The relatively small glaciers in the United States today, such as those in the northern Rocky Mountains, are concentrated in valleys and are called valley glaciers. The glaciers that covered Illinois were parts of huge ice sheets that extended over much of the North American continent and are called continental glaciers. They spread over most of Canada, then pushed southward to bury New England and a great area in the north-central part of the United States north of the Ohio and Missouri Rivers. Formation of Gravel and Sand Deposits As the glacial ice edged slowly southward from Canada, it froze fast to and picked up soil and loose pieces of rock, with enormous force tore away huge chunks of bedrock, and mixed and ground these materials together (fig. 17). Into Illinois the glacier carried rock materials from Canada, Wisconsin, Minnesota, and Michigan; other rock fragments were picked up in Illinois as the ice front advanced. When the glacier melted, it left behind its load of rock flour and rock fragments, much of it as a gray clay containing pebbles, cobbles, and boulders. Geologists call such deposits glacial till. [Illustration: Figure 16—Extent of the exposed deposits of the Wisconsinan, Illinoian, and Kansan glaciers in Illinois, and the unglaciated areas of the state.] UNGLACIATED WISCONSINAN GLACIER Freeport Fulton Peoria Decatur Charleston ILLINOIAN GLACIER Kewanee Waterloo Carbondale Harrisburg KANSAN GLACIER UNGLACIATED Hardin The ice in the continental glaciers usually crept forward, sometimes slowly, sometimes more rapidly. Whether the front of a glacier moved forward or back depended on the balance between the rate of forward motion of the ice and the rate of melting. When the ice advanced faster than it melted, the front of the glacier moved forward. When the glacial ice melted faster than it moved forward, the front of the glacier receded. When the rates of melting and advance were about equal, the front of the glacier stood still or moved back and forth in a narrow zone. [Illustration: Figure 17—Striated boulder. Scratches and flattened surfaces were caused by abrasion by other rocks while boulder was embedded in glacial ice.] When such a more or less stationary front existed, an enormous amount of clay, silt, sand, pebbles, and boulders was deposited in a belt only a few miles wide along the front of the glacier, creating a line of hills and ridges that extended for many miles. Such belts, called end moraines, can be seen today in many parts of Illinois. The building of end moraines often was accompanied by the release of great quantities of water (meltwater) from the melting ice. The water, laden with rock debris, flowed from the front of the glacier in many streams.