Around 10 million years ago, Jackson Hole's first large freshwater lake was impounded by east-west fault movement in what is today the southern end of the park. Geologists call this fault-scarp dammed body of shallow water Lake Teewinot and it persisted for around 5 million years. The resulting Teewinot Formation of lakebed sediments sits directly on the Colter and consists of limestones and claystones mixed with volcanic material and fossilized clams and snails. All told, sediments in the Tertiary period attained an aggregate thickness of around 6 miles (10 km), forming the most complete non-marine Tertiary geologic column in the United States. Most of these units within the park are, however, buried under younger deposits.