The rocks of southwestern Pennsylvania are almost entirely sedimentary and were deposited for the most part in the Mississippian, Pennsylvanian, and Permian Periods. The Allegheny Mountain Section, east of the west flank of Chestnut Ridge, consists of strata that are Devonian, Mississippian, and Pennsylvanian in age. The area is characterized by ridges, steep-sided gorges, and valley floors. 

Pittsburgh Low Plateau

The broad Pittsburgh Low Plateau occupies all of Fayette County west of Chestnut Ridge and is underlain by Pennsylvanian and Permian Strata. 

Mississippian System

The Mississippian System falls into three lithologic units, which are quite distinctive. The lowest is the Pocono Formation, which is predominately sandstone; followed by the Loyalhanna siliceous limestone; and the Mauch Chunk formation, which is a mixture. 

Pennsylvanian System

The Pennsylvanian System is divided into the Pottsville series largely composed of: 

  • The Allegheny group consisting of: 
    • Coal
    • Fire Clay
    • Limestone
    • Sandstone
    • Shale
  • The Conemaugh group consisting of: 
    • Limestone
    • Red and Gray Shale
    • Small Amounts of Low Grade Fine Clay
    • Thin Coal Beds
    • Sandstone
  • The Monongahela group consisting of limestone, shale, sandstone, and several beds of workable coal
  • Sandstone and sandy shale

The latter group is present only in the western part of the County. The Washington group of the Permian System consists of shale, sandstone, limestone, and several impure or thin beds of coal.