Pennsylvania, US

Paleontology and geology

The Precambrian: During the Precambrian, Pennsylvania was mainly a low, featureless plain sloping gently southeastward toward the sea. Most of Pennsylvania's exposed Precambrian rocks are metamorphic and igneous, and thus lack fossils. The Paleozoic: Paleozoic rocks are well represented in Pennsylvania. Warm, shallow seas covered much of the state through the early Paleozoic, when Pennsylvania lay at the eastern edge of Laurentia. The seas teemed with marine organisms whose fossils can be found today in many rocks throughout the state. The Carboniferous witnessed the collision of the former continent of Gondwana with Laurentia to form the supercontinent of Pangea. A huge mountain range—part of which would become the Appalachians—arose from this tectonic activity. Vast deltas formed as the newly-uplifted mountains eroded and streams carried the sediments westward across the state. The shallow seas retreated, leaving much of the state as a low-lying, swampy plain, covered with scale trees (lycopods), ferns, and horsetail rushes. By the end of the Paleozoic, the swamps had dried out and Pennsylvania had become an upland region where erosion predominated over deposition. The Mesozoic: Mesozoic rocks are present mainly in the rift basins of the eastern part of the state. These fault valleys developed as tectonic activity pulled apart the supercontinent of Pangea and began opening the Atlantic Ocean. Red beds are the dominant rocks of the Mesozoic in Pennsylvania and are formed from sediments deposited in rivers, streams, lakebeds, and in the rift basins. Lava flows are interbedded with the red beds. Fern spores and pine pollen are common in these rocks, as are footprints left behind by dinosaurs that roamed the landscape. The Cenozoic: The Early Cenozoic is poorly represented in Pennsylvania. There are a few outcrops in the far southeastern corner of the state that have been identified as Tertiary by some geologists, but as Cretaceous by others. The Late Cenozoic is well represented by glacial materials deposited by the Quaternary (Pleistocene) ice sheets that covered much of the state during this time. Fossil plants, including willow and sedge, indicate a tundra-type environment for parts of the state not covered by ice at the end of the Pleistocene.