The shales and siltstones are eroded more easily than the sandstones. Thus, as erosion proceeds, the slowly eroded sandstones form ridges while the shales and siltstones are eroded more rapidly to form the lowlands. The Appalachian Mountain Section occurs in central Pennsylvania where it is the topo-graphic expression in all or parts of 22 counties. The Section occurs as a northeast-southwest, band that curves from Bedford and Fulton Counties in the southwest to Lackawanna, Carbon, Monroe, and Pike Counties in the northeast. The Section includes all of Huntingdon, Mifflin, Juniata, Perry, Snyder, Union, Northumberland, Montour, and Schuylkill Counties.
Water eroding the ridges and valleys is carried away mainly by the Susquehanna River.
The Potomac and Delaware Rivers drain some of the section in the southeast and northeast, respectively. Typical topography can be viewed almost anywhere within the Section. Aug Park Gorge, Onoko Falls, Pinchot Falls, Pinnacle Rock, Prospect Rock, Pulpit Rock, Raymondskill Falls, Schuylkill Gap, Silverthread Falls, Spring Mountain Thrust Fault, St. Clair Fern Fossil Locality, Stony Ridge, The Cliff, The Pinnacle, The Tubs, Twin Falls, Wallpack Bend, Whirlpool Canyon Falls, Wind Gap, and Wolf Rocks. Scenery of the Section can be experienced by visiting one of the many State Parks situated within the Section. While travelling in the Section, have a picnic at one of the State Forest Picnic Areas .