Depression formed as underlying limestone bedrock is dissolved by groundwater. Sinkholes vary greatly in area and depth and may be very large. The two main varieties are those caused by the collapse of a cavern roof, and those caused by the gradual dissolving of rock under a soil mantle. Collapsed sinkholes generally have steep rock sides and may receive streams that then flow underground. Soil-mantled sinkholes are generally shallower; they may become clogged with clay and hold a small lake.