One of the most fascinating points of interest in the Anza-Borrego Desert State Park is the system of caves known as the Arroyo Tapiado Mud Caves. Arroyo Tapiado, translated from Spanish, means “walled wash.” The mud caves are found along the walls of this wash canyon. One of the most extensive mud cave systems in the world, they contain approximately 22 known caves and 9 slot canyons.
The mud caves are formed by fluvial erosion caused during periods of heavy rainfall. When this infrequent rainfall occurs, it cuts channels into the mud hills that are commonly found in the Pseudokarst topography of this arid region. The channels cause erosion and form canyons with unstable and undercut walls. As the channels deepen, the walls cave in. Because of the cohesive consistency of the mud in this particular area and its ability to swell to several times its original dry volume, it adheres to itself and to the canyon walls, creating natural bridges and, sometimes caves, as it dries.