Mount Umunhum (Ohlone for "Resting Place of the Hummingbird") is a peak in the Santa Cruz Mountains, located in Santa Clara County, California. It is the fourth-highest peak in the Santa Cruz Mountains, after Loma Prieta, Crystal Peak, and Mt. Chual. Most of the mountain is located within the Sierra Azul Open Space Preserve. Geology Mt. Umunhum consists of a geologic formation that is known as the Coast Range Ophiolite. This ophiolite is a piece of Middle Jurassic oceanic crust and mantle that has been obducted onto land and largely been converted to serpentinite. It has been extensively faulted and diapirically uplifted and eroded to form Mt. Umunhum and extensive outcrop areas throughout the Santa Cruz Mountain foothills and elsewhere. The Coast Range Ophiolite is also known to underlie the rocks of the Great Valley throughout central California. The exposure on the northeast face of Mt. Umunhum is one of the best Bay Area exposures of the Coast Range Ophiolite. Mount Umunhum and nearby Loma Prieta are pushed upward by local collision forces associated with a left bend in the San Andreas fault. Starting near Uvas Canyon, the fault stops trending northwest but instead bends west-northwest for about eight miles before continuing its northwest direction. This left bend is renowned among geologists as a archetypal restraining bend which acts to oppose strike-slip motion along the fault. From the perspective of Mount Umunhum and other mountains atop the Sierra Azul block, the Pacific Plate is trying to shear them off the North American Plate but can't break them off. Rather, the resulting transpression force pushes up the local terrain and helps explain why these are the highest peaks in the Santa Cruz Mountains. History The Cube, a radar tower which housed an 85.5-ton AN/FPS-24 radar, was completed in 1962 and used to watch for hostile aircraft during the Cold War. Its signal caused electronic interference in many radio, TV and sound systems within its 250-mile range on each rotation. The summit was closed to the public due to hazardous materials and unsafe conditions caused by partially demolished structures from the former Air Force station. Plans were made to clean it up and restore it for public use. "In 2009, with federal funding, Midpeninsula Regional Open Space District removed 3,000 cubic yards of hazardous materials, including lead paint, asbestos, fuel storage containers and PCB transformers; and deconstructed 13,680 tons of concrete, asphalt,