The Pocono “Mountains” are the southernmost region of the Catskills, a part of the Allegheny Plateau. All of these are part of the Appalachians. The Poconos are the remains of an uplifted sedimentary plain created during nearby mountain building when the Africa and North America plates collided 400 million years ago. The Pocono Escarpment, a sandstone wall, divides the Appalachian Plateau, forming the Pocono Plateau on the northwest, which stands higher than Glaciated Low Plateau.
The Taconic Orogeny (mountain building) in the Ordovician Age (430 million years ago) was the first of three plate collisions on the east coast of North America. The “core” of North America and Greenland was tilted 90 degrees clockwise of its current alignment and was along the equator when it collided with smaller continental masses called the Taconian Terrane. As that land was accreted (joined) to the continent, the edges were overthrust and folded. During the Silurian Age (400 million years ago) these mountains eroded and their sediments filled a low area called the Appalachian Basin.
Avalon Terrane volcanic arc formed in Africa on Gondwana, the southern supercontinent. Avalonia rifted away and collided with tropical Laurasia (North America + Europe). The collision tore Avalonia into pieces now located from Newfoundland to Georgia, and in Ireland, England and France. Avalon was history by the end of the Carboniferous Age (300 million years ago). The Catskill and Pocono region were lowlands or shallow seas closest to the Acadian Highlands built by that collision, so they collected deep sediments as those mountains eroded.
At the end of the Paleozoic Age (245 million years ago), Laurentia (North America) was beginning to collide with the African plate on Gondwana to form the Pangaea supercontinent. It had been forming for 80 million years. Crust between the continents was thrust inland and folded upward to form huge mountains. The Appalachian Basin was folded into the Appalachian Range, with its foreland basin uplifted to form the Catskill-Pocono Plateau.
The Pocono Plateau was buried in ice by the Pennsylvanian glaciation during the Carboniferous Age, by the Illinois glaciation during the mid-Pleistocene Age (350,000 to 130,000 years ago), and by the Wisconsin glaciation, which ended only 10,000 years ago. Recent glacial scour over resistant sandstone is responsible for the absence of deep or wide stream valleys, but the landscape includes U-shaped glacier-carved valleys, as well as terminal moraine deposits.
The Pocono Escarpment, an eroded east facing wall of the plateau, is the most dramatic topographic feature of the Poconos. The relief in the 1,200 to 2,300 foot altitude uplands is usually less than 200 feet, with some hills standing 600 feet above adjacent terrain. Swamps and peat bogs have developed where glacial scour created local depressions. Streams drain from the interior of the plateau toward the margins. An ancient creek from a post glacial lake drained toward the Delaware River, contributing to erosion of the Delaware Water Gap. Bedrock has low porosity and permeability, but secondary fracturing and faulting has created groundwater reservoirs.