A canyon (occasionally spelled cañon) or gorge is a deep ravine between pairs of escarpments or cliffs and is most often carved from the landscape by the erosive activity of a river over geologic timescales. Rivers have a natural tendency to cut through underlying surfaces so will eventually wear away rock layers to lessen their own pitch slowing their waters; given enough time, their bottoms will gradually reach a baseline elevation—which is the same elevation as the body of water it will eventually drain into. This action, when the river source and mouth are at much different base elevations will form a canyon, particularly through regions where softer rock layers are intermingled with harder layers more resistant to weathering. Most canyons were formed by a process of long-time erosion from a plateau or table-land level. The cliffs form because harder rock strata that are resistant to erosion and weathering remain exposed on the valley walls. The word canyon is Spanish in origin (cañón, , meaning big caña, cane in English). The word canyon is generally used in the United States, while the word gorge is more common in Europe and Oceania, though gorge and ravine are also used in some parts of the United States and Canada. The military derived word defile is occasionally used in the United Kingdom.