.P. from the upper pink silt and 2 6, 800 ±700 (W-871) years B.P.
from the lower gray silt. The lake sediments, therefore, were included in the
Farmdalian Substage and assigned to the Farmdale Silt.

When the name "Farmdale Silt" was replaced by the term "Robein Silt" to
correct a duplication in nomenclature (Willman and Frye, 1970), the deposits at
Wedron were included in the Robein Silt. As the Robein Silt is dominantly silt
rich in organic material and in many places contains beds of peat, the formation
is made more uniform by differentiating the distinctive lacustrine sediments as
the Peddicord Formation.

The laminated clays and silts in the "Lake Kickapoo deposits" occur at a
stratigraphically higher position and they are retained in the Wedron Formation.
It is possible, however, that other sediments included in the "Lake Kickapoo de-
posits, " particularly in the areas near Wedron and Seneca, will be found to belong
instead to the Peddicord Formation.

The Peddicord Formation is Farmdalian in age, as shown by the radiocar-
bon dates previously mentioned and by its stratigraphic position. At Morris the
formation is overlain locally by silt rich in organic material that has been as-
signed to the Farmdalian Robein Silt, but the oldest overlying Woodfordian till is
the pink till of the Bloomington Drift. The base of the Peddicord Formation is not
exposed at Morris.

At Wedron the Peddicord Formation is overlain by sand, gravel, and till
of the Lee Center Till Member of the Wedron Formation (Willman and Frye, 1970,

6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 467

p. 191), formerly called Shelbyville Drift. The Peddicord Formation overlies the
Sangamon Soil on Illinoian drift (Willman and Payne, 1942, p. 148, 307), except
where the drift is eroded and it rests directly on the Ordovician St. Peter Sand-
stone.

The stratigraphic relations of the Peddicord Formation, therefore, place it
as Wisconsinan in age but older than the earliest Woodfordian glaciation. The
nearest Wisconsinan drift definitely identified as Altonian (pre-Farmdalian) in age,
and thus older than the Peddicord Formation, is about 18 miles north of the Morris
North Section. However, Lake Peddicord could have existed during later Altonian
and Farmdalian time, and it might have been formed by Altonian glaciers reaching
the Ticona Valley farther west and depositing drift that blocked the drainage.

Robein Silt

The Robein Silt is represented in the Morris North Section by a truncated
bed of yellow- gray silt, as much as 1 foot thick, that contains dark gray to black
streaks rich in organic material (bed 3) . The organic material is not abundant
enough to be radiocarbon dated. The Robein Silt is rarely found this far back from
the front of Woodfordian moraines, but the deposit is correlated with the Robein
on the basis of its organic content and stratigraphic position. It contains mol-
lusk shells that indicate an environment different from that of the Peddicord For-
mation, as will be described in the discussion of the paleontology. The tight
folding of the organic streaks in the silt suggests that permafrost conditions ex-
isted during the advance of the Woodfordian glaciers.

Morton Loess

The truncated 1-foot thick bed of tan- gray, loess- like silt (bed 4) over-
lying the Robein Silt in the western part of the exposure strongly resembles the
Morton Loess and is correlated with it on the basis of stratigraphic position.
The Morton Loess is composed of silt blown from the outwash of the advancing
Woodfordian glaciers.

Wedron Formation

Tiskilwa Till Member

The Tiskilwa Till Member (beds 5 and 6) consists of 1 to 2 feet of sand
overlain by the pink sandy till characteristic of the member and of the Blooming-
ton Drift throughout most of central northern Illinois. The till is as much as 4
feet thick, but it is truncated by the overlying Yorkville Till Member in the east-
em part of the exposure. The top of the till is at an elevation of about 540 feet,
which is 50 or 60 feet lower than it is in many exposures farther east along the
Illinois Valley where the till forms a persistent layer about 20 feet thick (Willman
and Payne, 1942).

