The glacially produced features and numerous fossils found in the ancient limestone bedrock are one of the park's biggest attractions.  Four hundred million years ago, most of the United States was covered by a shallow saline sea. It was during this period

that the limestone strata at Clark Reservation were laid down. These layers of limestone contain the fossil remains of plants and animals which lived millions of years ago, including honeycomb and horn corals, brachiopods, and sponges.
 

More recently, about 20,000 years ago, the last of several glacial advances occurred in this area. A mile-thick ice sheet extended from Canada to Pennsylvania. About 10,000 years ago, this glacier began to retreat. During its recession, meltwater torrents carved out spectacular features at Clark Reservation, including the glacial plunge pool basin (containing the lake) and a system of amphitheaters and ravines. Geologists speculate that the glacial waterfall contained a volume of water greater than the American Falls at Niagara. The lake which occupies the plunge basin is classified as a glacial cataract lake, and is one of the finest examples of its kind in eastern North America.