The north frontal fault system of the San Bernardino Mountains is made up of a number of disparate structural elements, each of which accommodates range-front deformation in a manner dictated by its geometry. A two-stage history of late Cenozoic structural development is proposed for the northwestern San Bernardino Mountains: the range was first uplifted on low-angle structures and later modified by high-angle faulting. Evidence for Pliocene onset of deformation and uplift in the westernmost San Bernardino Mountains is found in the provenance and character of the associated sediments. Thrusting uplifted the northern range front during a pulse of deformation spanning late Pliocene through middle Pleistocene time. Uplift in the westernmost San Bernardino Mountains was accomplished contemporaneously by tilting, warping, and arching. Nature and timing of deformation are consistent with the hypothesized formation of a transpressional welt across the San Andreas fault, which may have affected both the San Bernardino and San Gabriel Mountains. High-angle faulting replaced thrusting and warping as the dominant style of deformation in the northwestern San Bernardino Mountains beginning in middle to late Pleistocene time. Pleistocene left-lateral faulting in the westernmost San Bernardino Mountains has accomplished north-south crustal shortening by squeezing the San Bernardino Mountains block eastward. Northwest-trending right-lateral faults, characteristic of the Mojave block prior to range-front uplift, have reasserted and incorporated themselves in the complex zone of range-front deformation. Local extension resulting in minor graben formation, appears to have been associated with lateral motion on the north frontal fault zone in Fifteenmile Valley (Sky Hi Ranch fault zone) and the Cleghorn fault zone. Arcuate patterns of faulting in the western San Bernardino Mountains can be explained in terms of the pattern of faulting predicted for secondary faults near the end of a strike-slip fault. In this case the "end effect" would be produced by a change in slip rate on the San Andreas fault in Cajon Pass, possibly related to motion on the San Jacinto fault. All faulting in the study area is interpreted as the product of compression across the San Andreas fault. A weathered erosion surface was developed on the crystalline terrane over most of the area in response to humid conditions during the late Miocene(?), at which time the region was characterized by an upland surface of subdued relief. This weathered erosion surface is a useful index to structural deformation in the northwestern San Bernardino Mountains. Late Cenozoic stratigraphy constrains the timing of deformation and uplift in the northwestern San Bernardino Mountains. The late Miocene to Pliocene(?) Crowder Formation was deposited by drainages carrying distinctive volcanic and metamorphic clasts from the Victorville area southward, across the site of the western San Bernardino Mountains. The late Miocene beds of the Punchbowl Formation are faulted against the lower Crowder Formation, but are overlain by the upper Crowder Formation. The Punchbowl and Crowder Formations share the same age and paleocurrent direction, yet differ markedly in sediment character and provenance. The relationship between these two units remains an unsolved stratigraphic problem, which seemingly requires substantial lateral structural translation. The middle to late Pliocene onset of deformation and uplift is recorded in the stratigraphic sequence by the appearance of fine-grained sediments, new clast lithologies, and northerly paleocurrent directions. The volcanogenic eastern facies of the Crowder(?) Formation is believed to be a syntectonic deposit indicative of ponding that accompanied the reversal in drainage direction brought on by incipient uplift of the western San Bernardino Mountains. The fine grained, lacustrine character of the Harold Formation can be interpreted in the same way; the base of the Harold Formation is <2.75 my old on the basis of paleomagnetic constraints. The Old Woman Sandstone in Lucerne Valley records an abrupt change from fine-grained sediments indicative of incipient uplift to coarse, angular debris signalling the emergence of the range front as a topographic element. The influx of range-front debris is estimated to have occurred <2.5 my ago. Several small, deformed patches of fine-grained sediment in Arrastre Canyon appear to be of similar origin. The Quaternary stratigraphy of the western San Bernardino is dominated by the Harold Formation, Shoemaker Gravel and Older Alluvium, which underlie the Victorville Fan. These units were shed northeast off the San Gabriel and western San Bernardino Mountains, and record their uplift. The Harold Formation contains the earliest appearance of San Gabriel Mountains crystalline basement lithology within the stratigraphy of the Mojave block. The Victorville Fan sequence is believed to be time-transgressive, reflecting the northwestward movement of the San Gabriel Mountains crystalline terrane along the San Andreas fault. The Older Alluvium capping the Shoemaker Gravel in Cajon Pass records the Brunhes/Matuyama polarity reversal of 730,000 y B.P. The Older Alluvium can be divided into dissected and undissected facies believed to predate and postdate the polarity transition respectively. This crude chronology can be extended to sediments on the flanks of the Ord Mountains that define a late Pleistocene drainage system tributary to the ancestral Mojave River. The Pleistocene units contain evidence of progressive growth and integration of drainage in the Western San Bernardino Mountains in response to uplift during Pleistocene time. Detailed geologic mapping of the northwestern San Bernardino Mountains permits slip rates and offsets to be calculated for important range-front faults. The Cleghorn fault has a cumulative left-lateral offset of 3.5 to 4.0 kilometers, and a slip rate of about 3.0 mm/yr. The Sky Hi Ranch fault zone has a late Pleistocene right-lateral offset of approximately 0.5 kilometers, with a slip-rate on the order of 1 mm/yr. The faults along the west flank of the Ord Mountains have a vertical slip-rate of less than 1 mm/yr. The Cleghorn fault is classified as "active", under the criteria set forth by the State of California in the Alquist-Priolo Act of 1972, and is considered capable of a M[subscript s] 6.8 earthquake. Parts of the north frontal fault system on the west flank of the Ord Mountains and the Sky Hi Ranch fault zone are classified as "potentially active", in the terminology of the Alquist-Priolo Act, and are thought to be capable of a M[subscript s] 6.6 to 6.8 event. The Tunnel Ridge lineament and Arrastre Canyon Narrows fault zones are considered tentatively active, and should be examined in detail prior to development of adjoining areas. Clearly, the San Andreas fault poses the greatest seismic hazard to the communities in the study area.