The upper surface of the till has a prominent boulder pavement, with boul-
ders as much as 2 feet in maximum diameter. Many of the boulders of igneous
and metamorphic rocks are so weathered that they disintegrate into sand upon ex-
posure. As boulders of this size are not common in the Tiskilwa Till, they may
represent a lag concentrate from the Maiden Till Member (Willman and Frye, 1970),
which is missing in this exposure. On the other hand, the weathered character of

FARMDALIAN LAKE DEPOSITS AND FAUNAS 7

the boulders may indicate exposure during the major ice withdrawal that followed
deposition of the Tiskilwa Till Member and before the readvance that deposited
the Maiden Till Member.

The absence of the Lee Center Till Member, the earliest Woodfordian till,
which normally occurs below the Tiskilwa Till Member, may result from erosion,
but, as deposits definitely related to the Lee Center Till Member have not been
identified east of this area, the Woodfordian ice front may not have retreated this
far before the Bloomington readvance. If this is true, the sand and silt deposits
of bed 5 may be outwash from the first advance of Woodfordian glaciers.

Yovkville Till Member

The upper gray clayey till (bed 7) of the Morris North Section is similar in
lithology to the Yorkville Till Member, which is the surface till of the Marseilles
Morainic System and of the area east of the moraine (fig. 1) . Although a pebble
concentrate shows the upper surface of the till at the Morris North Section has
been eroded, the surface of the till is about 225 feet lower than it is on the crest
of the moraine north of Marseilles, where the till is at least 150 feet thick.

Henry Formation

Two feet of sand and gravel with a silt layer at the top (bed 8) forms the top
of the Morris North Section. It is a remnant of the beach deposit that commonly
occurs along the top of the escarpment at the Cryder Lake shoreline (Culver, 1922) .
It is a surficial sand and gravel deposit assigned to the Henry Formation.

PALEONTOLOGY

The molluscan fauna of the Peddicord Formation at the Morris North Sec-
tion was collected during October 1970 and was handpicked from the outcrop;
bulk samples were taken in June 1971, and the fauna of the overlying Robein Silt
also was sampled in bulk in June. The faunas from the Peddicord Formation and
the overlying Robein Silt at the Morris North Section are described. The two
faunas are contrasted and compared with the fauna from the Farmdalian lake de-
posits (the Peddicord Formation) at the Wedron Section, which previously was the
only lacustrine fauna of Farmdalian age described from Illinois (Leonard and Frye,
1960) . The specimens from the Peddicord Formation and the Robein Silt are in the
collections of the Illinois State Geological Survey, cataloged under the prefix 55P

Peddicord Formation

The fossil assemblage recovered from the Peddicord Formation at the Mor-
ris North Section is described below.

Amnieola gelida Baker. This branchiate gastropod, assumed to be extinct,
is known as a fossil from Illinois and Ohio. Baker described this gastropod (1921,
p. 22) as a subspecies of Amnieola lustviea, but he later raised it to specific rank
(1928, p. 110). The type locality is given as "Morris, Grundy County, Illinois,
in Pleistocene deposits, " and it was included in a list prepared by Baker and re-
ported by Culver (1922, p. 159) from a former strip mine called "Bells Stripping"

8 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 467

1 mile southeast of the Morris North Section. Baker somewhat doubtfully records
the species from Wisconsin, for after saying "Wisconsin records: None, " he
goes on to say "Wisconsin distribution: Known only from Spring Lake, east of
Green Lake, Green Lake Co., " which would indicate that the species was living
there at that time. La Rocque lists the species from Ohio but mentions no speci-
fic locality or stratigraphic horizon.

From the known physiological requirements of the several members of the
genus, and from the associates of Amnioola gelida, it may be safely inferred that
this species required a permanent, relatively cool pond or lake in which to live.
At the Morris North locality the deduced habitat of a cool lake of relative perma-
nence fits the inferred requirements of A. gelida.

Although not as numerous as Amnioola precursor, A. gelida is relatively
common in the sediments studied; it may be considered as having been a success-
ful species at the time the lake was in existence.

Although the types of the species came from Morris, the specimens we
collected there from the Peddicord Formation are shorter and more slender than
those described by Baker. If the type locality is "Bells Stripping, " it is in de-
posits younger than the Peddicord Formation.

