Blackhawk Mountain in southern California rises above southeastern Lucerne Valley at the eastern end of the rugged 4,000-foot escarpment that separates the San Bernardino Mountains on the south from the Mojave Desert on the north. Its summit is a resistant block of marble thrust northward over easily eroded uncemented sandstone and weathered gneiss. Spread out on the alluvial apron at the foot of the mountain is the prehistoric Blackhawk landslide, a lobe of nearly monolithologic marble breccia from 30 to 100 feet thick, 2 miles wide, and 5 miles long. The Blackhawk landslide and an adjacent older landslide, the Silver Reef, have many peculiarities of form and structure in common with the historic Elm, Frank, and Sherman landslides; and in lithology, provenance, size, and “coefficient of friction” they strongly resemble many of the monolithologic breccia deposits of possible landslide origin found associated with Tertiary faults and fanglomerates in the southwestern United States and elsewhere.
The older rocks of the Blackhawk area consist of gneiss, quartzite, Carboniferous marble, and Cretaceous quartz monzonite, which originally underlay a landscape of relatively low relief. Uplift of Blackhawk Mountain, first by overthrusting from the south, and then by monoclinal folding along a northwest-trending axis, led in the late Tertiary and Quaternary to deep erosion of the mountain front accompanied by the growth northward of extensive alluvial deposits interspersed with several large-scale landslides, the largest and most recent of which is the Blackhawk landslide.
Both the geological evidence and, in the case of the Elm and Frank landslides, the eyewitness reports suggest that the Blackhawk landslide and its congeners started as huge rockfalls, which were launched into the air and then traversed the gently inclined, relatively smooth slopes below as nearly nondeforming sheets of breccia sliding at high speed on a relatively thin, easily sheared lubricating layer. These facts suggest the hypothesis that landslides of this type acquire such high speed in their descent that at a sudden steepening of slope they leave the ground, overriding and compressing a cushion of trapped air upon which they traverse the gentler slopes below with little friction, much as the slipper in a thrust bearing slides on a cushion of oil with no metal-to-metal contact. This air-layer lubrication readily accounts for the low friction, high speed, and nonflowing motion of these large landslides and explains many otherwise puzzling details of their form and structure, such as the striking three-dimensional jigsaw puzzle effect seen in the pervasively fractured larger blocks, the transverse corrugations, soil schlieren, and certain of the peculiar debris cones on the landslide surface, the moraine-like ridges along the sides, and the low rim, steep scarp, and transported debris at the distal end. Thus, it appears that under the right circumstances massive avalanches like the Blackhawk can slide for miles on nothing more substantial than an ephemeral layer of compressed air.