Pitted sandstone on Rabbit Mountain near Lyons, Colorado, USA — both art and history.
Photo by keagiles. 

This stone is part of the Dakota Formation, which formed along the edge of the Cretaceous Sea (this area, near Lyons, Colorado, USA, was sometimes above the shoreline and at other times below sea level). These marine sands were bioturbated (burrowed) by ancient critters (like fairy shrimp), and over time the burrows became filled with softer, more erodible material than the surrounding sand (the fill usually was clay-sized and composed partly of fecal pellets and other material left by the animal [sometimes the animals actually lined the burrows with the fecal pellets]). When the sandstones were exposed to weathering after the inland sea retreated, the exposed part of the burrow weathered more quickly than the surrounding rock, which results in these pits. — This information was provided by geologist Barb Echohawk.