Twenty thousand years ago, a great sheet of ice called the Superior lobe covered this area. At its farthest advance, it formed the St. Croix moraine, a series of high hills to the west, south, and east. When it melted, it left behind vast deposits of gravel, sand, and clay. After the Superior lobe retreated, another glacier, the Grantsburg sublobe, moved in from the southwest, filling the lowlands with ice. As the climate warmed again, the ice melted and created a lake that covered most of Anoka County, including this spot. Fine sand carried in the meltwater settled to the bottom. The lake disappeared when its water found an outlet into the Mississippi River, leaving the Anoka Sand Plain, an expanse of sand measuring 2,200 square km (850 square miles). Beginning about eight thousand years ago, drier weather and wind formed sand dunes in some areas of the Anoka Sand Plain, and dunes often rise 9m (30 ft) or more above the surrounding land. Meanwhile, low-lying areas of the glacial lake basin are gradually filling with organic matter and evolving into bogs. As the glaciers retreated, they also left behind huge blocks of ice buried by gravel, sand, and clay, and when the blocks melted they left depressions known as kettles. Melting ice created thousands of kettle lakes in Minnesota, and in many places melting glaciers also deposited outwash plains of sand.