This Earthcache looks at the formation of Southern California’s second highest peak and the North America’s second steepest terrain and a rain shadow.
This location provides a wide overlook of the San Gorgonio pass to the north and the Coachella valley to the east, including, on clear days, the Salton Sea. To the west and up another 2,000 feet or so is Mt. San Jacinto. If you can make the hike, I would highly recommend climbing the peak. I have read that you can see the Channel Islands on clear days, but good luck finding a clear day in Southern California.
Mt. San Jacinto rises about 9,000 feet above the desert in less than six miles. Only Telescope Peak in Death Valley has a steeper topographic change in North America. The steep mountainside is the result of both a sinking valley and a growing mountain.
The creation of Mt. San Jacinto begins about 135 million years ago, the end of the Jurassic. At that time the North American Plate was moving over the Farallon Plate pushing the Farallon down into the earth creating a subduction zone similar to plate margins of the Pacific North West of today.
The melting Farallon plate created huge masses of molten magma beneath the North American plate called plutons. These plutons migrated up toward the surface. In some cases they erupted at the surface as volcanoes.
About 70 million years ago, the late Cretaceous, most of the Farallon plate had disappeared beneath the North American Plate removing the source of magma for the region. The plutons cooled into huge masses of granitic rock called a batholith (“deep rock”). Mt. San Jacinto sits at the north end of the Peninsular Batholith which extends hundreds of miles to the south. At this point, the Peninsular Batholith was still deep underground.
Following the disappearance of the Farallon plate beneath the North American Plate, the San Andreas Fault formed between the Pacific Plate and the North American Plate. Movement along the San Andreas moved the San Jacinto Mountains about 155 miles northwest. Along the way, erosion continually removed rock above the batholith.
About 23 million years ago, the Miocene, the spreading center that the North American Plate moved over began to separate Baja California from North America. This began a clockwise rotation of what would become Baja California and began the uplift of the San Jacinto Mountains. At the same time, the movement of Baja California away from North America began forming a depression that extends from the Gulf of California all the way up Imperial Valley and Coachella Valley to San Gorgonio Pass. As spreading deepened the valleys, the rising mountains filled in depression. About 7.5 million years, the Gulf of California formed and has been widening ever since.
This configuration of a rising mountain and a deepening valley continues today and has created the magnificent mountain you stand on and the exceptionally steep mountain side in front of you.
This tall mountain range also forms a rain shadow creating the large desert to the east that includes Palm Springs and the Imperial Valley.
A rain shadow is formed when tall mountains force wet moist air up. As the air moves up, it cools and looses its capacity to retain moisture forming rain. As the air moves across the top of the mountain it then descends back down where it warms and increases its potential to retain moisture and less rain forms.
Here, moist air from the Pacific is blown on-shore where it is forced up by the the mountain ranges all along Southern California. At these higher elevations, the air cools and drops its moisture as rain in the mountins. Once the air crosses the mountain ranges, the air drops back down and warms. Now, not only has the air lost alot of moisture by raining in the mountains, but it has also warmed and can hold more moisture. Thus the landward side of the mountians rarely gets water and a desert is formed.
Send me a note with :
As always you can post any pictures of the area.
- The text "GCQHZT Mt. San Jacinto Earthcache" on the first line
- The number of people in your group.
- Comparing the vegitation at the top of the mountain and the vegitation down in the valley and the possible reasons for the difference.
The following sources were used in generating this cache:
- Images: USGS and NPS
- Paul Remeika and Lowell Lindsay. Geology of Anza-Borrego: Edge of Creation. San Diego: Sunbelt Publications, 1992
- USGS, http://pubs.usgs.gov/publications/text/Farallon.html
- San Jacinto DUSEL Detailed Site Information, San Jacinto Dusel S2 Proposal, University of California Irvine, April 20, 2005, http://www.ps.uci.edu/%7ESJNUSL/
- NPS, http://www.nps.gov/grba/EdPages/RAG/Unit%204.pdf