An Outlier of Appalachia

Driving southwest on I-65 from Louisville, Kentucky, one soon climbs from the Devonian sediments of  Greater Louisville into the early Mississippian limestones that underlie most of central and western Kentucky; this rather steep ascent, known as the Muldraugh's Hill Escarpment, yields an elevation gain of 600 feet.  Leveling out near Elizabethtown, the highway continues southwestward to Bowling Green; along this segment, one notices hills to the west that, composed of upper Mississippian limestones, comprise the Dripping Springs Escarpment.  Mammoth Cave, perhaps the most extensive network of caves and underground streams on the planet, courses through this limestone.

This layer of upper Mississippian limestone curves northwestward at Bowling Green and continues into southern Illinois and southwestern Indiana, bisected by the Ohio River.  Interior to this broad shield of limestone is a less extensive cap of Pennsylvanian sandstone, bordered by its Pottsville Escarpment.  Streams throughout this unglaciated Tristate plateau (known as the Shawnee Hills, especially in Illinois) have sculpted the Carboniferous (Mississippian and Pennsylvanian) sediments into a maze of ridges and valleys, typical of the Appalachian Plateau to the east.  Indeed, the component rock formations of the two plateaus are identical, suggesting that they were once connected.

Today, the Glaciated Plain of eastern Indiana and western Ohio, the Lexington Peneplain of northern Kentucky and the Highland Rim of central Kentucky stretch between the Shawnee Hills Province and the Appalachian Plateau.  These intervening regions consist of older, horizontal marine sediments, exposed by glacial and stream erosion; the exposed rocks of Greater Cincinnati and the Lexington Peneplain are of Ordovician age, rimmed by Silurian deposits in eastern Indiana and western Ohio, Devonian sediments in central Ohio and Greater Louisville and lower Mississippian limestones in central Kentucky.  In the distant past, the upper Mississippian limestones and Pennsylvanian sandstone of the two plateaus likely covered these older geologic deposits (i.e., the Appalachian and Shawnee Hills Plateaus were contiguous).