page_title: Geology of Pennsylvania
section_title: Appalachian Mountain
hierarchical_section_title: Geology of Pennsylvania / Ridge and valley / Appalachian Mountain
caption_attribution_description: English: View west from Clarks Knob in Pennsylvania's ridge and valley region.
context_page_description: The Geology of Pennsylvania consists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economic advantages and geologic hazards and plays an important role in shaping everyday life in the state. They are: the Atlantic Coastal Plain Province, the Piedmont Province, the New England Province, the Ridge and Valley Province, the Appalachian Plateau Province, and the Central Lowlands Province.
  A majority of the rocks in Pennsylvania exposed at the surface are sedimentary and were deposited during the Paleozoic Era. Almost all of the metamorphic and igneous rocks are confined to the southeast portion of the state. A total of four orogenies have affected the rocks of the Commonwealth including the Grenville orogeny, the Taconic orogeny, the Acadian orogeny, and the Appalachian orogeny. The Appalachian event has left the most evidence and has continued to shape the landscape of the state. The Pennsylvania terrain has also been affected by continental rifting during the Mesozoic era.
  Pleistocene glaciers have also repeatedly visited the state over the last 100,000 years.
context_section_description: Standard long, narrow, and steep-sided ridges with narrow valleys define the state in LANDSAT photos. Many of the valleys have karst features due to carbonate rocks that reside in them. Road-building generally follows the valleys and rarely cuts across the ridges. The Pennsylvania Turnpike used tunnels cut through the ridges rather than scaling the mountain tops. Mount Nittany, Tuscarora Mountain, Jacks Mountain, Wills Mountain and Sideling Hill are five prominent mountains in this section.
  The section contains Cambrian- through Pennsylvanian-aged sediments all deposited into the Appalachian Basin. During the Appalachian orogany, these sediments became folded, faulted, and moved around. Only during the past few million years has the landscape we see today taken shape. The relatively softer or easily weathered rocks became valleys while the harder and erosion-resistant rocks became the mountain ridges. The development of this landscape continues to this day.  Uplift of the province has caused rivers to cut water gaps through the mountain ranges, and has continuously presented new softer rocks in the valleys to be eroded away.
caption: View west towards Appalachian Ridge country from Clarks Knob.
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