of the city of Santa Barbara, and then west, paralleling the coast, to the city of Lompoc and Vandenberg Space Force Base. The Santa Ynez River flows just north of the mountains, paralleling them for most of their length. Before reaching Lompoc the mountain range diverges into two low ridges, separated by Jalama Creek, which then vanish into the Pacific Ocean. The mountains parallel the Channel Islands to the south, another east–west trending range, a geologic extension of the Santa Monica Mountains; the two ranges are about thirty miles apart. Principal summits in the Santa Ynez range include Divide Peak, , La Cumbre Peak, (above Santa Barbara), and Santa Ynez Peak, . There are several important passes, including Gaviota Pass in the western portion of the range, through which runs U.S. Highway 101 via the Gaviota Tunnel; and San Marcos Pass near Santa Barbara, through which runs California State Route 154 capped by the magnificent Cold Spring Canyon Arch Bridge near the summit. The highest point, an unnamed and unmarked crest colloquially called Peak 4864, is located right above the eastern terminus of the range, near Lake Casitas and Matilija Canyon. Geology Geologically the mountains are young and mostly of sedimentary origin. The most common rock types in the range are sandstones and shales, with some limestone in the western portion of the range. Volcanic rocks can be found at the very western extremity of the range, near Point Arguello (Tranquillion Mountain Volcanics), while some schists of the Franciscan Assemblage are exposed in a number of locations. The Santa Ynez Mountains were uplifted in Pliocene and Pleistocene time, beginning about five million years ago, along the Santa Ynez Fault, a feature which is considered to be active. The left bend in the San Andreas Fault is proposed to have constrained northward moving lithosphere to cause uplift of the range. Being a young uplift, the slopes are steep and the topography is extremely rugged; the gradient is highest on the northern side above , adjacent to the trace of the fault. Dramatic sandstone formations, including outcrops of the resistant Coldwater and Matilija formations, are visible at many locations in the range. Biology The predominant ground cover is chaparral, with coastal sage scrub, oak woodland and grasslands at lower elevations. Isolated stands of conifers (mostly Coulter pines) and other evergreen trees such as bay laurel, tanbark oak and madrone grow on the