From the Appalachian Plateau to the Central Plains and from the Great Lakes to the north edge of the Gulf Coastal Plain, almost all of the exposed bedrock was deposited in the Paleozoic Era. This second major division of geologic history, 600 to 225 million years ago, stretches from the appearance of shelled marine life to the rise of mammal-like reptiles.
Though older, Precambrian rocks underlie these Paleozoic sediments, they are exposed only in the northernmost region of the Great Lakes (where the glaciers scraped off younger rocks) and in southeast Missouri, where an uplift of this ancient basement has been carved into the St. Francois Mountains. Throughout the Paleozoic Center of North America, younger rocks are primarily limited to poorly compacted sands, gravels and loess from the Pleistocene Epoch; these recent deposits are found along the major river channels, across the Coastal Plain and atop the Paleozoic bedrock in the northern Midwest, where a thick layer of glacial till obscures the underlying strata.
To the north, the Paleozoic Center is bounded by the Precambrian igneous and metamorphic rock of the Canadian Shield while, to the south, it blends into the sandy soils of the Gulf Coastal Plain. On its east side is the Precambrian spine of the Smokies and Blue Ridge Mountains and, to its west, covering most of the western High Plains, are the deposits of the Cretaceous Sea and Tertiary debris from the Rockies and western volcanoes. It is in the Heartland where the Paleozoic is preserved!
