Ancient, Late Cenozoic Glaciations in Pennsylvania
Evidence exists for at least three ancient glacial advances into northwestern and northeastern Pennsylvania that far predate the capability of radiocarbon dating to establish a numerical chronology for these events. In northeastern Pennsylvania, there are deeply eroded remnants of glacial deposits and erratics beyond the Late Wisconsinan glacial limit, and glacial lake sediments exposed in deep strip mine operations have both reversed and normal magnetic polarities. Bordering such ancient glacial margins are large, in-place, tower-like masses of residual orthoquartzite rock (tors) composed of orthoquartzite sandstone and conglomerate. The tops of these bedrock eminences are extremely unlikely to have been shielded by soil, snow cover, or significant vegetation cover during their exposure histories. They provide ideal targets for the determination of values of cosmogenically-produced radionuclides of 10Be, 26Al, and 36Cl. Diamond-drilling has been undertaken, and will continue, as we obtain core samples from tor tops in sites that strategically mark ancient glacial margins. Control samples will also be taken from scoured tor sites within the Late Wisconsinan, Late Glacial border that retain some glacial striations on the bedrock surface.
Green Mountain, Pennsylvania, area tor. Areas of deeply-weathered tors occur beyond putative margins of ancient glaciations in northeastern Pennsylvania, and should contain high cosmogenic radionuclide values. Alternately, expected values may be less; cold-based ice may have protected preexisting tors from erosion, concomitantly shielding them from cosmogenic radiation dosage during one or more episodes of glaciation.
G. Michael Clark
Department of Earth and Planetary Sciences
1412 Circle Drive
Knoxville, TN 37996-1410
Phone: (865) 974-6006
Email: clarkgmorph@utk.edu