The Pennsylvanian Period lasted from 320 to 286 million years ago. During the Pennsylvanian Period, widespread swamps laid down the thick beds of dead plant material that today constitute most of the world's coal . The term Pennsylvanian is a U.S. coinage based on the frequency of rocks of this period in the state of Pennsylvania; internationally, the terms late Carboniferous Period or Silesian Period are preferred.
Although most artist's conceptions of the Pennsylvanian Period emphasize its prolific swamps, these were characteristic only of the equatorial regions. The Southern Hemisphere, which was dominated by the huge continent Gondwana, underwent a series of ice ages during this period. These ice ages sequestered water in times of ice growth and released it in times of melting , causing the ocean to cyclically regress (uncover coastal lands) and transgress (cover coastal lands) around the world. Repeating sequences of sedimentary rock layers record these changes in sea level.
From the bottom up, a typical sequence is sandstone , shale, coal, limestone , and sandstone again. Each such unit is termed a cyclothem and was formed as follows: (1) As ice melted in Gondwana, seas rose globally. Rivers and streams deposited sand and gravel in the coastal lowlands as they sought new equilibrium profiles (i.e., stable altitude-vs.-distance cross-sections). This sand layer eventually became sandstone. Although the coastal zones where sandstone deposition was taking place at any one time were narrow, larges areas were blanketed by these sediments as the seas rose and coastlines swept slowly inland. (2) As the rising sea neared a given location, a lush coastal swamp developed. This deposited a thick layer of dead leaves, tree trunks, and other organic material rich in carbon that would eventually form coal. (3) When the sea finally submerged the swamp, a shallow marine environment appeared. The remains of shelly marine animals built up on the sea floor and eventually became limestone. (4) Ice began to build again in Gondwana, and sea levels began to drop in a new phase of regression. (5) Erosion of re-exposed coastal lands scraped off the topmost sediments left by the last transgression, including some of the limestone layer. (6) Ice began to melt again in Gondwana, triggering a fresh cycle of transgression.
As many as 90 cyclothems have been found in one place, one on top of the other. Each such cylothem records a complete climatic cycle like the one described above.
The first reptiles evolved during the Pennsylvanian Period. These were small (about a foot long) and outnumbered by the amphibians, which were prosperous, diverse, and achieved lengths of up to 15 ft (4.6 m). Insects also throve; dragonflies with 2.5-ft (0.76 m) wingspans were common. Over 1,000 species of Pennsylvanian cockroach have been identified, giving this period the alternative, informal title of the "age of cockroaches."
See also Archean; Cambrian Period; Carbon dating; Cenozoic Era; Continental drift theory; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Geologic time; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Paleozoic Era; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Precambrian; Proterozoic Era; Quaternary Period; Silurian Period; Tertiary Period