During the Cambrian, Pennsylvania sat just slightly south of the equator as the Iapetus ocean transgressed (spread inland) across North America.  Although much of North America remained uncovered and exposed to erosion, Pennsylvania was covered by relatively shallow waters that provided an environment for sediment deposition and the development of life.  Northwest Pennsylvania was covered with sandy sediments, while the rest of the state was covered with carbonate sediment. This is how the limestone that we find in central Pennsylvania was formed.

The Ordovician began with a bang...literally.  East of North America and across the Iapetus Ocean subduction began and a volcanic arc was formed.  Some of the volcanic material that was spewed fell onto North America and can be found in the exposed carbonate rocks of central Pennsylvania.  Over the course of 40 million years, the volcanic arc moved closer to North America. 

During the late Ordovician, material from the volcanic arc and from the floor of the Iapetus Ocean was thrust upon the North American plate.  This event, called the Taconic orogeny, caused mountains to form where material was thrust onto the continental plate.  With all this added weight, the plate margin subsided creating the Appalachian basin west of the mountains.  Since the Taconic mountains lay east of Pennsylvania, the basin covered the state and filled with water and sediments.  Originally, the sediments were silt and clay, but they eventually turned to clastic sand and gravel by the end of the Ordovician.  This is the origin of the sandstone and shale exposed in many places in central Pennsylvania.

The Taconic mountains continued to be a source of sediment throughout the first half of the Silurian.  Sand and gravel (both made mostly of quartz) were deposited on the eastern and western parts of Pennsylvania, creating a very erosion-resistant sandstone.  This sandstone is called the Tuscarora Formation, and makes up the crests of some of the long ridges in Pennsylvania's Appalachian Mountains today.  Intense weathering of the Taconic Mountains carried iron into the marine basin, which formed the iron deposits that were mined in Pennsylvania through the 1800's. 

During the second half of the Silurian, the Taconic Mountains were so eroded that they were no longer a major source of sediment.  Carbonate deposition resumed in the Appalachian basin, and limestone and dolomite formed again in central Pennsylvania.  To the northwest, halite and gypsum were crystallizing out of a shallow evaporating basin.  These evaporite deposits are extensively mined in New York and Ohio.

Carbonate deposition continued into the beginning of the Devonian.  But after a few million years, two landmasses, Avalonia and Europe, collided with North America.  This collision, called the Acadian orogeny,  created the Acadian mountains to the east of Pennsylvania and closed the Iapetus Ocean.  Sediments were eroded from the Acadian mountains into the Appalachian basin.  We see these deposits today as black and gray shales and siltstones.

As deposition into the basin continued, an alluvial plain was formed which allowed course-grain sediments to settle on the alluvial plain, fine-grained sediments near the shore, and very fine-grained sediments in deeper water farther west.  As the alluvial plain grew and the shoreline moved westward across Pennsylvania, non-marine sediments were deposited on top of marine sediments.  Alternating rock layers now show that a regression was in progress.

Many of the sediments that were deposited at this time can be seen today as red clastic rock.  Iron oxide creates the red color and indicates that the sediment was deposited above sea level and in a seasonally dry climate.  Sediment from the wet seasons are also evident in the fossils found in certain layers of rock.

The name "Carboniferous" refers to two very important periods for Pennsylvania - Mississippian (older) and Pennsylvanian (younger).  Precipitation increased and seasons were less distinct during the beginning of the Mississippian, so sediment was no longer red, but instead gray due to a lack of oxidation.  Later, Acadian sediment deposition declined, and fine-grained sediments turned red once again from oxidation possibly due to the return of seasonal climate differences.  At this time, carbonates were still being deposited in western Pennsylvania in a shallow sea. 

Toward the end of the Mississippian, waters were leaving Pennsylvania and the Appalachian basin, leaving behind swamps and lush forests.  The climate was warm and moist since Pennsylvania still sat only 5-10 degrees south of the equator. 

As the Pennsylvanian began, plants such as lycopsid ( type of tree), as well as ferns and tree ferns, were dropping great quantities of dead trunks, leaves and other matter into the swamps.  This plant material was protected from oxidation and rotting by the water.  All of this plant matter gradually accumulated as peat and was covered eventually by sedimentation.  New swamps would form and repeat this process.  Over time, these peat marshes were compacted into coal, which later gave rise to the coal mine industry in Pennsylvania.

Ten feet of peat produces only one foot of coal, so a massive amount of plant matter must have been available in these Pennsylvanian swamps to produce the hundreds of feet of coal that is found in Pennsylvania.

During the Permian, Africa collided with North America in what is called this Alleghanian orogeny.  This orogeny was much larger than either the Taconic or Acadian orogenies, and it represented one of the last collisions in the formation of Pangaea.  It deformed the Appalachian basin forming mountains that were 150 miles wide, 750 miles long, and 2.5 miles high.  The western part of these Alleghanian mountains (not the Allegheny Mountains of today) was located in eastern Pennsylvania. 

The Alleghanian orogeny also formed thrust faults, fractures, anticlines, and synclines throughout Pennsylvania.  Great amounts of sediment were eroded from the Alleghanian mountains and deposited in the Appalachian basin, but little evidence is left of that because it has since been eroded.  Pennsylvania has never again been a depositional environment.