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 or a fault, or a zone of weakness along a bedding plane in sedimentary rock. Erosional caves can be found in almost any kind of rock, including limestone. It should be noted that erosion can be a very active process in solution caves as well, but occurs after the cave has drained and surface streams are pirated into the cave.
Erosion tends to produce tall, canyon-like passages.
Some of the most spectacular erosional caves known are formed in granitoid rocks in some of the mountainous regions of the western United States. The five photos below are from one such cave system, where water has carved out almost a mile of canyon passage with deep potholes like the one 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.