The Carboniferous period of geologic history is an important one for Pennsylvania. The rocks from this period are so well preserved and important that geologists all over America refer to the later half of the Carboniferous period as the Pennsylvanian period! The reason the rocks are so well preserved in Pennsylvania lies in the geography of the area at the time. Eastern Pennsylvania was still pretty mountainous at the time – mountains that formed during the Taconic and Acadian Orogenies. Western Pennsylvania, however, was submerged beneath a shallow sea that was cut off from the ocean by those eastern mountains. Sediments washed off of the mountains into that sea, creating a thick pile of well-preserved sedimentary rocks. The margins of that sea were thickly-vegetated swamps. Those swamps would later play a major role in the history of the state because those deposits of dead and decayed plant material ultimately were turned into Pennsylvania’s famous coal deposits! Pennsylvania coal fueled the both the steel-making mills and the cement plants during the industrial revolution.
Further north, the Catskill delta was depositing in New York.
Far off in the distant east, though, on the other side of the ocean, subduction of the oceanic plate was closing the gap between North America and Africa/Eurasia. The light at the end of that tunnel was the headlight of an on-coming train!