Liquid water, acting under the force of gravity, has sculpted spectacular landscapes across our globe and waterfalls are among the most beautiful and inspiring. Most waterfalls develop where a stream passes a geologic boundary, crossing from a hard, resistant bedrock to a softer, less resistant one; this causes an abrupt drop in the level of the stream flow. The "fall line" of the southeastern U.S. offers an excellent example of this process; here, rivers flowing from the Southern Appalachians to the Atlantic all possess significant waterfalls at the boundary of the Piedmont (underlain with hard igneous and metamorphic rock) and the Coastal Plain (composed of soft sedimentary deposits).
Once waterfalls form at these geologic boundaries, they begin to erode upstream and, over thousands of years, produce rugged gorges through the hard bedrock; an excellent example is provided by the Niagara Gorge, which formed (and continues to develop) as the falls cut their way upstream through the Niagara Escarpment. In other areas, waterfalls form when the walls of river valleys are altered by glaciers or landslides; tributaries that once descended through side canyons now drop precipitously to the valley floor; Yosemite Falls is perhaps North America's most spectacular example.
Some waterfalls occur beneath the surface of our planet, developing at sinkholes where the roof of a cave has collapsed. In these karst landscapes, thick layers of soluble bedrock (limestone or dolomite) lie just beneath the surface, often covered by a relatively thin veneer of sandstone. Cracks in the overlying rock allow rainwater and snowmelt to percolate into the limestone, eventually opening vast, underground networks of streams and caves. Surface streams drain toward these sinkholes and, once the underlying cave is spacious enough, a waterfall forms.
