The Minnesota River Valley preserves a vast record of geological and human history. Rocks formed 3.8 billion years ago, among the oldest in the world, lie exposed on the valley floor as remnants of a fiery young earth, when molten rock from the planet’s interior pushed against the crust, deformed it, and created mountains four miles high. Over eons, water and ice eroded those mountains into a subdued plain. At the close of the last ice age, 12,000 years ago, mile-high glaciers melted and formed Glacial Lake Agassiz to the north, whose outlet, Glacial Lake Warren, sent a torrent that carved the broad valley, streamlined rock outcroppings, and filled the air with the roar of rushing water and tumbling boulders. Today the gentle Minnesota River flows quietly in the shadow of that prehistoric past. Across the river, spearheads, knives, hide-scrapers, and other stone tools found with extinct forms of bison show that humans occupied the valley 6,400 years ago.