ABSTRACT During the Pleistocene, the Laurentian Ice Sheet extended southward into northwestern Pennsylvania. This field trip identifies a number of periglacial features from the Appalachian Plateaus and Ridge and Valley provinces that formed near the Pleistocene ice sheet front. Evidence of Pleistocene periglacial climate in this area includes glacial lake deposits in the Monongahela River valley near Morgantown, West Virginia, and Sphagnum peatlands, rock cities, and patterned ground in plateau areas surrounding the Upper Youghiogheny River basin in Garrett County, Maryland, and the Laurel Highlands of Somerset County, Pennsylvania, USA. In the high-lying basins of the Allegheny Mountains, Pleistocene peatlands still harbor species characteristic of more northerly latitudes due to local frost pocket conditions. Pleistocene fauna preserved in a cave deposit in Allegany County, Maryland, record a diverse mammalian assemblage indicative of taiga forest habitat in the Ridge and Valley province.

ABSTRACT With waterfalls and the deepest gorge in Pennsylvania, Ohiopyle State Park provides opportunities to observe a variety of habitats and three-dimensional (3-D) exposures of the Pennsylvanian sandstone most responsible for shaping Laurel Highlands landscapes. Evidence for the relationship between bedrock, ancient climates, and the landscape can be observed at some of the most scenic natural features of the park: Baughman Rock Overlook, Cucumber Falls, Ohiopyle Falls, Meadow Run Waterslide and Cascades, and Youghiogheny River Entrance Rapid. Channel azimuths and lateral variations in thickness of upper Pottsville fluvial/deltaic sandstone suggest that deposition was influenced by deformation of this part of the Allegheny Plateau during the Alleghanian orogeny. Geologic features of Pottsville sandstone outcrops include a 10-m- (~33-ft-) long Lepidodendron fossil and a 3-D exposure of a meter-high Pennsylvanian subaqueous sand dune and scour pit. Cosmogenic age dating has indicated very slow erosion of hard sandstone in an upland location at Turtlehead Rock and informed estimation of Pleistocene/Holocene waterfall retreat rates of Ohiopyle and Cucumber Falls. Bedrock exposures supporting scour habitats along the Youghiogheny River occur only in a limited area of Youghiogheny Gorge where knickpoint migration and bedrock erosion were relatively recent. Geologic factors, including locations of major tributaries, development of bars that constrict river flow, and proximity of Homewood sandstone outcrops as sources of boulder obstacles in the river, contributed to the class, location, and nature of whitewater rapids in the lower Youghiogheny River.

Abstract This guidebook chapter outlines a walking tour that provides an introduction to the geological, archaeological, and historical setting of Pittsburgh, with an emphasis on the use of local and imported geologic materials and resources in the eighteenth and nineteenth centuries. The focus is on downtown Pittsburgh, the low-lying triangle of land where the Monongahela and Allegheny Rivers join to form the Ohio River, and Coal Hill (Mount Washington), the escarpment along the Monongahela River to its south. Topics include the importance of—and concomitant effect of—historic coal use; use of local and imported geologic materials, including dimension stone used for buildings and gravestones, and chert used for gunflints and millstones; the frontier forts built at the site; and the ubiquitous landslides along Coal Hill.

The Pennsylvanian rocks of the central Appalachians record a progressive change in paleogeography and paleoenvironment, from extensive sea (Pottsville time), to relatively small bay (Allegheny–lower Conemaugh time), to entirely river-influenced lowsalinity bay-lake (upper Conemaugh–Monongahela time), to relatively small lakes of fluvial plain (Dunkard time). Sediments derived mainly from the southeast were dispersed into the elongate sea-bay-lake by prograding deltas. Sediments first filled the unstable geosynclinal trough-basin of southern West Virginia, and then the northeast-trending Dunkard basin of northern West Virginia which developed as a depression in a relatively stable platform. Sea transgression during Conemaugh time was in part tectonically controlled, but shifting delta lobes influenced the distribution of marine shell beds. The West Virginia deltaic complex evolved from a wave-dominant delta with fringing barrier islands (Pottsville time) to a fluvial-dominant delta (Conemaugh and Monongahela time), and facies of Conemaugh and Monongahela rocks are similar to those found in modern shallow-water deltas. Major anticlines were growing structures influencing northeast-trending drainage and facies. Tectonic warping of plateau nearly normal to hingeline orientation of N. 50° E. explains the vertical stacking of sandstone belts approximately 30 mi. wide trending northwest. However, supply frequently over-whelmed basin subsidence burying growing structures under sediment. Consequently, channel sandstones commonly trend across present fold axes and exhibit an offset stacking arrangement resulting from differential compaction.