Waterfalls- waterfalls form as streams flow from soft rock to hard rock (alternating bands). This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.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 where the process of rejuvenation creates enough potential energy for vertical erosion to recommence closer to the rivers mouth. 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.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.Waterfalls develop when a change of lithology (rock type) takes place along the river’s course resulting in differential erosion. When the rock type of the river’s channel changes from a resistant rock to a less resistant one (e.g. granite to limestone), the river erodes the less resistant rock faster producing a sudden drop in the gradient of the river with the resistant rock being higher up than the less resistant rock. As the river flows over the resistant rock, it falls onto the less resistant rock, eroding it and creating a greater height difference between the two rock types, producing the waterfall. When water flows over the waterfall it creates a plunge pool at its base and the splashback from the falling water undercuts the resistant rock. The unsupported rock is known as the cap rock and it eventually collapses into the plunge pool causing the waterfall to retreat upstream. Over thousands of years, the repeated collapse of the cap rock and retreat of the waterfall produces a gorge of recession

Gorges- A number of natural forces form gorges. The most common is erosion due to streams or rivers. Streams carve through hard layers of rock, breaking down or eroding it. Sediment from the worn-away rock is then carried downstream. Over time, this erosion will form the steep walls of a gorge. The flooding of streams or rivers increases the speed and intensity of this erosion, creating deeper and wider gorges.

	Created by Liv Pearce over 10 years ago

OCR AS geography revision