The rocks in that area are made of sandstone. During our winter rains, acidic rain water seeps into and sits in the rocks. The next summer, the dry heat pulls the moisture back out of the rock, leaving behind these holes.
Located in Diablo Foothills Regional Park ebparks.org/parks/diablo_foothills
Check out my Little Yosemite Trail tour for more pictures of the area.
January 31, 2015