Clays are composed of various minerals. Of these, the so-called clay minerals—complex substances composed mostly of alumina, silica, and water—generally are the most important. They impart the property of plasticity and also cause clays to become hard when fired. Most clays are what geologists call unindurated (unhardened) rocks. Clay that has been indurated and occurs in layered deposits is commonly called shale. The layers may be from a fraction of an inch to several inches thick. Most Illinois shales are not plastic when dug from freshly exposed deposits, but they become plastic when crushed and kneaded with water. The clays and shales of Illinois are the basis of a huge and important industry. Early Uses in Illinois Clays and shales are useful because they can be made plastic by adding water, formed into desired shapes, and fired to a rock like hardness. As a result, various kinds of bricks, drain tile, pottery, and other useful products are made from them. In its early years, Illinois had many widely distributed potteries that used clay from nearby deposits to make a variety of jugs, crocks, and bowls that served in place of many present-day glass or metal articles. Drain tile has been of major importance in the development of the state. Early settlers found many low lying, swampy areas and tracts of land that drained poorly after heavy rains. Ditches were dug to carry away the water from some areas, but others were drained by means of drain tile—pieces of fired clay pipe several inches in diameter and about a foot long that were laid end to end in trenches below plough depth and then covered with earth. Water seeped into the tile, which discharged it into ditches. Tile factories, built throughout Illinois near clay or shale deposits, did an active business. Gradually, however, as more and more farm land was drained the demand slackened and many tile factories went out of business. Although there are fewer factories, much drain tile is still manufactured in Illinois. Many of the early tile plants also made bricks to be used for making foundations, buildings, sidewalks, and other structures. The bricks were made by hand-operated equipment. Some of the old hand-molded bricks may still be seen in older buildings. Now the brick-making process is highly mechanized and even though there are fewer plants they produce more bricks. Clay and Shale Deposits Illinois shales are a part of the bedrock—that is, they are associated with indurated rocks such as sandstone and limestone. Most clays are surficial rocks occurring in deposits near the surface, where they lie above the bedrock. Exceptions are certain clays found in extreme southern Illinois and the underclays, also called fireclays, that occur beneath coal seams and are part of the bedrock.