Seasonally, some 58 million years ago, in the Green River Region of Wyoming, torrential rains would occur in the mountains. Upon flowing down the mountain sides, wide shallow lakes formed in the valley, which would evaporate during the dry seasons (similar to what happens in Africa today). Millions of fish lived and died in a short period of time. This happened in a cycle of every 20 thousand years. Layer upon layer of oil shale formed, then a reddish sandstone which contained no fish was deposited above the oil shale by a large river. Sometime later volcanic eruptions uplifted the land in areas where there were faults in the Earth, exposing fossil fish laden cliff faces.