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 depend upon the type of rocks that are being eroded by the water. This typically occurs in areas where alternating bands of rock, made up of soft and hard rock, form the bedrock. Some types of rocks (shale, for example) wear away more easily than others (such as sandstone or limestone).
Waterfalls form when water falls onto soft rock after flowing over hard rock. Falling water and rock particles erode the soft rock below, forming a plunge pool. Processes of erosion, such as hydraulic action, abrasion and corraision further erode the plunge pool and the back wall of the waterfall, undercutting the hard rock above. eventually, the hard rock will no longer be supported and it will collapse. The waterfall continues to retreat leaving behind a steep-sided gorge.
The diagram above shows the formation of a waterfall.
Formation of a waterfall
The video below shows an overhang beneath Summerhill Force waterfall, Bowlees, Teesdale.
Another example of a waterfall is High Force on the River Tees. At 22 metres it is the tallest waterfall in England. The River Tees tumbles over the Whin Sill, a layer of a hard rock called dolerite, which is known locally as whinstone. This resistant rock lies on top of softer sandstone, shale and limestone. The retreating waterfall has created a gorge 500m long.
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.