Rocks under the soil in LaSalle County are sedimentary in their origin and include in ascending order the Oneota dolomite, the New Richmond sandstone and the Shakopee dolomite of the Prairie du Chien group of Lower Ordovician age; the St. Peter sandstone, and the Platteville-Galena limestone and dolomites of Middle Ordovician age; the Maquoketa shales and limestone of Upper Ordovician age; the Silurian dolomite; Devonian limestone; and the Pottsville, Carbondale, and McLeansboro formations of the Pennsylvanian series. The unconsolidated boulder tills, gravels, silts, and buried soils represent the deposition of probably three periods of glaciation in the Quaternary period. Within Illinois Valley and its tributary valleys, sediments composed of sand and gravel, silt, and alluvium have accumulated partly because of glacial drift and partly due to continuing erosion and water transport.
The geology underlying LaSalle County can be easily observed while visiting Starved Rock and Matthiessen State Parks. The following passage is reprinted from Time Talks: The Geology of Starved Rock and Matthiessen State Parks, a publication of the Illinois Department of Natural Resources and the Illinois State Geological Survey.
(543 to 248 million years ago)
All of these parks’ rocks formed during the Paleozoic Era, but only Ordovician and Pennsylvanian Period rocks remain. Rocks from other Paleozoic periods, if deposited, have been eroded away.
During the Paleozoic, the Earth’s appearance was much different. For instance, North America was located at or near the equator and had a tropical climate. Large-scale events occurring over millions of years were altering the shape of the Earth’s crust. Shifting and folding from tectonic movement uplifted some areas and caused others to sink.
Starved Rock and Matthiessen State Parks are located on the northern flank of the Illinois Basin, a broad, spoon-shaped depression that subsided during Paleozoic time. The Basin extends across most of Illinois and parts of Indiana and Kentucky. As the Illinois Basin settled downward, the sea advanced northward. Over time, great thicknesses of marine and coastal sediments accumulated, were compressed, and eventually became the bedrock you see today.
(490 to 443 million years ago)
The oldest rock layer in the park is the Shakopee Dolomite, which forms the bed of the Illinois River. This dolomite originated from materials that accumulated on the ancient sea floor early in the Ordovician Period. The Shakopee’s irregular upper surface tells us that, for a time after its formation, it was above sea level and exposed to erosion.
Then, about 480 million years ago, the sea rose, and beach and nearshore sands were deposited over the Shakopee, forming the St. Peter Sandstone. These sands originated from weathered Canadian rock carried by ancient rivers and deposited into Ordovician seas. Although the rock originally consisted of several minerals, gradually most of the less resistant minerals have been weathered away, leaving a uniquely pure quartz sand. After still more cycles of water transport and deposition, over millions of years, the individual sand grains were worn to a remarkably uniform size.
As the St. Peter Sandstone formed, the Illinois Basin continued to sink. By about 460 million years ago, the shore of the ancient sea was far from the park area, and sand was no longer being deposited. As the water got deeper, shells accumulated from dead marine organisms, and the rocks they formed are collectively named the Platteville Group. These hard, well-cemented limestones and dolomites lie over the St. Peter Sandstone. They are exposed in some of the upper reaches of the canyons at Starved Rock Park and along the banks of the Vermilion River in Matthiessen Park.
(325 to 286 million years ago)
During the early to middle Pennsylvanian Period, the park area was still near the margins of the shallow, tropical sea that alternately rose and fell. During times of deeper water, carbonate material accumulated to form limestone and dolomite. When sea levels were lower, materials eroding from mountain ranges far to the east were deposited as sediments, forming shales and sandstones in shallow water or across vast river deltas.
Sandstone formed from sands deposited in river channels, near river mouths, and along beaches and nearshore waters. Shales formed from mud deposited in quiet-water bays and lagoons or on river floodplains. The alternating sandstone and shale beds you see today show that the river and shoreline positions shifted. Sometimes sand was deposited, and, at other times, mud.
Resources to learn more about the geology of LaSalle County
Illinois State Geological Survey's website: https://www.isgs.illinois.edu/outreach/geology-resources/build-illinois-last-500-million-years
Learn how glaciation and erosion have shaped La Salle County and the Illinois River valley by ordering Time Talks: The Geology of Starved Rock and Matthiessen State Parks from the Illinois State Geological Survey: https://shop.inrs.illinois.edu/st-parks.html.
Geologic Field Trip to the Starved Rock State Park Area, LaSalle County, Illinois
Download the guidebook for a field trip held on October 6, 2013 in conjunction with the 50th Anniversary Annual Meeting of The Clay Minerals Society held at the University of Illinois at Urbana - Champaign: http://www.clays.org/annual%20meeting/50th_annual_meeting_website/program.html.