Lake Cut Bank was a glacial lake in present-day Montana during the late Pleistocene, created when the Keewatin ice sheet blocked the Cut Bank Creek valley and formed an ice-dammed basin in the Missouri and Sun river region. Extending for about 20 miles, it was fed by local runoff and widespread glacier melt, including water from the Two Medicine, Cut Bank, St. Mary, and Milk River areas. Its existence is shown by granitic pebbles and boulders dropped from icebergs, thick laminated silts marking the old lake floor, and stratified sand and gravel. Evidence also points to an earlier higher lake level before drainage, after which Cut Bank Creek cut its inner gorge.