Explain the formation of a waterfall.

The formation of a waterfall occurs due to a difference in bands of soft and hard rock. The hard rock, e.g. granite, is eroded at a far slower speed than that of the soft rock, e.g. sandstone. This process is called differential erosion. The river erosional processes that make this happen are corrasion (the scraping of larger rock material against the river banks and bed) and hydraulic action (a process by which the sheer force of the river’s water flow against the rock produces mechanical weathering, which results in rock particles becoming dislodged and transported by the river). These processes cause the softer bands of sandstone to be worn away by at a faster rate than the hard bands of granite, which causes an overhang. This overhang causes undercutting to the soft rock beneath because the river energy is so high. This eventually gives way and creates a formation directly below the overhang called a plunge pool. The erosional processes of potholing and corrosion will continue to deepen this pool. This process then repeats and the waterfall will continue to retreat upstream as a result of the differential erosion. Eventually, a gorge may form due to the steep sides of the rock either side of the riverbed.