Cache Creek Wilderness - Geology

Geology

The wilderness is within the upper basin of the Cache Creek drainage area and is composed of sandstone, shale, chert and conglomerate belonging to the Franciscan group, Upper Jurassic age. These sediments are of marine origin with extensive folding and faulting. In some places, dikes of serpentinized rock are intruded into the sediments. This is overlain by unmetamorphized sandstone and shale of Cretaceous age, also of marine origin and also folded and faulted with thick serpentine rock interbedded.

Serpentine rock forms from peridotite that has received intense heat and pressure when down near a subduction zone of Earth's tectonic plates. Geologists believe that the rock then surfaces at these zones because of its lower density than the surrounding rocks. Serpentine soils have high levels of metals such as asbestos, copper, mercury and magnesium, among others, that are toxic to many plants. Some plants have not only adapted, but have evolved to grow almost exclusively in serpentine soils. The leather oak (Quercus durata var. durata) is one example. Many of these serpentine-adapted plants are also endemic to the state, as this type of soil is not widespread outside of California. Approximately 20% of the state's rare and endemic plants grow in serpentine soils.