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 provides detailed itineraries of three of the geological field trips related to the 2017 joint meeting of the GSA Northeastern and North-Central Sections in Pittsburgh. The first chapter outlines a walking trip of downtown Pittsburgh and the escarpment to its south, consisting of seven “Pitt stops” investigating geological, archaeological, and historical aspects of the Gateway to the West. Venturing further afield, the second chapter describes a trip that explores periglacial features as far as the Upper Youghiogheny River basin in Maryland and the Laurel Highlands of Pennsylvania. The third chapter investigates hydrologic aspects of the 1889 Johnstown, Pennsylvania, flood, largely following the progress of the flood from its point of origin to the city of Johnstown.

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.

Abstract The Pittsburgh region has long been recognized as one of major landslide activity. This results from the geology and geomorphic processes shaping the region. The underlying bedrock of flat-lying interbedded strong and weak sedimentary strata has been acted upon by erosion, stress relief, and mass wasting, including creep and landsliding processes, to produce masses of marginally stable colluvial rock and soil on many of the steep hillsides common to the region. Landsliding often involves re-activation of such rock and soil masses. Recent landsliding is often triggered by heavy precipitation and by human activities, i.e., slope excavation, fill placement, and changes in long-established patterns of surface and subsurface drainage. This field trip has four stops, all within 20 mi of downtown Pittsburgh. Each stop is along a transportation corridor (railroad, local road, and two along an interstate highway). Each stop has various sized examples of the types of landslides common to the region. Most of these examples involve reactivation of unrecognized colluvial landslide masses.