The city of Pittsburgh, PA, is located west of the Appalachian Mountains in a moderately to deeply dissected portion of the Appalachian Plateau Province. The relatively flat surface of the plateau is dissected by local drainage from the three principal rivers of the region, the Allegheny, Monongahela, and Ohio. The formation of Pittsburgh's three rivers has a long history dating back to before the Pleistocene Epoch, linked closely to the advance and retreat of continental glaciation, and subsequent meltwaters filling the river channels and eroding the landscape. Pittsburgh was not glaciated; however, periglacial activity and sand-gravel outwash represent two major results of glaciation that affected Pittsburgh and its surroundings.
Western Pennsylvania is associated with the western-most formation of the Appalachian Mountain chain. The Alleghany Orogeny had the most effect on southwestern Pennsylvania. Most of the associated compression was recorded by rock deformation to the east, with some deformation occurring in western Pennsylvania in the form of series of nearly flat-lying, gently warped Paleozoic sedimentary rocks. Rocks cropping out in the Appalachian Plateau Province range in age from Devonian to Permian. Surficial bedrock of southwestern Pennsylvania is associated with deltaic depositional environments that were cyclical in nature, resulting from fluctuating sea levels. Pennsylvanian strata of the region are dominated by thin cyclic sequences of sandstone, shale, claystone, coal, and limestone.
Pittsburgh's strategic location helped to shape westward expansion during the early formation of the United States, largely because of the rivers, which served as an inexpensive, yet efficient means of transportation. The region was considered a stronghold for the emerging country because of its tactical location and later due to its abundance of natural resources, which include coal, natural gas, oil, salt, limestone, sand and gravel, and water.
Geologic hazards present in Pittsburgh and the surrounding area include slope instability, expansive shales and slags, mine subsidence, acid mine drainage, pyritic acid rock, and flooding. Slope instability results from low-shear-strength colluvial deposits and the local Pittsburgh red beds, a notorious claystone responsible for numerous landslides. Because of the region's steep topography, abundant rainfall, low-shear-strength rocks, and soils with low residual strength, landslides have resulted in major property damage and loss of life.
Infrastructure is significant in Pittsburgh. The city began and grew because of the natural river systems, supplemented by man-made canals. Today, the region has 23 navigation locks and dams. The early system of canals was later replaced by rail systems for the shipment of bulk commodities. Allegheny County, the county encompassing Pittsburgh, has more bridges than any other county in the nation. In addition to bridges, the city has 11 tunnels that facilitate vehicular transportation and two locally famous inclines, which were originally used to transport workers up and down the steep topographic feature known as Mount Washington. However, the existing infrastructure of roads, bridges, tunnels, railways, and navigation locks and dams is aging.
Today, Pittsburgh has transcended the legacy name, “Steel City.” Boasting a vibrant downtown, the city has nationally recognized universities and medical centers. While coal continues to be a dominant energy source for the numerous coal-fired power plants in the region, there is resurgence in exploration for natural gas from shales using updated hydrofracturing methods that is changing the energy landscape of the region. Many environmental remediation projects are under way in the region, related to treating acid mine drainage from legacy coal mining, reclaiming land areas of former steel mills, and remediating past production sites of nuclear materials. Maintaining and replacing Pittsburgh's aging infrastructure of roads, bridges, tunnels, dams, and river navigation structures will be a major challenge and will generate work for many years into the future.