Contemporary with the Waterpocket Fold's formation was the development of an intermontane (between mountains) basin in the area. Lake Uinta filled this basin with stream water derived from the north and south. This large lake existed from about 58 million until 35 million years ago and is responsible for creating the Flagstaff Limestone and Green River Formation, which locally reach a thickness of around 200 feet (60 m). Elsewhere these formations have a combined thickness of over 9,000 feet (2,740 m). The Flagstaff is a white-colored, fossil rich layer that is composed of limestone, tufa and conglomerate that erodes into ledges and slopes.