Geology of Illinois State Parks by Max Reams

Author:Max Reams [Reams, Max]
Language: eng
Format: epub
Published: 2013-08-08T04:00:00+00:00

Central Lowlands Province

Till Plains Section

Springfield Plain: Red Hills State Park

Geology of the Springfield Plain

References: Weibel, Reinertsen, and Reed, 1989; Grimley and Phillips, 2007 and 2011.

Leighton, et al., (1948) describe the Springfield Plain as flatter with shallower stream valleys than the Galesburg Plain to the west. Streams have low slopes, contain alluvium and terraces, with tributaries that have wide, V-shaped valleys. The southern boundary is drawn where till is so thin that bedrock controls topography. Glacial moraines are more obvious in the western portion but are broad and low. Irregular gravelly ridges and hills, possibly kames and crevasse-fillings are present. Till is thinner than the Galesburg Plain and thick loess occurs along the Illinois Valley but thins rapidly to the southeast.

The Springfield Plain is floored by Pennsylvanian Period sedimentary rocks (Figure 2.) Sandstone, coal, shale, and limestone were deposited in regular sequences or cycles. This refers to repetition of thin layers of these rocks as sea level rose and fell. Sea level changes are linked to the advance and retreat of glaciers in the Southern Hemisphere. As glacial ice accumulated on continents, sea level dropped.

During warmer periods, ice melted and streams returned water to the oceans. The Pennsylvanian Period shoreline ran more or less east-west through Illinois. Stream deposition resulted in delta deposits of sand, swamps of peat and mud, muddy lagoons, and marine accumulations of shells and mud. Rise and fall of sea level allowed these environments to deposit their sediments over each other in successions of thin layers that eventually hardened into rock.

The Pleistocene Ice Age deposits here are Illinoian. A controversial series of ridges, termed “ridged drift”, resembling kames and eskers are scattered over the plain. These ridges have puzzled geologists for some time, especially concerning their exact timing during the Pleistocene.

The sand and gravel making up ridged drift form generally elongated features that parallel the direction that glacial ice flowed. Perhaps they are crevasse fillings (kames), subglacial ice tunnel fillings (eskers), or similar features found in Wisconsinan glacial deposits of the Kettle Moraine area of Wisconsin. Grimley and Phillips (2007 and 2011) describe several complex glacial processes involved in constructing ridged drift features.

Loess varies from over 20 feet thick near the Mississippi and Illinois Rivers to less than five feet in the center of the plain. Topographic relief is low (relief refers to the difference in elevation between the highest and lowest point in a region.)

Weibel, Reinertsen, and Reed (1989) describe the Springfield Plain as “generally flat and has tabular (table-like) uplands, its surface is gently undulating (rolling) in places and modern drainage is shallowly entrenched in it...drift is less than 25 feet thick beneath the tabular uplands but exceeds 100 feet in buried bedrock valleys”.

Distinguishing the Springfield Plain from the Mount Vernon Hill Country to the south can be a challenge. A.C. Phillips, Illinois State Geological Survey (personal communication, 2010) indicates that the Springfield Plain has obvious moraines that are absent in the Mt. Vernon Hill Country.

The Springfield’s uplands are flatter than more rounded upland ridges of Mt.

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