REVIEW OF EVIDENCE FOR WESTERN- AND EASTERN-SOURCE EARLY ICE ADVANCES IN ILLINOIS

Deposits of early Pleistocene ice advances have been recognized in the subsurface throughout much of Illinois, although they occur at or near land surface in only a limited area of western Illinois. Widespread truncation of these early deposits during later glacial advances precludes interpretation of landscape features. A review of the current status of knowledge suggests guidelines for future studies.

Past studies relied on stratigraphic position and the presence or absence of buried soils, intercalated waterlain sediments, and calcareous and noncalcareous zones to interpret the relative ages of pre-Illinois episode tills. Pebble lithologies were used to interpret source areas. Particle-size data aid in characterization but confirm that diamictons from both western-source and eastern-source (Lake Michigan lobe) ice advances, as well as Illinois Episode diamictons, are texturally similar (silty clay loams). In addition, a nearly ubiquitous gray color characterizes most pre-Illinois and overlying Illinois Episode tills in western Illinois. Mineralogical composition of the clay fraction in unaltered diamictons aids in distinguishing eastern-source and western-source diamictons: eastern-source sediments are illitic and dolomitic, whereas western-source sediments are calcitic and less illitic. However, in many areas a high degree of weathering can obscure such interpretations. In western Illinois, the presence of accretion-gleys on both pre-Illinois and Illinois episode sediment surfaces suggests that poorly drained landscapes persisted through time. Preliminary cross sections in western Illinois suggest that Illinois Episode sediments are a relatively thin veneer overlying thick pre-Illinois sediments. Several areas with alternating weathered and unweathered sediment sequences remain to be interpreted in terms of glacial advances. It has generally been assumed that the western- and eastern-source pre-Illinois episode deposits were contemporaneous, but age control is lacking, and sediments reflecting evidence from both source areas have not been found in the same locality.

This review indicates that much work remains to be done, both to fully characterize western-source and eastern-source pre-Illinois sediment sequences and to interpret the timing of their deposition.