Pleistocene deposits of the north-western Allegheny Plateau, U.S.A

Abstract

In North America Pleistocene ice sheets entered the lowland now occupied in part by Lake Erie and expanded far southward in several lobes in the Interior Plains, but less far in two lobes in the Allegheny Plateau to the east in north-eastern Ohio, north-western Pennsylvania and western New York. This maturely dissected plateau, composed of essentially horizontal Carboniferous strata, has a summit level ranging from about 1200 ft in the west to about 1800 ft in the east. In recent years excavations as deep as 135 ft for strip mines (coal), highways and quarries (limestone) in that part of the plateau in Ohio and Pennsylvania have exposed complete sections as much as a mile in length, which reveal lower drifts, mainly till, that have been previously almost unknown. Each till sheet has a characteristic texture and mineral composition and separate sheets can be traced for more than 100 miles, and treated as rock stratigraphical units. The younger tills are much higher in clay than the older tills. The feldspar content of the sand fraction ranges from about 5 per cent in the oldest till to 30 per cent in the youngest till. Carbonates range from less than one per cent in the oldest till to about 15 per cent in the youngest. Within each till sheet there is a decrease in feldspar and an increase in quartz toward the margin. Statistical studies, in part by trend-surface analysis by computer methods, confirm the regularity of this variation, which is ascribed to dilution by local feldspar-poor sandstone. The dilution is much greater in the older tills; at their margins 90 per cent of the quartz content may be locally derived. The lowest till, of pre-Illinoian age, entirely a sub-surface unit, is everywhere thoroughly weathered. The overlying till of probable Illinoian age, consists of two sheets, the upper one of which has a thick palaeosol. The bulk of the drift is early Wisconsinan in age and consists of three or more till sheets. It is named (in Pennsylvania) the Titusville Till and has a 14 C age of about 40 000 years. It is sandy, very compact and oxidizes to a distinct olive-brown colour. The weathering on it is not as intense as that of the two lower tills. The oldest till of late Wisconsinan age, the Kent Till, is a distinctive coarse yellow-brown, sandy till. It is overlain by the chocolate-brown, silty, more highly calcareous Lavery Till, which in turn is overlain by the brown, silty-clayey Hiram Till. Each of the older tills, except the pre-Illinoian, extends farther south and south-east than the later tills, so that each forms the surface drift in a belt, ranging from 2 to 20 miles in width, in which its surface morphology and soil characteristics can be studied and its general character compared with that in the sub-surface.