The simplest definition of a waterfall is a place where a river or stream flows over a vertical drop or a steep cliff into a pool below. It is essentially a "water-drop" in the path of a flowing body of water. Waterfalls are usually formed when a river flows from a layer of hard rock to a layer of softer rock; the water wears away (erodes) the softer rock more quickly, eventually creating a ledge that the water falls over. Gravity is the force that pulls the water down over the edge. There are different types of waterfalls based on how the water falls: a "plunge" waterfall drops straight down without touching the rock behind it, while a "cascade" falls over a series of rocky steps. Whether it is a tiny stream in a local park or the massive Niagara Falls, the core concept remains the same: it is water in motion being forced over a sudden change in the elevation of the land.