First Families of Chicago:
Local life about 400 million years ago
Revised and Copyright © 2006 Ellin Beltz
A brief overview of local geologyThe top of the oldest rock below Chicago lies about 2,000 feet below the city's pavements. It is a Precambrian red granite, an igneous rock that formed billions of years ago. We assume that there must have been some rock above it, because today granites are forming thousands of feet below modern volcanos. But we don't know what was above it because its top surface shows that it was exposed as a landscape. A gap in the rock record is called an "unconformity;" this one represents millions to hundreds of millions of years of erosion.
Geologists draw figures called "columns" to show the relationship of various rocks. Geologists also prepare maps of what the rocks would look like at the surface, if all the surface material was removed.
Click on the small map of Illinois (from the Illinois State Geological Survey) to go to a large version.
When you color in your bedrock map of Illinois, use a different color for each age of rock and color in the key and your geological column to match.
Click on the picture of the Illinois Stratigraphic Column to go to a larger version which has been annotated to correspond with localities mentioned in this text.
Geological columns are only approximations of reality. Some attempt to show all the possible rocks in a given area; others try to show just the formations which remain. This diagram is of the latter type. No full geologic column is ever exposed to view. Parts may be filled in by studying rocks brought up during excavation or drilling.
The vast majority of geologic time is Precambrian. During the Precambrian, what would become Illinois was part of an island arc. Perhaps that ancient island arc formed the same way they are today - by magma erupting upward from the subduction zone of two colliding oceanic plates. Deep below the volcanic upper surface, the pink granitic bedrock of Illinois and Chicago crystallized. Formally, geologists now assign the Chicago region to the eastern granite/rhyolite province of North America. Granites form deep within the Earth and are slowly uplifted to the surface during collisions. Most granites are jointed by the stresses of being near the surface; perhaps our regional jointing dates to this early time. Finally everything on top of this granite and the surface of the granite itself were eroded.
During this time too, the Baraboo, Wisconsin Syncline formed, providing more evidence of an ancient tectonic collision in the Midwest.
On top of the eroded upper surface of Chicago region Precambrian basement rocks are rocks of Cambrian age. The Mount Simon Sandstone provided well water for the city stockyards and many surrounding communities for over a century before they were depleted. The only outcrops of Cambrian Period rocks near Chicago are in Ogle County, near Dixon, IL. They are on the upthrown side of the Sandwich Fault and can reach 30-40 feet of outcrop.
Cambrian sandstones received their pure quartz sands from the oldest areas of North America, the Canadian Shield and the Laurentian Mountains. During the Cambrian, the Chicago area was alternately a beach and near-shore marine system. Not very many fossils are preserved in these environments because sand is so abrasive.
Over the Cambrian rocks lie the rocks of the Ordovician Period. These are most easily observed in LaSalle County at Starved Rock State Park, Matthiessen State Park, and along the Illinois and Michigan (I and M) Canal at Split Rock. The most famous formation of this Period is the St. Peter Sandstone which forms the bluffs along the Illinois River at Starved Rock. The contact between the topmost Ordovician rock, the Neda oolite, and the bottom-most Silurian rock is exposed near the campground in Kankakee State Park.
Click on the map to go to a larger image which shows bedrock and county boundaries.
The area which we call the Chicago Region is itself part of a larger geologic region, regionally known as the Niagaran Escarpment, marked by outcrops of Silurian dolomites and limestones. Niagara Falls cuts through this rock which is exposed in a giant ring around the middle three Great Lakes and centered on the state of Michigan.
Silurian rocks are are exposed
Blocks of Silurian limestones and dolomites were used in construction of the old Chicago Water Tower, the Chicago Tribune Building, the Michigan Avenue Bridge and as parts of Chicago's lake front revetments and inner city Metra overpasses.
The Ordovician and Silurian rocks record a long period of time during which the Chicago area was covered by a warm shallow ocean. During the Silurian, great reefs grew around the present-day state of Michigan. The restricted sea within the reefs occasionally dried up - resulting in vast salt deposits under Michigan - and some interesting differences in the fossil fauna of Illinois, Wisconsin, Indiana and Michigan.
Under the City of Chicago, the rock record pauses in another great unconformity. Erosion has removed all of the Devonian, Mississippian, Pennsylvanian, Permian -- as well as the entire dinosaur era and mammal era except for Pleistocene glacial till. Whether the Chicago region had or did not have these layers is speculative. However, nearby areas may provide some clues.
