THE FINGER LAKES: A LONGER LOOK

It is well known that the Finger Lakes troughs of central New York are eroded primarily into shales of the Hamilton and Genesee Groups of Middle and Upper Devonian age, but the details of that lithologic control have never been elaborated. The Devonian strata thin northwestward across the Finger Lakes region. Eastern coarser-grained facies also grade northwestward into shales, although with numerous lateral fluctuations. The two largest Finger Lakes troughs cut across the widest exposure belt of thick erodible shales, which grade eastward from Cayuga Lake into more resistant siltstones and sandstones, and thin drastically westward from Seneca Lake under more resistant Upper Devonian clastic strata. The smaller eastern and western Finger Lake troughs are eroded across much narrower shale exposure belts.

Late Paleozoic epeirogenic uplift, southward tilt, and erosion combined to strip an unknown extent of the former cover of Paleozoic strata from the Canadian Shield and regions southward. Published studies report that the basal Pennsylvanian quartz-pebble Sharon conglomerate in northern Ohio, which rests uncomformably on Mississippian sandstone and shale, includes silicified Devonian fossils derived from sources now north of Lake Erie, implying regional ancestral south-flowing drainage systems. Also, in northwestern Pennsylvania, lower Pennsylvanian Pottsville strata disconformably onlap northward over Mississippian strata that had been previously eroded into a cuestaform subaerial topography. Therefore, it can be hypothesized from the evidence to the west that as early as late Paleozoic time, a cuestaform fluvial landscape had evolved over the Finger Lakes region, well adjusted to lithologic controls. Much later, the Finger Lakes troughs were deeply eroded into and across an ancient set of river valleys by multiple Pleistocene ice sheets.