Tafoni
(c) 2002 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Tafoni are these rounded pits, big enough to put your head into. They form by a physical process called cavernous weathering, which starts when water brings dissolved minerals to the rock surface. When the water dries, the minerals form crystals that force small particles to flake off the rock. Tafoni are most common along the coast, where seawater brings salt to the rock surface (see an example from San Francisco). The word comes from Sicily, where spectacular honeycomb structures form in the coastal granites. Here on this mountainside near New Idria, California, it's groundwater that brings minerals to the surface, where the strong morning sun makes them crystallize. Another picture from this locality is available as a free wallpaper image.
The photo below shows tafoni on the south-facing side of Griswold Canyon, in the same region. Notice that in both places, the surface layer of rock is harder than the interior. This hardened crust is essential to make tafoni; otherwise, the whole rock surface would erode more or less evenly.
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