At the base of a waterfall a rounded pool is found. This is called a plunge pool. The force of the falling water causes the water to swirl in the pool in a movement called eddying. As the water falls the hydraulic action may cause water to splash back against the back wall causing further erosion. This causes undercutting of the back wall. Finally the overhang of hard rock is unstable and unsupported due to undercutting. It collapses and the rocks fall into the plunge pool. Waterfalls continue to erode backwards in a process called headward erosion. This is when the hard and soft rock is all worn away and the river returns to its original slope. There are two main types of waterfall: Cataracts are large vertical falls of water and cascades are waterfalls that fall over a series of smaller steps or drops.