Gorges and canyons are an erosional feature of water cutting into soft underlying rock. Limestone gorges often form on plateaus or areas uplifted by the action of plate tectonics. The erosive effects of water on stone has spectacular results, such as the world famous Grand Canyon. There are many large gorges and canyons on the Colorado Plateau, including Dead Horse Canyon and numerous narrow slot canyons. Limestone gorges and canyons appear around the world wherever there are thick limestone formations with rivers running over them.
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Layers of sedimentary limestone, often interspersed with sandstone and shale, make up the base material of limestone gorges. The rock was formed from sediment falling on the floors of the ancient shallow seas that covered much of the Earth 200 to 300 million years ago. The sediment was deposited over long periods of geological time. Pressure and chemical reactions eventually compressed it into solid rock. Limestone is primarily composed of calcite from the shells of marine animals, including diatoms, mollusks and corals.
Limestone is made up of the alkaline mineral calcite, called calcium carbonate. Calcite, with its basic pH, reacts chemically with the slightly acidic water flowing over it. Acidic water percolating into the stone's crevasses and cracks makes it crumbly and susceptible to erosion. The softened stone then erodes more easily, facilitating the formation of gorges and canyons over periods of geological time. The actions of the wind and water erode the stone into small abrasive particles that further accelerate the weathering process.
Gorges are often found on uplifted plateaus of limestone where the water flows downhill. The running water cuts into the stone aided by the river's load of sediment, sand and small rocks. A gorge will start to form as a waterfall retreats upstream, eating away at the limestone base as it goes. The falls move backwards, in the opposite direction of the water's flow. Steep gorge wall formation is aided by the freezing and thawing of water to loosen large sections of limestone that tumble to the bottom.
Gorges alternatively are formed by the collapse of cave ceilings in areas of limestone karst topography. Karsts are areas with large above-ground limestone outcroppings and underground caves. The caverns are carved by the action of rainwater draining through cracks and joints in the permeable limestone layers. The weak acid of the water makes the limestone soluble and more likely to collapse. Once the cave implodes, the actions of running water enlarge and define the gorge.
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