The area now occupied by the San Cruz Mountains was, in pre-hisortic time, the entrance to a vast inland sea that encompassed central California’s San Joaquin Valley. These mountains lie on a large section of land known as the “Salinian Block” and are atop the junction between the Continental plate and Pacific plate sections of the earth’s surface. Over a period of from one to two million years, by a process called subduction, the Continental plate has forced itself beneath the Pacific plate. This process has progressively forced the edge of the Pacific plate upward, forming the Santa Cruz Mountains. Unlike the other, older Pacific mountain ranges, these mountains are very young and are made up mostly of sedimentary rock, known as sandstone, which is very prone to movement. Solid rock ledges and faces generally only populate sections of the upper elevations of the mountains. They were pushed up from the sea floor in the early stage of subduction.
Northwesterly movement of the Pacific plate at a rate of one to two inches a year along its junction with the Continental plate has resulted over time in a series of fractures in the Salinian Block. These fractures form what is commonly known as the “San Andreas Fault System” and include, along with the “San Andreas Fault”, other faults such as the “Zayante Fault.” It is this Zayante Fault that precipitated the “1989 Loma Prieta Earthquake” that badly damaged sections of the San Francisco Bay area and coastal cities along these mountains.
A place with that much ground shift would seem like an odd choice for a major metropolitan area but hey, it’s just so sunny around here.
That is a overly simplistic to wildly inaccurate description of the geology of the Santa Cruz mountains. But since this is a hiking blog I won’t bore you with the details.
To your point Tom, it’s the underlying geology and faulting that shape the hills and valleys and make this an attractive site for a major metropolitan area. I can deal with excessive ground shaking every hundred years or so if it creates such cool places to hike.