Iowa, US

Paleontology and geology

The Precambrian: A small amount of Precambrian rock is exposed in the northwest corner of Iowa. This rock is metamorphic and no fossils are known from it. The Paleozoic: Warm, shallow seas covered much of what is now Iowa during most of the Paleozoic. These waters were home to a myriad of marine organisms, and their fossils can be found in the rocks formed from sediments deposited on the ancient sea floor. Particularly abundant are brachiopods, trilobites, cephalopods, corals, molluscs, and bryozoans. Later in the Paleozoic, great coal swamps covered much of the state. The seas returned periodically, flooding the swampy coast. This resulted in a depositional pattern of alternating marine and non-marine sediments called cyclothems. Fossils of lobe-finned fish, amphibians, and plants have been recovered from the rocks formed from these sediments. The seas withdrew from the state by the end of the Paleozoic, and the surface of the future state of Iowa was exposed to a time of extensive erosion. The Mesozoic: The state remained above sea level for the early part of the Mesozoic before another shallow sea returned to flood the landscape. The shoreline of this shallow sea eventually retreated to the west, leaving behind isolated areas of evaporating seawater. When the brines in these areas became sufficiently concentrated, gypsum crystals formed and built up on the sea floor. River systems developed on the newly exposed land surface, and the floodplains and coastal lowlands became covered with lush subtropical vegetation. The sea returned late in the Mesozoic and flooded the area. Plesiosaurs swam in these waters, and their fossils have been recovered in the western part of the state. The Cenozoic: There are no known rocks or fossils from the early part of the Cenozoic in Iowa. The state lay above sea level, and this was a time of erosion. In the late Cenozoic, great continental glaciers flowed down from the north. The sediments left by the melting ice were deposited across most of the state. Among the most famous are vast deposits of wind-blown silt (loess) in western Iowa, thick enough to have their own hilly topography: the Loess Hills. Fossils of mastodons and mammoths have been found throughout Iowa.