Tunnel kilns, made from heat-resistant bricks, are actually tunnels big enough for a man to stand in. The unburned bricks are loaded on flat steel cars on top of a layer of refractory blocks that protect the steel from the heat. The cars enter the kiln and heating begins. As they move through the kiln, they carry the bricks through a firing area, then through a cooling zone, and finally out into the air. In some brickyards in the Chicago area, dried unburned bricks are carefully stacked by machines into piles about 17 feet high, 35 feet wide, and 115 feet long, which are known as scoves or scove kilns. A layer of burned bricks that is plastered with clay covers the sides of the scove. A jet of flame is directed through small tunnels at the base of the scove, and the heat fires the bricks. During 1963, more than 325,000,000 bricks were produced by Illinois brick plants. In the same year, the value of all the clay and clay products produced in Illinois was nearly $54,000,000. Besides brick and drain tile, the products of the clay and shale industry of Illinois include refractory brick, building block and tile, fire-proofing, sewer pipe, flue liners, stoneware, lightweight bloated burned clay aggregate for concrete, and a variety of unburned clays for special purposes, including bonding clay, refractory fireclays, absorbent for use on garage floors, and litter for animal cages. PEAT After the retreat of the last of the great ice sheets from Illinois, numerous ponds and lakes were left in northern Illinois, especially in the eastern section. Some of them were soon drained by natural processes, but others remained. In the shallow water along their shores grew various plants, chiefly reeds and sedges and, locally, a variety of moss. As the plants died, their partially decomposed remains were preserved beneath the water. Ultimately, the ponds and lakes were overgrown and more or less completely filled by the plants and their remains, giving rise to peat (fig. 22) bogs. Some peat bogs have been drained and are now used as farm land. Others remain and a few of them are the source of peat or humus for horticultural purposes. Producing operations are located in northeastern Illinois and in Whiteside County in northwestern Illinois. OTHER MINERAL RESOURCES