How is a waterfall formed?

A waterfall is formed primarily by the erosion of soft rock, which occurs in a process that involves the gradual wearing away of both soft and hard rock layers by the flowing water. As a river flows, it often encounters different types of rock along its course. When it meets a layer of softer rock, the current can erode that material more easily, leading to the development of a drop in elevation. Over time, the continued erosion of the softer rock can create a ledge, and as the water cascades over this edge, it forms a waterfall.

This process not only highlights the dynamic relationship between water flow and rock types but also illustrates the significant role of erosion in shaping landscapes. The presence of hard rock layers above the softer rock helps to create a dramatic drop-off, enhancing the visual impact of the waterfall itself. Thus, erosion of soft rock is the primary mechanism through which waterfalls are formed, making it the correct answer to this question.