Waterfalls commonly form where water rushes down steep hillsides in upland areas. They are typical of the upper valley, but can be found in the rivers lower courses. The height and number of waterfalls along a stream or river depends upon the type of rocks that are being eroded by the water. This typically occurs in areas where alternating bands of bed rock, made up of varying levels of resistance, form the bedrock. Some types of rocks (shale, for example) wear away more easily than others (such as sandstone or limestone).
As the river or stream wears away the weak rocks, they travel across the surface of stronger rocks. These more resistant rocks become the capstones to waterfalls. The number and thickness of these stronger rock units in a vertical sequence of rocks controls how many water falls there are and how much vertical drop there is on each waterfall.
A cross section of a typical waterfall
The diagram above shows the formation of a waterfall. Soft rock is undercut (1). This leaves a layer of hard rock which overhangs the layer of soft rock (2).
The video below shows an overhang beneath Summerhill Force waterfall, Bowlees, Teesdale.
The water flows over the overhang and creates a plunge pool in the soft rock below. As the water hits the plunge pool it will erode it by both hydraulic action (the force of the water against the bedrock) and by abrasion (where the material carried by the river acts like sand paper against the riverbed) (3). Eventually the overhang will collapse due to the erosion of the soft rock beneath it. The waterfall then retreats up stream (4). This creates a steep, gorge-like valley.
An example of a waterfall formed in this way is High Force on the River Tees. The River Tees tumbles over the Whin Sill, a layer of a hard rock called dolerite, which is known locally as whinstone.
Below is a video featuring Skógafoss which is a waterfall situated on the Skógá River in the south of Iceland at the cliffs of the former coastline.