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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

Quaternary Surficial Materials

Alluvial deposits .--Quaternary sand-and-gravel deposits are extensive in parts of the San Bernardino Mountains, including the following major outcrop belts:

  • alluvial deposits on the south margin of the mountains that accumulated within the intra-mountain drainages of City Creek, Plunge Creek, and Santa Ana River. In the Barton Flats area of the upper Santa Ana River, alluvial-fan deposits are interlayered with riverine deposits and glacial-outwash deposits, all of which have been deformed by landslide processes that have created a complex geomorphic setting of scarps and hummocky ground (Sadler and Morton, 1989)
  • the Big Bear Lake area, where lowlands now occupied by Big Bear and Baldwin Lakes have been the sites of Quaternary alluvial accumulation (note, however, that broad expanses of sedimentary material formerly interpreted as Quaternary in the Big Bear Lake region [for example, units interpreted by Richmond, 1960, as Quaternary talus deposits and by Sadler, 1981, as relict fanglomerate] are interpreted by Matti and others (1993) as mainly Tertiary in age)
  • multiple alluvial-fan units that extend north onto Lucerne Valley from the north front of the San Bernardino Mountains (Miller and others, 1998; Powell and Matti, 2000).

Glacial deposits .--Sedimentary deposits interpreted as morainal accumulations from late Pleistocene glaciers occur on the north side of San Gorgonio Mountain and to the west on San Bernardino Ridge (Sharp and others, 1959). Riverine glacial-outwash detritus is common on the upper parts of Barton Flats where it forms a sedimentary veneer overlying the Santa Ana Sandstone.

Landslide deposits .--Landslides are common in the San Bernardino Mountains. Large landslide complexes occur locally along the steep south margin of the range (for examples see Miller, 1979, and Matti and others, 1992b). Extensive landsliding has occurred in the Barton Flats area (Sadler and Morton, 1989) where older surficial gravel units that overlie the Santa Ana Sandstone have failed and slid northward toward the low-lying Santa Ana River, creating a complex pattern of crown scarps and depressions on the Barton Flats landscape. The steep terrane of Sugarloaf Mountain has shed numerous large landslide masses (McJunkin, 1978; Powell and others, 1983; Sadler and Morton, 1989), as has the precipitous west-facing slope of the Granite Peaks massif (Matti and others, 1982b). Spectacular landslides have originated on the north side of the San Bernardino Mountains. These include the well-known Blackhawk landslide, first recognized in the 1920's (Woodford and Harriss, 1928). This landslide traveled northward from the mountains a considerable distance onto the desert floor by riding a layer of compressed air (Shreve, 1968). Older landslides similar to the Blackhawk slide have been shed off of the north face of the San Bernardino Mountains throughout late Quaternary time (Sadler, 1981, 1982a; Matti and others, 1993).

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