Pine Valley Mountains

Pine Valley Mountains
Pine Valley Mountains and St. George
Highest point
Peak	Signal Peak
Elevation	10,365 ft (3,159 m)
Coordinates	37°19′11″N 113°29′30″W / 37.3197027°N 113.4916207°W / 37.3197027; -113.4916207Coordinates: 37°19′11″N 113°29′30″W / 37.3197027°N 113.4916207°W / 37.3197027; -113.4916207
Dimensions
Length	35 mi (56 km) N/S
Width	28 mi (45 km) E/W
Area	591 sq mi (1,530 km2)
Geography
Pine Valley Mountains
Country	United States
State	Utah

The Pine Valley Mountains are a mountain range in the U.S. state of Utah spanning Washington County north of the city of St. George. The highest point in the range is Signal Peak at 10,365 feet (3,159 m). The mountains are part of Dixie National Forest and are bordered to the south by the Red Cliffs National Conservation Area.

The Pine Valley Mountains formed from the Pine Valley Laccolith, which is the largest laccolith in the United States and perhaps the largest laccolith in the world. The laccolith was formed during a 20 million-year period of volcanic activity. After 4-5,000 feet of volcanics had been deposited on top of the Claron Formation, the magma vents were effectively sealed off. A final surge of magma, unable to find its way to the surface, instead pushed sideways along the weak seam between the Claron and the overlying volcanic layers. Thus was injected a 3,000 foot-thick layer of monzonite porphyry to form the Pine Valley laccolith. 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. Many volcanic cinder cones can still be seen in the foothills of the Pine Valley Mountains and these have been dated at around 20,000 years old. As a result of these lava flows, the valley for which the mountain range is named and in which the town of Pine Valley is situated, was formed when lava dammed off the Santa Clara River and formed a lake. Sediments eventually filled the lake until they reached the height of the lava dam. These sediments form the floor of the present day Pine Valley, Grass Valley, and Grassy Flat.

The mountains straddle the divide between the Great Basin watershed and the watershed of the Virgin River, a tributary of the Colorado River. The Chinamen's Canal tunnel at the north end of Grass Valley diverts the waters of Mill Canyon Creek from the Colorado River drainage system into the Great Basin drainage system, eventually finding its way into the Newcastle Reservoir via Pinto Creek.Zion National Park can be seen to the east from the mountains.