Fossils below ChicagoMuch of what we know about ancient life under the Chicago area comes from the fossil record. There are few to no fossils from the Precambrian Era and Cambrian Period in our area. And even in areas which are fossiliferous, only a few of the animals and plants which lived there in the past have been preserved and observed in the present. Some ecosystems are better represented in the fossil record than others and some plants and animals preserve better than others. So our fossil record shows us a fairly good picture of animals with hard parts (shells, spicules and segments) that lived in the warm shallow oceans of the Paleozoic Era.
Throughout geological time, certain animals only lived at particular times and no others. These are called "index fossils." Others occur in many periods in the fossil record. Some may stay the same for a long time ("conservative species") while others change features of their anatomy frequently like the trilobites.
The Illinois State Geological Society published illustrations of
A lot of plants and animals simply do not fossilize at all; some are found only rarely. Many groups of plants and animals that used to exist have gone extinct. Some have evolved and gone extinct and never been found yet in the fossil record. Some do not fit current classification schemes because we cannot figure out to which other group they belong. Much remains to be learned about the past; it is a fascinating study for people of all ages. Fossils found in the Chicago region include members of the:
All these fossils lived together in ecosystems and the ecosystems changed over time. Sometimes the Chicago area was exposed to erosion, quickly destroying the latest deposited soft sediments - just the places most likely to preserve fossils! So the fossil record is incomplete. It does not record well any animals which did not live in shallow marine or marginal marine environments and it usually only preserves those with hard shells, spines or body parts.
Fossils are preserved as molds or as casts which may be the whole body as in life, parts of the body, the body deformed, or a cast of a different material lying within a mold. Soft-bodied preservation is globally more rare than hard bodied preservation; Illinois has a widespread layer called the "Francis Creek Shale" which bears concretions which may preserve Coal Age plants and animals.
Illinois famous Mazon Creek Concretions are formed of siderite, a calcium carbonate mineral. Several times during the Coal Age animals and plants were drowned suddenly and instantly covered with mud. The negative charge of the clay minerals bonded with the still positive charge on the organic debris and the process of conversion to siderite and other minerals proceeded over vast amounts of time. Calcite in some concretions fluoresces under ultraviolet light and reveals even more detail. Metatamorphosed concretions come from the banks of the Vermillion River north of Lowell. During deformation at the end of the Paleozoic, the Francis Creek shale (and many other rocks) were bent into the structure known as the La Salle Anticline.
Precambrian Banded Iron Formations of Upper Michigan and Wisconsin's varved quartzites record an early environment dominated by organisms which produced layered structures called "stromatolites." Early metamorphism converted the layers of sediment (winter runoff) and hematite (organic remains). Some finely varved specimens have been suggested as confirmation for glacial conditions during this time.
Protozoa, of course, remain today. In fact, microscopic creatures far outnumber any of the higher orders everywhere on Earth. They just don't fossilize well. Recent work in high acid mine environments suggests that some "odd" metal deposits may have been concentrated by Protozoa. The Keewenaw, Michigan aberrant silver/copper intercrystals may have formed as the result of bacterial concentration of metals.
Foraminifers and radiolarians are single-celled floating oceanic plankton which occur in today's oceans in great blooms when the proper nutrients are available. Their dead bodies drift to the bottom and are preserved as chalks and cherts, respectively. Other modern plankton, diatoms and coccothithophores, enter the fossil record after Chicago region rocks were laid down. Index fossils called "Fusulinids" are a type of foraminifer.
Receptaculites, a spectacular sun-flower shaped inclusion in Paleozoic rocks was long assigned to sponges, but is now considered a member of the fossil algae. Large, round masses of Receptaculites are found in rocks near Lowell in LaSalle County.
The classes of sponges are divided on spicule shape, the symmetry of spicule arrangement and mineralogy of their spicules. Sponges were one of the most common reef-building organisms in the Paleozoic.
An ancient member of the reef-builders, the Archaeocyathans, may be related to the sponges. They are usually associated with algal fossils and seem to have been more resistent to silty water than other sponges. They disappeared entirely during the Cambrian, although their remains might be found along the Sandwich Fault in northwestern Illinois.
The closely related sea fans, sea whips, sea pens, and soft corals are rarely found as fossils although as ever closer attention is paid to microfossils, their parts and pieces will probably be found.
