Vaca Mountains - Geology

Geology

Bedrock throughout most of the Vaca Mountains are sedimentary rocks of the Great Valley Sequence that are Cretaceous in age (65-145 ma). These rocks are represented by shale and interbedded sandstone that are thought to have been deposited in a deep-marine setting as submarine fans, sourced by sediment shed during uplift of the ancestral Sierra Nevada. There is also minor conglomerate that may represent the fill of ancient submarine canyons developed at the heads of these fans. Because the sedimentary layers that make up the Vaca Mountains strike parallel to the crest of the range and dip down to east, these strata have eroded to create a steep escarpment on the west side with a much more gentle slope on the east side.

There are also isolated outcrops of prominent black rocks called the Putnam Peak basalt, in particular near Vacaville in the southern part of the range, at Putnam Peak in the middle part, and in Putah Creek Canyon at the northern terminus. This basalt dates at 15-16 ma, which makes it Miocene in age, so it is much younger than the Great Valley Sequence. It is thought to represent remnants from an ancient volcanic eruption on the east side of the Sierra Nevada of an especially fluid flow of lava that spread across a large part of central California.