Diablo Foothills Regional Park Trails Professional Review and Guide
"If you enjoy a challenging route, take this roller-coaster ride through the foothills just west of Mt. Diablo State Park. Using the Castle Rock, Shell Ridge Loop, Briones-to-Mt. Diablo Regional, Twin Ponds, Hanging Valley, and Stage Road trails, you’ll make two connected loops through the park, with a short jaunt into neighboring Shell Ridge Open Space, enjoying oak savannas, wildflower-dotted hillsides, and a variety of birds. You climb on a single-track trail, past a gate that prevents access by bikes and horses during wet weather. Meeting the Castle Rock Trail, a dirt road, you turn sharply right, then stay left at a fork. The road climbs on a gentle and then moderate grade. Wildflowers on display include California poppies, paintbrush, Ithuriel’s spears, asters, bellardia, Mariposa lilies, and clarkias. At the next junction you also stay left, passing through a blue oak savanna. Gaining a ridgetop, you wind uphill through chaparral—chamise, black sage, toyon, California sagebrush— with a view, left, of impressive sandstone cliffs."