Can you find traces of the three ancient lakes that helped form Hagerman Valley and preserved the fossils found here? The first, known as 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 geologist call the Glenns Ferry Formation.
About 50,000 years ago the ancestral Snake River was dammed by lava flows from the eruption of McKinney Butte to the north. Water trapped behind the lava dam turned the 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 are present in the fossil beds and parts of the Snake River Canyon today, but most of the formation was scoured away by the breach of the third ancient lake - Lake Bonneville.
Massive Lake Bonneville covered present-day western Utah until about 15,000 years ago, when the natural dam shorelines overtopped and ruptured. The resulting enormous flood lasted about eight weeks, widened and deepened the Snake River Canyon, and carved new alcoves, side canyons, and the valley before you.