About 1.1 billion years ago, a great rift valley formed across the North American continent from the Lake Superior region southwest to Kansas, and as it opened, basaltic lavas erupted into it and accumulated to a thickness of up to 20 kilometers in the Lake Superior region. The dark-grey basalt rock that form the St. Croix River gorge came from these rift lava flows. Although continental rifting with volcanism often leads to the breakup of continents and the formation of intervening ocean basins, the Midcontinent Rift in North America did not reach complete continental separation. About 520 million years ago in the Late Cambrian period, Minnesota lay near the equator and shallow seas covered the region, where great thicknesses of mainly quartz sand were deposited. These deposits became the sandstone bluffs along the river valley and the rock faces exposed on the highway road cuts to the south. While most of the region lay flat, the basalts in the Taylor Falls area stood high as rocky, cliff-ringed islands in a tropical sea. About 12,000 years ago, at the end of the Ice Age of the last two million years, melting glaciers receded northward and sent tremendous quantities of sediment-laden glacial meltwaters into the St. Croix River valley. At Taylor Falls, a colossal torrent of abrasive currents carved down through the Cambrian sandstones and deep into the rift basalts now seen on the valley floor, and the spectacular potholes in the lava rock in Interstate Park attest to the high velocity of the glacial waters that carved the St. Croix River gorge.