Illinois State Museum


Ordovician Sea Geology

During the Ordovician Period, Illinois was part of an ocean-covered platform. It stretched from the ancient Taconic Mountains, in what is now Pennsylvania and New York, to the Transcontinental Arch — an uplifted band of Precambrian rocks that ran from Minnesota to Colorado. The water in this area was probably no more that 61 meters (200 ft.) deep, and in most places, much less.