Early geologic research into glacial deposits held that the Ice Age in North America had only four periods of glaciation, but fourteen episodes are recognized today, five of them in Minnehaha County: late Wisconsin, Illinoian, and pre-Illinoian 1, 2, and 3. The Cactus Hills consist of sand and gravel left more than 300,000 years ago by streams flowing from a receding pre-Illinoian glacier, marking the place where the ice front stalled before continuing its retreat. These outwash deposits were originally more extensive, but later erosion by late Wisconsin meltwater removed much of the original material. Remnants of the deposit, including the Cactus Hills, remain at higher elevations on both sides of the Big Sioux River valley, and geologically they are part of a series of kames and terraces extending northeast toward Garretson, South Dakota.