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 The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

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.

A complex sequence of deformation produced the major central and southern trends of the Appalachian fold-and-thrust belt in the Roanoke recess, Virginia. The incipient recess first experienced the Appalachian-wide stress field, then shifted to far-field effects from incremental counterclockwise rotation of the shortening direction, which resulted in the Central Appalachian fold belt, and then shifted to incremental clockwise rotation, which produced the Southern Appalachian fold-and-thrust belt. We analyzed joints, veins, normal and reverse faults, stylolites, and paleoseismites from Mississippian strata at the structural front of the Southern Appalachian fold-and-thrust belt, and the adjacent Appalachian Plateau west of the recess. We distinguished seven deformational events using orientations, intersection relationships, fault-slip directions, and mineralization histories. Five of these sets represent late Paleozoic deformational events (A1–A5), with shortening directions that show an evolving Alleghanian fold-and-thrust belt in the recess. A1 (shortening trend 085°–265°) is consistent with the previously determined Appalachian-wide stress field and incipient layer-parallel shortening strain in middle Mississippian carbonates. A2 (trend 145°–325°) is a newly recognized event, herein called the Princeton event, which is consistent with dominant orientations of previously determined layer-parallel shortening strain, clastic dikes in Upper Mississippian strata, and stylolites. These far-field effects may mark high-angle basement faulting associated with development of the foredeep bulge during incipient thrusting along the Pulaski thrust system far in the hinterland. A3 (trend 120°–300°) corresponds to initiation of the major Central Appalachian deformation, which resulted in fold-and-thrust belt structures such as the North Mountain fault and Wills Mountain anticline, while A4 (trend 160°–340°) is associated with Southern Appalachian (e.g., St. Clair thrust and Glen Lyn footwall-syncline) structures. A5 (trend 010°–190°) represents late Alleghanian deformation of the Glen Lyn syncline, likely associated with blind thrusting coeval with emplacement of the nearby Pine Mountain thrust sheet. Two post-Alleghanian fracture sets, PA1 (joint trend 150°–330°) and PA2 (joint trend 060°–240°), are orthogonal; PA2 is younger. These joint sets are associated with strike-slip and normal faults that are compatible with some fault-plane solutions from the nearby Giles County seismic zone.

Continental crust is recycled into orogenic forelands by the distinct but inter- related processes of tectonic imbrication and sedimentary dispersal. Tectonic loading by the orogen drives flexural subsidence of a foreland basin, and the orogen provides a source of sedimentary detritus to fill the basin. Detrital zircons in Pennsylvanian-age sandstones in the Appalachian (Alleghanian) foreland basin reflect an Alleghanian orogenic source of recycled and primary detritus from Grenville-age basement rocks and Iapetan synrift rocks, which also yield pre-Grenville recycled craton-derived detrital zircons. Minor contributions are from Taconic- and Acadian-age plutons and accreted Gondwanan terranes. Ages of the detrital zircons show that most of the synorogenic clastic-wedge sediment was recycled from older continental crustal rocks. Cratonward thrusting of crystalline thrust sheets over the foreland recycles continental crustal rocks from the continental margin and thickens the continental crust. Along the Alleghanian foreland, the Blue Ridge thrust sheet of crystalline basement rocks and Piedmont thrust sheets of metasedimentary rocks represent imbrication and thickening of rocks of continental crustal composition. Both tectonic imbrication in foreland thrust sheets and sediment dispersal into the foreland basin recycle and thicken continental crust.

Abstract The Hudson Valley fold-thrust belt of eastern New York State involves a relatively thin sequence of shallow-marine Silurian and Lower Devonian strata. Because of the thinness of this sequence, structures of the belt are relatively small. Thus, first-order ramps and flats, and fault-related folds can be seen in their entirety at a single roadcut. A field trip in the southern half of the Hudson Valley fold-thrust belt, from the latitude of Catskill to the latitude of Rosendale, provides an opportunity to see many examples of these structures, and to discuss the three-dimensional architecture of fold-thrust belts in an orocline. In particular, we will see how along-strike changes in stratigraphy affect fold wavelength and the depth of detachment horizons. The trip also provides the opportunity to examine the relationship between mesoscopic structures (e.g., solution cleavage and veining) and first-order structures.