The Silicon Valley runs the entire length of the Santa Clara county from north to south in bay area.
It is surrounded by the hills of the Diablo Range on the East, and the Santa Cruz Mountains on the West.
Salt marshes and wetlands lie in the northwestern part of the county, adjacent to the waters of San Francisco Bay.
The following google earth map shows the details of Silicon Valley.
The grey white portion of the map represents the total Bay area. At lower end is Gilroy. There is Morgan Hill in between San Jose and Gilroy. Both Gilroy and Morgan Hill are away from San Jose. Thus San Francisco in North and San Jose in south limit the expanse of main Silicon Valley.
Geology
Santa Clara Valley was created by the sudden growth of the Santa Cruz Mountains and the Diablo Range during the later Cenozoic era. It is is a structural valley, created by mountain building as opposed to an erosional valley, or a valley which has undergone the wearing away of the Earth’s surface by natural agents.
The underlying geology of the Santa Cruz Mountains was also formed by the sediment of the ancient seas, where marine shale points to Miocene origin.
The Franciscan formation in tliis area consists of sandstone, silt- stone, shale, conglomerate, radiolarian chert, and altered basaltic and andesitic lavas and agglomerates. These types are intruded by basalt and andesite and by serpentinized nltrabasic rocks, diorite, gabbro, and dia-base. Glaucophane-bearing metamorphic rocks occur locally at the contact of bodies of serpentine and in isolated areas a few feet to a few hundred feet in diameter.
Sandstone and Shale. Sandstone, and smaller amounts of inter-bedded siltstone and shale, comprise more than 90 percent of the Fran-ciscan formation in this area. The total thickness of these rock types cannot be estimated, but there is at least 6,000 feet exposed east of the Calaveras fault, and the total thickness is undoubtedly much greater. The typical fresh sandstone is a dark-gray medium-grained massive rock, cut by many tight and almost invisible joints and fractures filled with calcite.
AAHien weathered, it is greenish gray, greenish brown, or buff, and the joints are so closely spaced that it is often impossible to determine the attitude except where the sandstone is interbedded with shale or where the exposures are unusually good. The rock is commonly rather thick bedded, but thin-bedded sandstone and siltstone occur in many places.
The sandstones can easily be distinguished from those of other formations by the almost ubiquitous occurrence of flakes of pearly transparent muscovite.
Today one can still find evidence of this in the Santa Cruz Mountains, where shark’s teeth and the remains of maritime life are still found as high as Scotts Valley, a city nestled in the mountains (Ref: https://en.wikipedia.org/wiki/Santa_Clara_Valley#Geology)
Active Earth quake Zone
There are two main faults
1. Rodgers Creek Fault, running from near Santa Rosa into the San Pablo Bay, and
2. the Hayward Fault, stretching from below San Jose through the Oakland and Berkeley hills into West Contra Costa County
Since 1979, there have been four earthquakes of magnitude 6 or greater, leading up to the recent 7.1 Loma Prieta earthquake. Geologists are now concerned that the strain along the faults has built up again and that more large earthquakes are possible. If the level of earthquake activity during the next few decades is similar to activity between 1836 and 1911, then the probability of a magnitude 7 earthquake in the next 30 years is about 75 percent.
| 1979 | M 6.0 Undetermined fault Coyote Lake Earthquake |
| 1980 | M 6.0 Mt. Diablo-Greenville fault Livermore Earthquake |
| 1984 | M 6.3 Calaveras fault Morgan Hill Earthquake |
| 1989 | M 7.1 San Andreas fault Loma Prieta Earthquake |
| 2001 | M 5.1 West Napa fault Napa Earthquake |
| 2007 | M 5.6 Calaveras fault |
Environmental Aspects
37,388 metric tons of Mercury were extracted from the New Almaden mine south of San Jose and north west of Santa Cruz during the 19th century. The area, closed for many years, resulted in pollution of the Guadalupe River and South San Francisco Bay. After final cessation of intermittent mining operations during the 20th century, the area was purchased by Santa Clara County for a park and designated a National Historic Landmark.