Following I-70 eastward from central Missouri to central Ohio, as we did today, one crosses relatively flat terrain, broken only by creek and river valleys. After all, the Pleistocene glaciers scoured the bedrock of this region and left a thick layer of glacial till as they retreated into Canada. In Missouri, from Columbia to St. Louis, their effects were limited; as a result, the terrain is a bit more hilly and outcrops of bedrock are seen in some areas. Columbia sits on a thick bed of Mississippian limestone which is evident at roadcuts and along stream beds throughout the city and its surrounding countryside; another prominent outcrop of bedrock is seen along the deep Loutre River valley, where a seam of Ordovician sandstone crosses the highway.
Once the traveler moves east of the Mississippi River, however, such outcrops all but disappear, primarily due to a thick layer of overlying glacial till. While Carboniferous rocks underly I-70 throughout all of Illinois and western Indiana, narrow swaths of Devonian sediments run through Indianapolis, Indiana, and Columbus, Ohio, and the Interstate runs above Silurian bedrock through eastern Indiana and western Ohio, these basement rocks cannot be seen from the highway. Indeed to catch a glimpse of the Paleozoic sedimentary rocks that underlie most of the Glacial Plain, one must visit deep river valleys and gorges (such as Clifton Gorge, near Springfield, Ohio).
Though geology stares us in the face across the American West, New England, the Upper Great Lakes and the Appalachian Chain, it hides beneath glacial till throughout most of the Midwest. Of course, that till (and the rich soil that it produced) once supported a vast tallgrass prairie and now nourishes the Great American Cornbelt.
