Glacial ice was the most powerful geological force shaping Minnesota, as continental glaciers spread over the state at least four times during the last million years in long cold periods, with each advance followed by widespread retreat. The last glacier to invade Minnesota from Canada came by the low valley of the Red River of the north, moved southward across the state, and advanced as far as Des Moines. During its slow retreat 11,000 years ago, the Glacial River Warren formed the valley in which the Minnesota River now flows, and this valley remains an impressive reminder of the volume of water it once carried. Big Stone Lake, now the headwaters of the Minnesota River to the northwest, formed behind a delta-like barrier of sand and silt deposited across this ancient drainage channel by South Dakota's Whetstone River.