The potholes at the St. Croix Dalles were carved into dark volcanic basalt formed by lava eruptions 1.1 billion years ago, basalt related to flows along the North Shore of Lake Superior. About 500 million years ago, shallow tropical seas buried this basalt beneath sand that later became sandstone. During the last two million years, glaciers repeatedly advanced through the Lake Superior basin and covered the Taylors Falls area. Near the end of the most recent glaciation about 12,000 years ago, meltwater gathered at the southwestern edge of a receding glacial lobe in the Superior basin to form Glacial Lake Duluth. When that lake drained, immense floods rushed south through the St. Croix River valley as Glacial River St. Croix because ice still blocked the basin's eastern outlet to the lower Great Lakes. Rapid erosion of spillways around Glacial Lake Duluth released tremendous amounts of water in a short time, cutting through glacial sediments and sandstone and deepening the valley. Where the current struck the hard basalt, fierce rapids, swift turbulent water, and heavy sediment loads provided the abrasive force needed to carve potholes. These cylindrical holes formed as eddies behind large boulders or other obstructions swirled pebbles and cobbles in place against the bedrock. Their unusual depth and abundance show the enormous power of Glacial River St. Croix.