Sulphur, Indiana, in the southern part of the state, was once at
the edge of an inland sea. A visit to the road cut at the junction
of Interstate 64 and State Road 37 in Crawford County contains the
proof of the former marine environment and provides a wonderful
location for fossil hunting and hiking. The area is a rich source
of fossils of various types from the microscopic foraminifera to
large brachiopods, crinoids, trilobites, and even shark teeth.
The layers of rock show great diversity, and to see why, we need
to understand the history of the rocks in Indiana. Glacial material
covers two-thirds of the state of Indiana, making it difficult to
see the original bedrock. Only in the south-central part of the
state is bedrock exposed at the earth's surface along stream and
river valleys, road cuts, and excavations (quarries and surface
mines).
Sedimentary rocks are laid down in parallel layers called
“beds”. Beds vary in thickness and can cover small areas or extend
for hundreds of miles. Beds that commonly occur together are called
"formations”. Formations can be hundreds of feet thick, and
hundreds of miles long or wide. A collection of formations can be
lumped together into "groups." Groups are often hundreds, or even
thousands of feet thick and can be traced hundreds of miles.
This map shows bedrock groups in Indiana: Map
of the bedrock layers of Indiana
Groups are commonly shown on statewide maps, because formations,
and especially beds, would be too thin to draw without making a
very large map. Like formations, there is nothing special about the
group names. They are useful when you want to talk about large
collections of rocks--which is what we are going to do.
Environmental conditions affect which type of rock forms, and the
rocks along this mile-long out-crop and road cut provide a snapshot
of Indiana as a tropical region on the edge of an inland sea.
Geologic time is divided into categories, each with its own
characteristics. The rocks on the outcrop formed during the
Carboniferous period (350 to 290 million years ago). In North
America, the period is called the ‘Mississippian period’. During
the Mississippian period, shallow seas covered the interior of
North America and receded again —several times. Limestone, shale,
and sandstone formed from the buried remains of plants and animals
living in the shallow waters. In Pennsylvania great deposits of
sandstone and shale were laid down by erosion of the coastal
highlands. In the far west, the Madison and Redwall limestones of
the Grand Canyon were forming. And in southern Indiana, which was
located only 10-15 degrees from the equator, a delta system formed
small islands and reefs as the water lapped the shores.
Located along the eastern edge of the Illinois Basin in south
central Indiana, the road cut contains rocks from the lower
two-thirds of the Chesterian section, from the Reelsville limestone
to the Haney limestone. The rocks span the Stephensport and West
Baden groups of the series and were found in outcrop and identified
even before the road cut was made. The road cut allows access to
the formation and provides uninterrupted study of the column of
these rocks in detail, and many local universities conduct field
trips to the area.
HOW SEDIMENTARY ROCKS FORM
Rivers, oceans, winds, and rain runoff all have the ability to
carry the particles washed off of eroding rocks. Such material,
called detritus, consists of fragments of rocks and minerals. When
the energy of the transporting current is not strong enough to
carry these particles, the particles drop out in the process of
sedimentation. This type of sedimentary deposition is referred to
as clastic sedimentation.
Materials weathered and eroded from the land are first deposited
on the continental shelf, mainly in tidal environments, in deltas,
and along beaches. The rest of the sediment continues out to sea.
Winds and currents carry fine-grained particles offshore where they
ultimately settle to the ocean floor. Over eons of time these rocks
form layers.
KINDS OF ROCKS, MINERALS, AND FOSSILS FOUND HERE
When geologists talk about a column of rocks, they discuss the
rocks from the bottom upward. Beginning at the lowest level of the
road cut and working upward the first two formations belong to the
West Baden group and the remaining layers are part of the
Stephensport Group.
The West Baden Group: The West
Baden group was named for outcrop rocks in West Baden, Orange
county, Indiana and is primarily made of shale and sandy limestone
showing cross-bedding. The fossils of this group of rocks include a
variety of fascinating foraminifera and conodonts. The two types of
rocks in the road cut from the West Baden group are the Reelsville
formation and the Elwren formation.
The Reelsville formation makes up the lowest layer of the
road cut. The Reelsville Limestone is a dark gray sandy limestone,
full of microscopic sized shells and ocean creatures suggesting
deeper water than the Elwren formation,.
The Elwren Formation is the final layer of the West Baden
Group. The Elwren Formation rocks include thin-bedded fine-grained
sandstone, cross bedded sandstone, and green-gray and red-brown
shale and mudstone. The rocks here suggest deposition in a marginal
marine environment with some subaerial exposure.
The Stephensport Group:
Named for an outcrop found in Stephensport, Breckinridge County,
Ky., the group consists of about equal parts of limestone, shale,
and cliff-forming sandstone.
Beech Creek The lowest layer of the Stephensport group is
the Beech Creek Limestone, a gray limestone with many crinoids,
brachiopods, and other larger fossils. The Beech Creek Limestone
contains the first appearance of the brachiopod Coelidium
explanatum and also several species of the blastoid genus
Pentremites (P. cervinius Hall, P. elegans Lyon, P. tulipaformis
Hambach).
Big Clifty Formation presumably derives its name from Big
Clifty Creek in Grayson County, Ky. where the formation was
described as a buff to cream color heavily-bedded and
cross-laminated fine-grained sandstone. A major part of the Big
Clifty Formation on the outcrop in Indiana is a prominent
cliff-forming sandstone that for many years was erroneously
referred to as the Cypress Sandstone. Above the sandstone is a gray
shale and mudstone that was for a time called the Indian Springs
Shale, and later assigned to the Golconda. When the name Big Clifty
was adopted for use in Indiana, the gray shale and mudstone
designated the Indian Springs Shale Member was included in the Big
Clifty formation. The Big Clifty Formation varies from a very black
and oily shale at the base, to a fine-grained sandstone, to the
gray shale with fossils and interbedded limestone near the top. The
sandstone of this formation forms extensive cliffs. The
fossiliferous shale and limestone of the Indian Springs Member at
the top of the Big Clifty contain wing plates of the crinoid
Pterotocrinus.
Indian Springs shale The Indian Springs shale unit is easily
distinguished for the fossils it contains. Here it is gray, with
many fossils and topped by tannish shale and siltstone. Many
studies have been done on the Indian Springs Member; the layer
holds many of the same bryozoans, brachiopods, and crinoids found
in the Haney limestone. The fossils found here, suggest that this
layer formed in a protected marine environment, such as a bay or
estuary.
HaneyThe final layer present at the outcrop is the Haney
formation. In Indiana the Haney Limestone is very fine grained
skeletal limestone and micritic dolomite with some shale included.
The formation is thick and contains abundant blastoids of the genus
Pentremites, crinoid plates, and bryozoans, including Archimedes.
The dolomite occurs in the limestone in the form of occasional
“vugs” or small hollows with the pinkish dolomite crystals lining
the interior. Dolomite in itself is an interesting mineral find,
with an aura of enigma because it has proven impossible to
reproduce Dolomite in the laboratory using experimental conditions
that replicate the natural environments under which it formed in
the past. The mineral is very collectible.
Information taken from personal studies of the site and from:
IGS
Map
of the bedrock layers of Indiana