clay

Water runs off a clay surface. Clay is an impermeable rock. This means that water flows over it. (A little water can enter the clay, but this only causes the clay particles to expand and become even more impermeable.) The more water that falls, the greater the flow over the surface; this causes small gullies to form on the clay. In addition to the amount of rain, a steeper gradient will also increase the quantity of overland runoff.

Very fine-grained sedimentary deposit that has undergone a greater or lesser degree of consolidation. When moistened it is plastic, and it hardens on heating, which renders it impermeable. It may be white, grey, red, yellow, blue, or black, depending on its composition. Clay minerals consist largely of hydrous silicates of aluminium and magnesium together with iron, potassium, sodium, and organic substances. The crystals of clay minerals have a layered structure, capable of holding water, and are responsible for its plastic properties. According to international classification, in mechanical analysis of soil, clay has a grain size of less than 0.002 mm/0.00008 in.

Types of clay include adobe, alluvial clay, building clay, brick, cement, kaolinite, ferruginous clay, fireclay, fusible clay, puddle clay, refractory clay, and vitrifiable clay. Clays have a variety of uses, some of which, such as pottery and bricks, date back to prehistoric times.