Amnioola precursor Baker. According to Baker (1928, p. 116), Amnioola
precursor is an extinct species of Amnioola, known only from Illinois and Wis-
consin. It has always been found in associations of mollusks that require perma-
nent cool water. The type locality is at Green Lake, Wisconsin, below 7 meters
of mud. Baker (1928, p. 117) considered A. precursor the ancestor of A. walkeri
Pilsbry, a living species widely distributed in the Upper St. Lawrence and Upper
Mississippi drainages. Although A. precursor is the most numerous of the spe-
cies recovered at the Morris North locality, it had not been found previously at
other localities reported by Leonard and Frye (1960) or by Leonard, Frye, and
Johnson (1971) .

Caryohium canadense Clapp. Described originally as a subspecies of
Carychium exile, C. canadense is distinctive enough to warrant specific rank.
It is a terrestrial species of northern affinities, and, according to Pilsbry, it re-
places C. exile to the north, where it is distributed in "the Canadian Zone, Maine
and Ontario to Michigan and Manitoba" (Pilsbry, 1948, p. 1059). C. canadense
is, according to its discoverer, "always some distance from water." The two ex-
amples recovered from the Morris North locality must have drifted on the water
as dead shells until they sank to become incorporated in the silts where they were
found. They attest to a cooler ambiance than exists there at present.

Gyraulus sp. A single juvenile shell, not clearly recognizable as to
species, represents this genus of aquatic planorbid pulmonate gastropods. These
animals live in ponds, lakes, and sluggish streams, where they are most frequent-
ly associated with aquatic vegetation. The habitat at Morris, therefore, must
have included a pond or lake.

Ostracoda. Numerous, as yet unidentified valves of several species of
ostracodes occur in the Peddicord Formation at the Morris North locality. These
calcareous valves, which protected the soft bodies of tiny arthropods, are readily
preserved in unleached sediments. Their presence in such abundance bears wit-
ness to an aquatic habitat of relatively permanent nature.

Pisidium compressum Prime. This small species of sphaeriid clam was
named from examples obtained from Fresh Pond, near Cambridge, Massachusetts,
but according to Baker (1928, p. 3 71) the typical form is confined to creeks and
rivers. It thrives (or at least formerly did so) in the small brook formed by flow
from the artesian spring in Meade County State Park, Kansas, but there is consid-
erable evidence that Pisidium oompressum does thrive in ponds, not to mention

FARMDALIAN LAKE DEPOSITS AND FAUNAS

the fact that the type locality is a clear, quiet pond. Only a few examples were
taken from the Morris Section, and it seems not to have been a prominent item of
the local fauna.

Pisidium pauperoulum Sterki . This is a variant of Pisidium nitidum , ac-
cording to some malacologists; La Rocque(1967, p. 334) lists it as P. nitidum form
pauperoulum Sterki. The consensus of authors who have studied the ecology of
this small clam is that its preferred habitat is quiet, shallow water with a mud or
sand bottom and with depths from 1 .5 meters to 5 .0 meters. Only a few valves
were recovered in our samples; it does not form a prominent part of the total fauna
at the Morris North locality.

Potamogetan seeds. Potamogetan is a weed that lives in ponds of per-

manent water. Two seeds found at the Morris North Section are tentatively re-
ferred to this genus (Martin and Barkley, 1961) .

Sphaerium laoustre (Miiller) . This clam, larger than Pisidium, forms a
conspicuous feature of the molluscan fauna at the Morris North locality. Sphaeri-
um laoustre, which is also widely distributed in Europe, is a highly adaptable
species and has been recorded in the southern United States, although most oc-
currences are northern. Herrington (1962, p. 20) stated that it is most plentiful
in small lakes and ponds but also occurs in large lakes, creeks, and rivers. It
seems to prefer muddy bottoms.

Strobilops labyrinthica (Say) (?). A single damaged shell is referred to
this species. As this gastropod is terrestrial in habit and lives in deciduous for-
est environments, the shell must have been carried into the lake at this locality.
Its presence, however, suggests a permanent arboreal vegetation near by.

