Castle Rock Regional Recreation Area is an eighteen-thousand-acre park in northern California’s East Bay, set in a sandstone canyon in the Diablo Foothills near Mount Diablo. Its defining landscape formed from sandy sediments deposited in an inland sea roughly 33 to 55 million years ago as the ancestral Sierra Nevada weathered in a warming climate. Today the area is valued for hiking, climbing, swimming, and wildlife viewing, with Castle Rock drawing both casual hikers and climbers. Seasonal closures protect a peregrine falcon nesting site, underscoring the area’s dual role as recreation land and protected habitat within a broader connected expanse of parkland.