ABSTRACT This one-day field trip will visit northern Erie County in northwestern Pennsylvania, USA, and is divided into two parts. The first part will highlight brittle and ductile deformation preserved in Upper Devonian bedrock. It is possible that this deformation occurred in relation to the Alleghenian orogeny during the late Paleozoic. However, some of this deformation displays possible evidence of postdating Wisconsin glaciation, but this interpretation is speculative and based largely on possible topographic disruption of Quaternary river terrace deposits. Morphologic characteristics of outcrops in creeks that expose these tectonic structures will be used as a starting point to discuss structure origin, timing, and their implications. The second part of the trip will visit Erie Bluffs State Park (EBSP) and will examine ongoing mass wasting of bluffs along the southern Lake Erie shoreline. The timing (as determined from dendrochronological analyses) and styles of mass wasting that have occurred at EBSP will be discussed and compared to two other locations along Pennsylvania’s Lake Erie shoreline. An appended road log serves as a geologic guide to the features and processes highlighted on this trip.

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 The Middle Paleozoic section of the Appalachian Plateau exhibits a mechanical stratigraphy defined by layers that emit seismic energy with unique signatures in response to a strain energy accumulated on time scales associated with local, regional, and plate-scale processes. The Earth is in a state of frictional equilibrium, which means that even small changes in effective stress cause brittle failure and the concomitant release of ambient seismic energy. Stress changes as low as 0.001 MPa, the level of stress changes during Earth tides or the transmission of a fluid pressure wave, can activate failure on critically oriented fractures. These phenomena lead to a release of ambient seismic energy, which can be mapped using seismic emission tomography (SET) methods to image fracture networks emitting coherent seismic waves. We used a buried array of 54 sondes to identify active fracture networks over a contiguous volume of 3.76 km 3 within Middle Paleozoic rocks hosting two Marcellus gas shale wells drilled under the Appalachian Plateau of Lycoming County, Pennsylvania, USA. We sampled ambient seismic emissions before and after two stimulations and found that the pattern was repeatable. The fracture patterns illuminated by ambient seismic emissions defined a mechanical stratigraphy populated by clouds of seismic activity separated by packages of beds emitting relatively less seismic energy. The unique attribute of the beds emitting less seismic energy is a lower least horizontal stress (S hmin ) relative to adjacent mechanical units in the section. These low stress beds include the bottom portion of both the Marcellus and Burket/Geneseo black shales. There are three thicker mechanical units carrying clouds of higher energy emissions. These three units include siltstones of the Brallier above the Burket/Geneseo package, silty shale beds of the Mahantango between the Marcellus and Burket/Geneseo packages, and Silurian-Devonian carbonates below the Marcellus package. In map view, emission patterns in the Brallier follow Alleghanian J2 joints. Patterns in the Mahantango are consistent with slip along columnar joint zones like those cutting upward in outcrops of shale on the Appalachian Plateau. In sum, SET reveals a mechanical stratigraphy based on the release of strain energy from three major units of the Middle Paleozoic section.