Unionid shell fragments. No unionid mussel shells sufficiently intact
to identify were recovered from the Morris locality, but the deposit contains
many fragments of nacreous plates of these animals, and an internal cast of a
mussel, probably an Anodonta, was secured. Because these mussels live in per-
manent water and require fish for the completion of their life cycle, their presence
has an important implication in the interpretation of the local environment.

Valvata trioarinata Say. This branchiate gastropod occurs with relative
abundance in the Peddicord Formation at the Morris North locality. It is a spe-
cies of essentially northern distribution, living from Ohio and Illinois to Great
Slave Lake. Valvata trioarinata adapts easily to a variety of aquatic habitats, in-
cluding lakes and streams with or without vegetation, to depths of nine meters in
a wide variety of bottom sediments. Its evident success in the local environ-
ment at the Morris North site is consistent with a cool, permanent pond or lake.

Robein Silt

The fauna recovered from the Robein Silt at the Morris North Section is
described below. This sediment is not abundantly fossiliferous, but its fauna
shows distinct variations from that of the underlying Peddicord Formation. Un-
fortunately, each of the several species is so sparsely represented that it is dif-
ficult to draw firm conclusions regarding the habitat.

Discus cronkhitei (Newcornb) . Damaged shells of four individuals were
recovered. Discus crorikhiteiis a terrestrial gastropod; its habitat today is main-
ly river floodplains, where it lives under fallen logs, bark, and other dead vege-
tation. It appears throughout the northern half of Illinois and at higher latitudes
and altitudes elsewhere in the United States; it is found as far south as Arizona,
where it lives in the higher elevations of the mountains. Although the sediments
at the site where these specimens were collected may be waterlaid, the species
must have lived in near-by woodlands.

10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 467

Gyraulus altissimus Baker. Only two shells of this aquatic pulmonate
gastropod were collected. La Rocque (1967, p. 493) wrote about the ecological
requirements of this extinct gastropod: "This species, from its association with
other mollusks in Pleistocene deposits, can be said to have preferred shallow
bodies of water with abundant vegetation. " He went on to show in detailed stud-
ies of Pleistocene ponds that after the development of vegetation these snails
increased several- fold over previous population levels. Potamogeton natans, a
pond weed, occurs in these deposits .

Eendersonia occulta (Say). Two well preserved shells of this terrestrial
gastropod were found. Eendersonia occulta lives in dry woodlands or on flood-
plains of creeks; it is extremely rare as a living mollusk in Illinois. Baker (1939,
p. 39) noted that it is known from only a single locality near Athens, Menard
County. The general distribution of living populations includes the north-central
United States, although the species has been reported as far south as Kentucky.
As a fossil, E, occulta is known from numerous localities in Pleistocene deposits,
from the Kansan Stage to the late Wisconsinan (Leonard and Frye, 1960, fig. 2;
Leonard, Frye, and Johnson, 1971, fig. 3).

Polygyrid fragments, large species. The large polygyrids are terrestrial
snails that frequent deciduous forests. As the fragments recovered do not allow
specific identification, about all that can be said is that their presence implies
the near-by areas were forested.

Potamogeton natans (? ) . Fairly numerous seeds of this pond weed were
recovered from the Robein Silt in the type section.

Strobilops labyrinthica (Say) . Two well preserved examples were found
in the sample. The species, found also in the Peddicord Formation in this type
section, was discussed above.

Succinea retusa Lea. Succinea is a genus of terrestrial gastropods, but
S. retusa (Oxyloma retusa), found at this locality, may be described as semi-
aquatic; it lives among sedges and other aquatic vegetation at the edges of ponds
and marshes in very wet places. Populations of S. retusa are distributed through-
out the northern half of the United States and over much of Canada; in Illinois S.
retusa is widely distributed, especially in the northern half of the state (Baker,
1939, p. 125).

Valvata tricarinata (Say) . One specimen was collected in the Robein
Silt. The species was discussed with the Peddicord fauna.

