Three ancient lakes helped form Hagerman Valley and preserve the fossils found there. The first, Lake Idaho, covered most of present-day southwestern Idaho about three to eight million years ago. Over time, the ancestral Snake River deposited sediment layers into this lake. Those layers are still visible today in the bluffs along the river and are part of what geologists call the Glenns Ferry Formation. About 50,000 years ago, lava flows from the eruption of McKinney Butte to the north dammed the ancestral Snake River. Water trapped behind the lava dam turned Hagerman Valley into Lake McKinney. Sediments brought in by the river formed the Yahoo Clay, which built up and filled the valley. Isolated outcrops of the clay remain in the fossil beds and parts of Snake River Canyon today, but most of the formation was scoured away when the third ancient lake, Lake Bonneville, breached. Lake Bonneville covered present-day western Utah until about 15,000 years ago, when its natural dam shorelines overtopped and ruptured. The enormous flood that followed lasted about eight weeks, widened and deepened Snake River Canyon, and carved new alcoves, side canyons, and the valley before you.