Coulee lakes are also of volcanic origin but they are formed by the flow of lava across the valley of some river so that it blocks the river flow and gives rise to lakes. Lake Tana of Absyssinia is a characteristic example. Several lakes of this type are found in Iceland. Such lakes are usually rectangular in outline but may be irregular also. Sink lakes are characteristic of limestone regions, where the water descending through the shallow holes produces large underground caves. In case of roof of such a subterranean cavern collapses, a basin is formed. Such a basin resembles a sink and has also been formed by the sinking of the land. The lakes formed in these depressions are known as ‘Sink Lakes’. These are usually small in size.