Named for its abundance of interesting rock formations, Castle Rock State Park offers a window into California's geologic past. Sculpted outcroppings of weathered sandstone (called bosses) are interspersed with caves and hollows (called tafoni) which are the result of thousands of years of weathering by cool, wet winters and hot, dry summers. There are also features known as fretwork: beautiful, lacy patterns of stone and rock, formed during the uneven erosion of sandstone and calcium carbonate. More mysterious are the features known as "cannon balls", naturally occurring round, reddish stones, whose origins puzzle geologists.

Climate
The majority of the park falls into the San Lorenzo Watershed, with the range in elevation running from the peak of Mount Bielawski at 3,820 feet to 960 feet in the San Lorenzo river valley. Temperatures in the park are more severe than in the rest of the San Francisco Bay area, and there is often a light dusting of snow during the winter in the higher elevations.

Vegetation
Today the vegetation in the park is a recovering mixture of wildlands, with a few remaining outposts of virgin wilderness. The combination of grasslands, mixed evergreen woodland, chaparral, and deep redwood forest also contains several unusual plant communities such as the rare Black Oak forest, Knobcone Pine forest, and White Alder forest. Although not an untouched wilderness, Castle Rock State Park is now in a protected state, as a key piece in the mosaic of preserved lands of the Santa Cruz Mountains.