The mantle, which surrounds the core, is a rigid zone of materials rich in iron and magnesium. These materials are not as dense as those found in the core. WHAT IS THE CRUST OF THE EARTH AND HOW IS IT DISTINGUISHED FROM THE MANTLE AND THE CORE? The crust, the top part of which includes the ground we walk on, is the hard, outer surface layer of the earth. Although the crust is about 6 to 30 miles deep, the scale (size) of figure 2 allows it to be drawn only as the thin outer line. The rocks and minerals that we are all acquainted with are found in the crust. These rocks are not as dense and do not have as high an iron content as do the rocks of the mantle and the core. WHAT IS THE EARTH’S CRUST UNDER ILLINOIS MADE OF? The earth’s crust under Illinois is made of: sedimentary rocks deposited by water, wind, or glaciers (limestone, sandstone, shale, glacial drift); igneous, or melted rocks (granite, gabbro); and metamorphic, or altered sedimentary and igneous rocks (quartzite, gneiss). These various types of rocks have a relatively thin covering of broken or weathered material called mantle rock and soil. WHEN AND HOW WAS THE LAND SURFACE OF ILLINOIS FORMED? The ground surface over most of Illinois was formed during the ice age (Quaternary Period) which lasted from about one million years ago up to nearly 5,000 years ago. As much as 90 percent of the state was covered by one of several sheets of glacial ice (fig. 3). The glaciers smoothed down and covered much of the old, rocky, hilly land surface and filled old valleys with loose, unconsolidated deposits. WHERE DID THE LOOSE, UNCONSOLIDATED DEPOSITS COME FROM AND WHAT ARE THEY MADE OF? In Canada vast thicknesses of snow and ice accumulated until the weight of the ice finally caused it to flow slowly outward, mainly to the south (fig. 4). Rocks and surface materials of all types were picked up and carried by the glaciers for great distances before being deposited. Pieces of granite, quartzite, and native copper among the many local rocks and minerals are found in glacial deposits and indicate that at least part of these deposits came from the Great Lakes Region. The materials deposited range from clay-size minerals to large boulders. [Illustration: Figure 3—Extent of the main glacial advances which began with the Nebraskan (a—the oldest) and ended with the late Wisconsinan (f—the youngest). Diagram “d” shows the major stream development during the time between the Illinoian and the earliest Wisconsinan glaciers. The heavy lines on the diagrams indicate major stream valleys that were present during these times.] a. Inferred limit of NEBRASKAN glaciation b. Inferred limit of KANSAN glaciation c. ILLINOIAN glacial advance d. SANGAMONIAN major drainage e. Maximum WISCONSINAN glacial advance f. Late WISCONSINAN Valparaiso front and Kankakee Flood HOW DID ILLINOIS GET ITS SOILS? [Illustration: Figure 4—Limits of ice age glaciers.]