<P> The California Bighorn Sheep Zoological Area near Mount Williamson in the southern Sierra was established to protect the endangered Sierra Nevada bighorn sheep . Starting in 1981, hikers were unable to enter the Area from May 15 through December 15, in order to protect the sheep . As of 2010, the restriction has been lifted and access to the Area is open for the whole year . </P> <P> The earliest rocks in the Sierra Nevada are metamorphic roof pendants of Paleozoic age, the oldest being metasedimentary rocks from the Cambrian in the Mount Morrison region . These dark - colored hornfels, slates, marbles, and schists are found in the western foothills (notably around Coarsegold, west of the Tehachapi Pass) and east of the Sierra Crest . The earliest granite of the Sierra started to form in the Triassic period . This granite is mostly found east of the crest and north of 37.2 ° N. In the Triassic and into the Jurassic, an island arc collided with the west coast of North America and raised a chain of volcanoes, in an event called the Nevadan orogeny . Nearly all subaerial Sierran Arc volcanoes have since disappeared; their remains were redeposited during the Great Valley Sequence and the subsequent Cenozoic filling of the Great Valley, which is the source of much of the sedimentary rock in California . </P> <P> In the Cretaceous, a subduction zone formed at the edge of the continent . This means that an oceanic plate started to dive beneath the North American plate . Magma formed through the subduction of the ancient Farallon Plate rose in plumes (plutons) deep underground, their combined mass forming what is called the Sierra Nevada batholith . These plutons formed at various times, from 115 Ma to 87 Ma . The earlier plutons formed in the western half of the Sierra, while the later plutons formed in the eastern half of the Sierra . By 66 Ma, the proto - Sierra Nevada had been worn down to a range of rolling low mountains, a few thousand feet high . </P> <P> Twenty million years ago, crustal extension associated with the Basin and Range Province caused extensive volcanism in the Sierra . About 10 Ma, the Sierra Nevada started to form: a block of crust between the Coast Range and the Basin and Range Province started to tilt to the west . Rivers started cutting deep canyons on both sides of the range . Lava filled some of these canyons, which have subsequently eroded leaving table mountains that follow the old river channels . </P>

What plate is the sierra nevada mountains on