Michigan, US

Paleontology and geology

The Precambrian: Some of the oldest rocks in North America are exposed in Michigan's Upper Peninsula. Among these are lightly metamorphosed sedimentary rocks that contain some of the oldest fossil evidence of complex life, including filamentous algae. The Paleozoic: Paleozoic rocks are well represented in Michigan. This part of North America lay near the equator in the Early Paleozoic and fossils indicate that trilobites and brachiopods proliferated in the warm, tropical Cambrian waters that covered the continent. This shallow sea persisted through the Ordovician, Silurian, and Devonian Periods. During this long interval of time, a typical marine fauna dominated by brachiopods, trilobites, crinoids, and corals thrived. Fossils of these organisms are abundant in Michigan’s Paleozoic rocks. The shallow seas that had covered Michigan for so much of its early history withdrew for the last time during the early part of the Carboniferous. In the Late Carboniferous, nearshore coal-forming swamps characterized Michigan. Fossils of ferns, scale trees (Lycophyta), and other plants dominate the fossil record of this time period. The Paleozoic and Mesozoic: The Permian, Triassic, and Cretaceous Periods are not represented in Michigan. Any sediments (and the fossils they may have contained) that were deposited during these time intervals were subsequently removed by erosion. The Jurassic Period (between the Triassic and Cretaceous) is known only from drill cores of red sandstone that contain fossil plant spores of Jurassic age. The Cenozoic: There are no sediments or fossils from the Tertiary, as erosion outstripped deposition in Michigan's continental environment; however, Quaternary glacial deposits are abundant. During this time, glaciers carved the Great Lakes and sculpted the present day landscape of lakes, hills, and swamps. Pine and spruce forests covered the region during times when the great ice sheets retreated. Fossils from this time indicate that mammoths, mastodons, musk ox, and giant beaver roamed the Quaternary lands.