Typically, a river flows over a large step in the rocks which may have been formed by a fault line. Over a period of years, the boundaries of this shelf will gradually break away and the waterfall will steadily retreat upstream, creating a gorge of depression. Often, the rock layer just below the more resistant shelf will be of a softer type, meaning undercutting, due to splashback, will occur here to form a thin cave-like formation known as a rock shelter under and behind the waterfall. Finally, the outcropping, more resistant cap rock will fall down under pressure to add blocks of rock to the base of the waterfall. These blocks of rock are then broken down into smaller boulders by attrition as they crash with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool.