The Pine Valley Mountains formed from the Pine Valley Laccolith, which is the largest laccolith in the United States and perhaps the largest 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 monzoniteporphyry 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, 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.
The Pine Valley Mountains support spruce-fir forests, including a large stand of virgin Engelmann spruce. The mountains also have numerous meadows up to in size. 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. Botanically speaking, Pine Valley Mountain is the most diverse mountain range in Utah. A floristic survey was conducted between 1985 and 1987 and 967 species were collected. Since then, additional species have been added bringing the total to nearly 1,000. Three species new to Utah were identified including bitter cherry, common sandweed, and Lemmon's onion. Nineteen ferns and fern allies are present, 18 gymnosperms, 150 species of monocots, with the remainder being dicots. The three largest families represented in the flora of Pine Valley Mountain include the Asteraceae with 160 species, Poaceae with 85 species, and Fabaceae with 63. The three largest genera and number of species collected are Eriogonum, 23, Cryptantha, 20, and Astragalus, 20. Some of the finds include:
Forked spleenwort, a small grass-like fern found at a single high-elevation location north of Mill Flat.
Grape fern found in the high meadows.
Young bristlecone pines are found along the very crest of the mountain especially near the summit of the Oak Grove trail and westward.
Sequoia tree. A single tree planted behind the Browse guard station is almost four feet in diameter.
Pistachios have escaped cultivation from old plantings near Leeds and Harrisburg. The rootstocks for those orchards were the Mount Atlas pistachio while the upper portions of the trees were the edible pistachio. Some of the tops have died and trees have resprouted from the rootstocks and produced viable seeds. Birds have helped move them into the southern foothills of Pine Valley Mountain where they have become naturalized, often growing as bonsai trees out of cracks in rocks and cliffs.
Dollarjoint cactus scattered along the southern foothills.
A yellow-flowered population of Bridges penstemon, along the south fork of Pinto Creek.
Three species of orchids in the genus Corallorhiza.
Pine Valley goldenbush, with its peculiar crinkled leaves, is only found in the Pine Valley Mountains, most commonly along the Whipple Trail in Hop Canyon.
Utah spikemoss has an extremely limited distribution in southwestern Utah and one location in Nevada, but is locally common on Pine Valley Mountain wherever a shady spot and Navajo sandstone can be found together.
All but one of Utah's conifers grow in the Pine Valley Mountains. Only the lodgepole pine is missing.
Recreation
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.
