Ghosts of industry
The Empire Mine State Historic Park in Grass Valley, once the richest gold mining site in California (1850-1956), is also home to some very cool hiking trails. They provide a nice escape from some of the Valley heat, and along the way you may stumble across the remains of a number of smaller mines, some of which, like the Orleans Mine, were absorbed into the Empire Mine operation.
The Orleans stamp mill was in operation until the early 1900s.
The Sand Dam area, now fenced off to visitors, held waste rock from the mining operations.