Ostracoda. Ostracodes of at least two kinds, neither of them identified,
were found in the Robein Silt at the Morris North Section, but they are more num-
erous in the Peddicord Formation.

Comparison of the Robein and Peddicord Fossils

Although the assemblages from the Robein Silt and the Peddicord Formation
include about the same number of items, several differences should be emphasized
First, the population density per unit volume of sample is much greater in Peddi-
cord deposits, from which it may be inferred that the habitat was the more favor-
able of the two because in neither locality does any mechanism seem to be oper-
ating to condense or disperse the faunas. Second, the Peddicord fauna lived in
a cool, large pond or lake; the fauna is composed entirely of gastropods and
pelecypods restricted to such a habitat. The Robein fossils seem also to have
come from a pond deposit, since they include two aquatic snails (Gyraulus altis-
simus and Valvata tricarinata), seeds of the pond weed Potamogeton, and valves
of ostracodes. However, the greater part of the Robein assemblage is composed

FARMDALIAN LAKE DEPOSITS AND FAUNAS 11

of terrestrial gastropods that drifted from near-by wooded land surfaces. From
this it may be inferred that the pond, although relatively permanent, was small
in area and possibly surrounded by a marshy zone.

Comparison of Peddicord Fossils from
Morris North and Wedron

The fauna from the Wedron Section (Leonard and Frye, 1960) comprises
nine species, all of which are mollusks of aquatic habits except one species of
Lymnaea that lives on the moist mud, near water rather than in it. The faunal
assemblage includes:

Amnioola leightoni Baker Lymnaea parva Lea

Amnioola lustrica Pilsbry Pisidium compression Prime

Gyraulus altissimus Baker Pisidium pauperculum Sterki

Helisoma campanulata (Say) Valvata sincera Say

Valvata tricarinata (Say)

The predominant species is Valvata tricarinata, followed by Amnioola
lustrica and Gyraulus altissimus. A. leightoni* Helisoma campanulata, and the
two species of Pisidium are present in goodly numbers, but Lymnaea parva and
Valvata sincera are poorly represented in the Wedron assemblage.

The ecological correlation between the fauna at Wedron and Morris North
is more real than apparent. For example, although the amnicolids at the two lo-
calities do not bear the same specific names, three of the four species are ex-
tinct and may represent nothing more than changes in form that result from local
conditions. Furthermore, all amnicolids are branchiate gastropods requiring per-
manent water for their life processes, and almost all of the North American spe-
cies have a northern distribution. The occurrence of Strobilops only at Morris
North and Lymnaea only at Wedron may be accidents of collection, since only
one or two specimens of each were found. Furthermore, the Wedron sample was
not treated properly for recovery of ostra codes or pond weed seeds, both of which
very probably occur in this lacustrine environment. In the light of these facts, it
may be concluded that ecologically the two faunas are almost identical; both re-
flect the presence of a lake of cool, permanent water. Radiocarbon dates at the
Wedron locality (26, 800 ±700 B.P., W- 8 71) are so close to those at Morris North
(24, 990 ±280 B.P., ISGS-61) that they make a discussion of the age interpreta-
tion of the faunas unnecessary.

SUMMARY

The newly described exposure in the Morris North Section confirms the
presence of an extensive lake in Farmdalian time that was related to the buried
Ticona Valley. This lake is herein named Lake Peddicord. The deposits that ac-
cumulated in the lake are largely buried by younger drift. Lake Peddicord is the
first of a series of late Pleistocene lakes that covered various parts of the Morris
Basin. Successively younger lakes of different areal extent include Lake Kickapoo,
Lake Illinois, Lake Wauponsee, Lake Morris, and Cryder Lake, the last two of
which are related to outlet stages of Lake Chicago.

The only area where these lake deposits were known previously is at
Wedron, and there they have at various times been included in the "Kickapoo

12 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 467

beds, " "Lake Kickapoo deposits," Farmdale Silt, or Robein Silt. The lake depos-
its at Wedron and at Morris North are here assigned to a new formation, the Ped-
dicord Formation. Their Farmdalian age is confirmed by a radiocarbon date of
24, 990 ±280 B. P. (ISGS-61).

