Roccia Dell'Orso, "Bear Rock," is a large outcrop on Sardinia with deep tafoni, or large weathering cavities, sculpting it. (more below)
Photo courtesy Martin Wintsch of Flickr under Creative Commons license
Tafoni are large rounded pits that 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. The word comes from Sicily, where spectacular honeycomb structures form in the coastal granites. Honeycomb weathering
is a name for cavernous weathering that produces small, closely spaced pits called alveoli.
Notice that 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.