Big Island Camp Fortunate stood near the base of the prominent island in Clark Canyon Reservoir. Before submergence in 1964, the singular mountain in the open plain described by Meriwether Lewis was a limestone rock outcrop that separated the two forks of the Jefferson River. The rock remains prominently exposed today because of uplift of the ground and slow erosion of the limestone in the arid climate of southwest Montana. This Mississippian limestone, known to geologists as the Madison Group, formed about 360 million years ago when South Montana lay beneath a tropical ocean and seashells accumulated on the sea floor, were buried, and were cemented into limestone. That ancient marine setting was inhabited by fish, sharks, squid, corals, crinoids, brachiopods, bryozoans, and many other animals, and fossil remains of some of the most common forms can still be seen in the rocks. Corals lived in mineralized cups called thecae and filtered the water for food; brachiopods were bottom-dwelling filter feeders with two valves; bryozoans formed twig-like colonies of animals with sticky tentacles; and crinoids were marine invertebrates related to starfish and sea urchins whose arms captured plankton. In the broader geologic history of the area, the Madison Group limestone was deposited during the Paleozoic Era, later sedimentary rocks were laid down with folding and thrust faulting during the Mesozoic Era, and in the Cenozoic Era sedimentary and volcanic rock were deposited as the Earth's crust extended and the Red Rock Valley and Tendoy Mountains formed.