Castlewood Canyon State Park, also a Designated Colorado Natural Area, encompasses 873 acres; it is within the northernmost extension of the Black Forest, and occupies an ecotone between plains grassland and Front Range forest communities along Cherry Creek. Canyon walls and adjacent caprock are excellent exposures of the Castle Rock conglomerate member of the White River Formation, coarse, erosion-resistant cobble cemented by volcanic ash. In the northwestern portion of the park, a loose alluvium known as the Dawson Formation lies exposed. Since the Dawson Formation is more erosive than Castle Rock conglomerate, a wider creekbed, floodplain containing large sediment deposits, more developed soils and obscured geologic features result in this area.

Ecologically, park uplands are dominated by ponderosa pine savanna, lower montane forest, mixed foothill shrubland, mixed grassland, riparian and wetland communities. Nearly barren rock outcrops and cliffs support lichen and moss communities. Ponderosa pine and Rocky Mountain juniper are the dominant savanna trees, commonly associated species include mountain mahogany, skunkbrush, currant pine dropseed, blue grama, prairie Junegrass and sunsedge. Douglas-fir is the common tree on talus slopes within the canyon. Shrublands occupy drier slopes and openings within the forest community, where Gambel oak, mountain mahogany, skunkbrush and snowberry are the dominant species. Along drainages, chokecherry and American plum commonly grow with Gambel oak. Grasslands present are an interesting association of shortgrass prairie, tallgrass prairie, montane grassland and introduced species. Dominant grass species include big and little bluestem, prairie sandreed, blue and side-oats grama, needle-and-thread grass, pine dropseed, prairie Junegrass, western wheatgrass and smooth brome. A variety of riparian and wetland communities are present along Cherry Creek, tributary drainages and on seeps and springs. Unique grotto or hanging garden wetlands occupy ceilings and floors of dissolution caverns along the western canyon walls. They contain mosses, liverworts, ferns and vascular plants species representative of the last ice age.

Many species of migratory and resident birds are known to frequent this area including the grassland species lark bunting, horned lark and western meadowlark, raptors, songbirds and waterfowl. Turkey vultures roost along the canyon rim, riding thermal updrafts created as the rocks warm each morning. A unique dry-land heronry remains, where great blue herons nest in Douglas-fir trees near an historic dam and reservoir site. Mule and white-tailed deer, coyote, raccoon, striped skunk, cottontail rabbit, porcupine, ground squirrel and red fox are common sights. Rare observations of black bear, mountain lion and elk may occur. Cherry Creek supports warm-water fish species including longnose dace, fathead minnow and black bullhead.