Geology About Peoria, Illinois

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Peoria, Illinois is located in the central part of the state, along the Illinois River. The geology of the Peoria area provides insight into the ancient environments, landforms, and geological processes that have shaped this region over millions of years.

This article explores the key aspects of Peoria’s geology, including its bedrock, topography, soils, minerals, and how the area has changed over geologic time.

Bedrock Geology

The bedrock underlying Peoria consists primarily of sedimentary rocks formed during the Paleozoic Era between 541 and 252 million years ago, when this area was covered by warm, shallow seas. The local bedrock includes the following formations:

  • Maquoketa Shale: A fine-grained gray shale formed ~450 million years ago during the Ordovician Period. It contains fossils of graptolites, brachiopods, trilobites and cephalopods. Up to 300 feet thick in the Peoria area.
  • Galena-Platteville Dolomite: Light gray to buff dolomite and limestone ~440-490 million years old (Ordovician). Contains fossils of brachiopods, bryozoans, cephalopods, corals and trilobites. Around 250 feet thick.
  • St. Peter Sandstone: A fine-to-medium grained sandstone formed ~500 million years ago during the Ordovician. Ranges from white to yellow-gray. Approximately 200 feet thick.
  • Prairie du Chien Group: A mix of dolomite, sandstone and shale 345-500 million years old (Ordovician-Cambrian). Up to 600 feet thick.

These Paleozoic rocks dip gently to the east-southeast in the Peoria area, descending to greater depths in that direction. They form an important regional aquifer system.

Glacial Geology

While bedrock shapes the foundation of the Peoria area, the overlaying sediments deposited during the ice ages have the greatest impact on the modern landscape. Peoria lies on the Bloomington Ridged Plain, a glacial landform characterized by flat plains punctuated by elongated moraines (ridges) formed when ancient glaciers retreated.

The last glaciers retreated from northeastern Illinois about 13,500 years ago during the Wisconsin Glaciation. As they melted, they left behind thick deposits of loose gravel, sand, silt, clay and boulders called glacial drift.

This poorly sorted material filled in the preglacial Teays River valley that formerly occupied the modern Illinois River valley.

Specific glacial deposits in the Peoria area include:

  • Loess: Fine wind-blown silt up to 15 feet thick mantles the bedrock across upland areas. Loess provides fertile soil.
  • Till: Unsorted, unstratified clay, silt, sand and gravel deposited directly by glacial ice. Forms moraines.
  • Outwash: Sorted sand and gravel carried by glacial meltwater streams. Fills valleys.
  • Lake deposits: Fine silt and clay that settled out in temporary proglacial lakes.

These glacial sediments provide parent material for Peoria’s fertile soils.

Topography and Landforms

Peoria’s terrain is characterized by relatively flat plains at an elevation of ~500-600 ft above sea level, with several prominent moraines rising 100-200 ft above the plains. These include Farm Creek Moraine and Mackinaw Moraine. Other features include:

  • Illinois River valley: Deep bedrock valley filled with glacial outwash that is now occupied by the Illinois River.
  • Ground moraines: Low, irregular hills of till that form undulating plains.
  • Lake deposits: Flat lowlands made of silt/clay from ancient lakes.
  • Streams: Creeks, rivers and ravines cutting across the landscape. Many follow the folds and fractures of the bedrock.

Overall, the topography slopes gently downward to the west-southwest toward the Illinois River valley, which provided a natural path for meltwaters to drain from the receding glaciers.

Mineral Resources

Economically significant mineral resources are limited in the Peoria area due to the lack of exposed bedrock. These include:

  • Sand and gravel: Abundant glacial deposits that are extensively mined for construction aggregate.
  • Limestone: Mined locally from Galena-Platteville formations for cement and aggregate.
  • Coal: Thin, sporadic seams occur in Pennsylvanian bedrock west of Peoria. Not economically viable.
  • Oil and gas: Small amounts of oil and gas have been produced from localized wells in the Peoria region. Limited potential.

While no metallic mineral deposits occur, the rich glacial soils made possible extensive agriculture in the Peoria area. The Illinois River also provided key transportation.

Geologic Hazards

Geologic hazards pose limited risks in Peoria:

  • Seismicity: Low earthquake hazard. Far from active faults so only small intraplate quakes occur. The last magnitude 5.2 quake centered near Peoria was in 1952.
  • River flooding: Periodic flooding affects lowlands along the Illinois River and its tributaries. Flood risk is managed by dams and levees.
  • Karst: Minor sinkhole risk where Prairie du Chien dolomite is near surface.
  • Other risks: Tornadoes occur but are not strongly associated with geology. Limited landslide, expansive soil and radon gas hazards.

Overall, Peoria’s geology is generally favorable and hazards are modest. The biggest influence is the fertile glacial sediments supporting agriculture.

Geologic History

Peoria’s geological history spans over 500 million years of Earth history:

  • Cambrian (~500-485 Ma): Sandstone, shale and carbonates of the Prairie du Chien Group and Jordan Sandstone deposited in a shallow sea.
  • Ordovician (~485-443 Ma): Continental collision led to the Taconic Orogeny, causing uplift and erosion. Limestones and shales later formed as seas returned.
  • Silurian (~443-419 Ma): Erosion occurred as the area was again above sea level. No rocks from this time period remain.
  • Devonian-Mississippian (~419-323 Ma): Ongoing erosion above sea level. Only Devonian chert clasts are found in glacial till.
  • Pennsylvanian (~323-299 Ma): Sea levels rose and swampy coal forests covered the area. Erosion later removed most Pennsylvanian rock.
  • Permian-Paleogene (~299-23 Ma): Regional erosion for hundreds of millions of years removed most Paleozoic rocks, forming the Peoria Bedrock Valley.
  • Neogene (23-2.6 Ma): The ancient Teays River flowed through the area prior to glaciation.
  • Pleistocene (2.6 Ma-11,000 years ago): Onset of glaciations and multiple advances/retreats leading to thick glacial sediment deposits and the modern landscape.
  • Holocene (11,000 years ago-present): Glaciers retreated leaving behind the Illinois River valley and conditions suitable for human settlement.

Peoria’s long geologic history provides a fascinating record of the changing environments, climates and rock formations that have shaped this region over deep time. The influence of ancient oceans, erosion and multiple ice ages created the landscape we see today.

In summary, this overview covers key aspects of Peoria’s geology including the Paleozoic bedrock, glacial sediments, landforms, mineral resources, geologic hazards and the area’s formation through various mountain building events, marine inundations and erosional periods spanning back half a billion years.

The area’s rich glacial soils and river access greatly influenced human settlement and agriculture in the region starting in the Holocene.

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