The molluscan fauna from the Peddicord Formation at Morris North signif-
icantly expands knowledge of Farmdalian aquatic faunas, previously known only
from Wedron. These faunas indicate the presence of a lake of cool, permanent
water, and confirm the fact that the Farmdalian, a time of extensive glacial with-
drawal, was characterized by a cool, moist climate.

REFERENCES

Baker, F. C, 1921, New forms of Pleistocene mollusks from Illinois: Nautilus,
v. 35, no. 1, p. 22-24.

Baker, F. C, 1928, Freshwater mollusca of Wisconsin: Wisconsin Geol. and

Nat. History Survey Bull. 70, pt. I, Gastropoda, 507 p., 28 pis., pt. II,
Pelecypoda, 495 p., pis. 29-105.

Baker, F. C, 1939, Field book of Illinois land snails: Illinois Nat. History Sur-
vey Manual 2, 166 p., figs. 1-8, 175 unnumbered figs.

Culver, H. E., 1922, Geology and mineral resources of the Morris Quadrangle:
Illinois Geol. Survey Extract Bull. 43B, 114 p.

Fisher, D. J., 1925, Geology and mineral resources of the Joliet Quadrangle:
Illinois Geol. Survey Bull. 51, 160 p.

Herrington, H. A., 1962, A revision of the Sphaeriidae of North America: Michi-
gan Univ. Mus. Zool. Misc. Pub. no. 118, 74 p., 7 pis., 2 figs.

La Rocque, Aurele, 1967, Pleistocene mollusca of Ohio: pt. 2, Ohio Geol. Survey
Bull. 62, 356 p., 8 pis., 208 figs.; pt. 3, Freshwater gastropoda, p.
357-553, pis. 9-14, figs. 209-408; pt. 4, Terrestrial gastropoda, p. 554-
800, pis. 15-18, figs. 409-624.

Leighton, M. M., 1928, Lake Illinois and the question of post- early Wisconsin
deformation in northern Illinois [abs.]: Geol. Soc. America Bull., v. 39,
p. 215.

Leonard, A. B., and J. C. Frye, 1960, Wisconsinan molluscan faunas of the Illi-
nois Valley Region: Illinois Geol. Survey Circ. 304, 32 p., 4 pis., 3 figs.

Leonard, A. B., J. C. Frye, and W. H. Johnson, 1971, Illinoian and Kansan mol-
luscan faunas in Illinois: Illinois Geol. Survey Circ. 461, 23 p., 4 figs.

Martin, C. M., and W. D. Barkley, 1961, Seed identification manual: Univ.
California Press, 221 p., 824 pis., 288 figs.

Pilsbry, H. A., 1948, Land mollusca of North America: v. II, pt. 2, p. ixlvii,
521-1113, 585 figs.

Sauer, C. O., 1916, Geography of the upper Illinois Valley and history of de-
velopment: Illinois Geol. Survey Bull. 2 7, 208 p.

Willman, H. B., 1940, Pre- glacial River Ticona: Illinois Acad. Sci. Trans.,
v. 33, no. 2, p. 172-175.

Willman, H. B., 1971, Summary of the geology of the Chicago area: Illinois
Geol. Survey Circ. 460, 77 p.

Willman, H. B., and J. C. Frye, 1970, Pleistocene stratigraphy of Illinois: Illi-
nois Geol. Survey Bull. 94, 204 p.

Willman, H. B., and J. N. Payne, 1942, Geology and mineral resources of the
Marseilles, Ottawa, and Streator Quadrangles: Illinois Geol. Survey
Bull. 66, 388 p.

Illinois State Geological Survey Circular 467

12 p., 2 figs., 3000 cop., 1971

Urbana, Illinois 61801

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25
(10-71-3000-36572)

CIRCULAR 467

ILLINOIS STATE GEOLOGICAL SURVEY

URBANA 61801