The Santa Ynez Mountains are an east-west range primarily in Santa Barbara County, California. They stretch from the Pacific Coast in the west to Ventura County in the east.

Geologically, the mountains are composed almost entirely out of sedimentary rock deposited in the Eocene and Oligocene, and were uplifted in the Miocene.

The oldest rocks in the mountains, however, are from the Franciscan formation, dating from the Cretaceous or upper Jurassic period. This layer extends up and down the coast of California, and is composed of a chaotic mix of sedimentary and metamorphic rocks, including greenstones, serpentines, limestones, and greywacke sandstones. This is also the deepest layer in the mountains, and is only visible at the surface on the northernmost slopes of the mountains, directly adjacent to the Santa Ynez Fault.

Above the Franciscan are the various sedimentary layers of these mountains (from oldest to youngest): the Jalama, Juncal, Matilija, Cozy Dell, Coldwater, and Sespe layers. These are all sandstones and shales, and were deposited in alternating deep- and shallow-marine regimes.

The Sespe formation, the youngest layer truly part of the mountains, has some distinctive colors. Pink arkosic sandstones and red/green siltstone are both present in this layer.

The highest point near Santa Barbara is La Cumbre Peak, which is cut out of the Matilija layer. Viewed from Santa Barbara or Goleta, the buff sandstones of the Matilija and Coldwater formations is readily visible, as are the mini-depressions and valleys where the Cozy Dell shales have eroded away.

This writeup is based partially on the Wikipedia article of the same name, but primarily on my Geology 2 course at UCSB and my own experience and trip into the mountains.

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