Describe the formation of a waterfall.

A waterfall is an erosional landform found in the upper course of a river. Its formation starts as water flows over rock eroding downwards by hydraulic action and abrasion (and perhaps corrosion). Differential erosion occurs as the less resistant rock is eroded at a faster rate. As it continues to erode forming the drop a plunge pool is created where further hydraulic action abrasion and attrition occurs, eroding and expanding the waterfall. As the river undercuts into the rock the above rock overhangs and is subject to weathering forces, weakening it. Eventually the overhang cannot be supported and falls as mass movement. The waterfall then retreats further upstream and leaves behind the distinct vertical gorge.