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Many of the reef inhabitants are filter feeders and sediment prevents feeding. The water is usually clear of silt or even wind-blown sediments. Modern reef environments are found in warm, shallow oceans. Fossil crinoids, brachiopods, trilobites, ammonoids, gastropods and corals are found in Thornton Quarry rock. These pictures are of a diorama at the Milwaukee Public Museum of plants and animals which lived on and formed Silurian-age reefs. 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.Ĭlick on the map to go to a larger image which shows bedrock and county boundaries.Ĭhicago is built upon a vast Silurian Reef. Peter Sandstone which forms the bluffs along the Illinois River at Starved Rock. The most famous formation of this Period is the St. 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.
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Over the Cambrian rocks lie the rocks of the Ordovician Period. Not very many fossils are preserved in these environments because sand is so abrasive. During the Cambrian, the Chicago area was alternately a beach and near-shore marine system.
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They are on the upthrown side of the Sandwich Fault and can reach 30-40 feet of outcrop.Ĭambrian sandstones received their pure quartz sands from the oldest areas of North America, the Canadian Shield and the Laurentian Mountains. The only outcrops of Cambrian Period rocks near Chicago are in Ogle County, near Dixon, IL. The Mount Simon Sandstone provided well water for the city stockyards and many surrounding communities for over a century before they were depleted. On top of the eroded upper surface of Chicago region Precambrian basement rocks are rocks of Cambrian age. Finally everything on top of this granite and the surface of the granite itself were eroded.ĭuring this time too, the Baraboo, Wisconsin Syncline formed, providing more evidence of an ancient tectonic collision in the Midwest. Most granites are jointed by the stresses of being near the surface perhaps our regional jointing dates to this early time. Granites form deep within the Earth and are slowly uplifted to the surface during collisions. Formally, geologists now assign the Chicago region to the eastern granite/rhyolite province of North America. Deep below the volcanic upper surface, the pink granitic bedrock of Illinois and Chicago crystallized. 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. During the Precambrian, what would become Illinois was part of an island arc. The vast majority of geologic time is Precambrian. Parts may be filled in by studying rocks brought up during excavation or drilling.
#CRINOID AND OOLITE FULL#
No full geologic column is ever exposed to view. Some attempt to show all the possible rocks in a given area others try to show just the formations which remain.
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Geological columns are only approximations of reality. 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.Ĭlick 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. Geologists also prepare maps of what the rocks would look like at the surface, if all the surface material was removed.Ĭlick on the small map of Illinois ( from the Illinois State Geological Survey) to go to a large version. Geologists draw figures called "columns" to show the relationship of various rocks. A gap in the rock record is called an "unconformity " this one represents millions to hundreds of millions of years of erosion. But we don't know what was above it because its top surface shows that it was exposed as a landscape. We assume that there must have been some rock above it, because today granites are forming thousands of feet below modern volcanos. It is a Precambrian red granite, an igneous rock that formed billions of years ago. The top of the oldest rock below Chicago lies about 2,000 feet below the city's pavements. Presented at "Make Tracks with Paleontology" - January 15, 2000 First Families of Chicago: Local life about 400 million years ago