Erosional Caves
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Erosional caves are those formed by the action of water or wind, carrying abrasive particles capable of carving rock. With respect to water, the processes involved are very much like those when a surface canyon is carved, and such speleogens as fluting and potholes may be formed. But in order to form a cave instead of just a canyon, something must fracture the host rock and allow an influx of water. This could be a joint, fault, or zone of weakness along a bedding plane in sedimentary rock.
Erosional caves can be found in almost any kind of rock, from hard granite to soft claystones, and even limestone. In fact, erosion can be a very active process in caves originally formed by solution caves as well but usually occurs after the cave has drained and surface streams are pirated into the cave. Erosion tends to produce tall, canyon-like passages.
Hard Rock Erosional Caves
Some of the most spectacular erosional caves known are formed in granitoid rocks in mountainous regions of the western United States. These four photos are from one such cave system, where water has carved out almost a mile of canyon passage with deep potholes like the one at Left. Below left, a caver uses the scalloping on the walls to climb over a deep pool. Center, two dikes cross in the passage wall. Dikes are molten material intruded into cracks in the igneous host rock. Right, the cave system in flood. Sand particles borne by floodwaters have played a large role in creating this system.
Wind Caves
Sandstone is soft and readily eroded. In some areas wind has sculpted large (but not deep or extensive) caves, as the one on the left. Note the large sand dune below the left-hand entrance. On the right is a smaller cave with extensive wind sculpting from Castle Rock State Park in California. Weathering in sandstone may also be assisted by water dissolving the carbonate cement that holds sandstone together.
Soil Pipe Caves
A very common erosional type of cave that forms in soft rocks or clay is known as a soil pipe or claystone cave. Often found in arid regions, these caves form from the erosive power of water on soft rock. Surface topography may include sinkholes such as are found in limestone karst, and often termed pseudokarst because it doesn't result from solutional processes. Some fairly lengthy soil pipe caves are known in the Rockies and in California's Mojave Desert.