Jellyfish are known from Mazon Creek concretions (often named "blobs" by collectors) and from a Mississippian outcrop in Brown County. They are not common elsewhere in North America.
Bryozoa are common in all modern oceans, from tropics to the arctic regions. They eat plankton, mostly diatoms and radiolarians. They prefer clear water as their tiny feeding filters are easily clogged by silt.
Brachiopods are an ancient and common group of filter-feeding, shelled animals has members are alive today that we can study to better understand the ecology and behavior of ancient brachiopods. While only the hard shells are usually preserved, some brachiopod fossils reveal internal anatomy and show us that most had long, fleshy anchors which they used to fasten themselves to rocks or other fixed objects.
There are two classes of brachiopods: the articulate brachiopods and the inarticulate brachiopods. Articulate brachiopods have two shells with a well-developed hinge and muscle system as well as a fleshy anchor or "pedicle." The top and bottom shells may be quite different. Each half of the top shell is identical to the other half of the top shell; while each half of the bottom shell mirrors its other half. The other class, the inarticulate brachiopods, have only one shell and pedicle. This class is represented now by Lingula, a mud-dwelling genus which is relatively unchanged from the Ordovician Period to the present day. All of the bivalve fauna on the typical fossil illustrations are brachiopods; the more familiar mollusks developed later and are not represented in Paleozoic Illinois fossil fauna.
Modern and ancient Echinoderms show radial symmetry, a hydraulic vasular system and tube feet. They have no eyes or respiratory, circulatory or excretory systems. They are a sister group to chordates (including vertebrates) because their developing cells are not locked into a predetermined pathway, but can change into other cells and other organs. Some Echinoderms can regrow lost arms; an ability which is being studied for human limb regeneration. Most Echinoderms display a pentameric (five pointed) symmetry.
Paleozoic Echinoderms found in midcontinent rocks include crinoids, blastoids, cystoids, carpoids, brittle stars, sea stars ("starfish"), and sea cucumbers.
Crinoids are especially common in Illinois rocks. They are usually preserved as pieces of the stem and the feeding arms; rarely is a whole animal found intact. The stem pieces look like shirt buttons, but they only have one hole -- in the center. Why we find mostly disarticulated stem pieces may be explained by the behavior of a modern crinoid; when threatened, it frees the topmost, flower-like part of its body, leaving the stem and holdfast to die. The top part of the crinoid can float freely and reproduce. It is unknown if Paleozoic crinoids could separate, but we do know one way that they moved around was to attach themselves upside down to floating debris. Typical Silurian echinoderms include Caryocrinites, Eucalyptocrinites and Pisocrinus.
Paleozoic Echinoids are rare. Echinoids, including the sand dollar, sea biscuits, sea urchins, heart urchins and others with spines are best preserved in Mesozoic and Cenozoic environments. Echinoid body plates are held together by loose sutures and tend to separate immediately postmortem.
Finding your first trilobite is usually a very exciting experience. Then you wonder how many "crinoids" and "horn corals" you've identified that were actually one or two lobes of this three-lobed animal. There is a trilobite mold on the front of the Tribune Tower building about 3-4 feet above the sidewalk and other bits and pieces of trilobites in the Michigan Avenue Bridge, in Joliet and Lemont building stones such as those used on the old Water Tower and Pumping Station and many other places.
Where to see fossils around Chicago
In the wildNo collecting is permitted on public lands in Illinois. Photographic collecting is always permissible and is highly encouraged to permit other fossil hunters to share your joy of discovery. Entering private lands always requires the permission of the owner. Some of my favorite fossils have come from gravel, including gravel along the edges of parking lots. Keep your eyes open!
You can look for fossils on public lands:
References and Literature CitedIllinois Geology - Useful Publications
Standard rock symbols . Complete Geologic Column . Ordovician fossils . Silurian fossils . Devonian fossils . Mississippian fossils . Pennsylvanian fossils . Illinois top of rocks map . LaSalle and Kanakakee geological column . LaSalle area cross-section . Northern Illinois top of rocks map
Take a field trip to see fossils...
Dinosaur Lab Tour . La Salle . Chicago's Michigan Avenue . Southern Illinois . Thornton Quarry . Wyoming
Travel in space and time...
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Ellin Beltz / firstname.lastname@example.org
March 14, 2006