Tafoni (singular: tafone) are small cave-like features found in granular rock such as sandstone, with rounded entrances and smooth concave walls. They often occur in groups that can riddle a hillside, cliff, or other rock formation. They can be found in all climate types, but are most abundant in intertidal areas and semi-arid and arid deserts. Currently favored explanations controlling their formation include salt weathering, differential cementation, structural variation in permeability, and the length of the drying period between wettings. They also frequently occur in granitic rocks.
Salt Point State Park from Kglavin at Wikimedia
Pauldineen Galleries at Flickr
Image from Wikipedia.