A final surge of magma, unable to find its way to the surface, pushed sideways along the weak seam between the Claron and the overlying volcanic layers.

[6] The contact between the top of the Claron and the bottom of the laccolith can be seen in several locations, most notably near the headwaters of Cottonwood Creek along the southwest corner of the mountain range (see photo below in the gallery), and at the headwaters of Leap Creek north of the Browse Guard Station.

After erosion exposed the laccolith, volcanic activity continued and the youngest flows are 1-1.6 million years old.

The Chinamen's Canal tunnel at the north end of Grass Valley diverts the waters of Mill Canyon Creek[citation needed] from the Colorado River drainage system into the Great Basin drainage system, eventually finding its way into the Newcastle Reservoir via Pinto Creek.

[10] The Pine Valley Mountains support spruce-fir forests, including a large stand of virgin Engelmann spruce.

[9] Several threatened, endangered, and sensitive species live in the mountains, including Bonneville cutthroat trout, Townsend's big-eared bat, and the pygmy rabbit, among others.

There are numerous opportunities for recreation in the mountains, including hiking, rock climbing, camping, wildlife watching, fishing, hunting, horseback riding, and boating on a reservoir.