San Bernardino Mountains
This web narrative on the geologic setting of the San Bernardino Mountains is taken from the following sources: Bailey and Jahns (1954); Dibblee (1982b); Matti and Morton (1993); Matti and others (1992a, 1992b); Miller (1946); Sadler (1981, 1982); Spotila and others (1999).
The San Bernardino Mountains consist of a high, east-trending elongate block that has been uplifted to its present elevation during the last few million years. The west, north, and south margins of this block are tall, precipitous escarpments whose geomorphology reflect the faulting tectonics that have elevated the mountains; the east margin of the block declines more gradually to the Mojave Desert floor.
The north side of the range is bounded by a series of south-dipping faults referred to as the north-frontal fault system (Meisling, 1984; Miller, 1987). The interior of the range is traversed by the east-trending and north-dipping Santa Ana Fault that separates the mountain geomorphology into two main terrains:
- to the north, an extensive, partly dissected plateau that forms the main mass of the San Bernardino Mountains;
- to the south, a more strongly dissected terrain that has been deeply eroded by major stream canyons that head into the interior away from the south edge of the mountains. The highest summits of the San Bernardino Mountains occur in this terrain, including San Gorgonio Peak (11,485 ft).
The southwestern and southeastern margins of the San Bernardino Mountains are traversed by several strands of the San Andreas Fault zone that in part form the geomorphic and structural boundary of the range. These faults separate geologic materials and geologic structures of the San Bernardino Mountains into three main domains (Matti and others, 1992a):
- Rocks in the southeastern San Bernardino Mountains south (outboard) of the Mission Creek strand
- Rocks between the Mission Creek and Wilson Creek strands
- Rocks of the main San Bernardino Mountain mass north (inboard) of the Mill Creek strand
Geologic materials of the San Bernardino Mountains mainly are ancient basement rocks that have been uplifted to their current elevations. These rocks include the following major groups:
- Very old crystalline rocks that are part of the ancient North American continental interior. These include igneous granitic rocks on the order of 1.7 billion years old that locally have intruded even older rocks that originally were deposited as sediments but ultimately were metamorphosed to gneiss before or during intrusion of the 1.7 b.y granites. These ancient North American basement rocks are only locally preserved because they have been obscured by subsequent geologic events.
- The ancient North American basement rocks ultimately formed a continental-margin platform for great thicknesses of marine sand, silt, mud, and calcareous sediment that began to accumulate on the platform about 1 billion to perhaps 900 million years ago (Cameron, 1981, 1982). These sediments accumulated during the Paleozoic Era for several hundred million years before deposition terminated and the sediment pile was warped, folded, faulted and dragged deeper into the Earth's crust where the sediments were metamorphosed to the metasedimentary lithologies (schist, metaquartzite, and marble) we now observe throughout large parts of the San Bernardino Mountains.
- Starting about 230 million years ago in the early Triassic Period, all pre-existing rocks in the San Bernardino Mountains area were injected by a succession of granitic magmas (plutonic bodies) that intruded and enveloped the older rocks. Magma injection was most intense and pervasive during the late Cretaceous Period, between about 80 and 70 million years ago.
- Between 70 million years and about 20 million years before present, little is known about geologic events or geologic materials in the vicinity of the San Bernardino Mountains. Presumably the area was undergoing gradual uplift and erosion of rock material that was transported by streams away from the region, but this history is not well understood.
- Starting about 18 to 20 million years ago in the early Miocene Epoch, stream and lake sediments began to accumulate locally in the vicinity of the San Bernardino Mountains (Meisling and Weldon, 1989) on a low-relief erosional surface that presumably had developed during the preceding geologic interval. Deposition in nonmarine environments continued intermittently throughout the next 10 to 15 million years (Miocene and Pliocene Epochs). In the San Bernardino Mountains, vestiges of these deposits can be seen in the western part of the range (Crowder Formation as used by Meisling and Weldon, 1989), in the vicinity of Running Springs in road cuts of State Highway 330, in the vicinity of Big Bear Lake, and in the drainage basin of the Santa Ana River (Santa Ana Sandstone as used by Sadler (1982, 1993).
- Uplift of the San Bernardino Mountains to their current elevation above the Mojave Desert floor and above lowlands to the south occurred in two stages: (1) block faulting and warping perhaps in the late Miocene (11 to about 5 m.y. before present) may have created an ancestral "proto" San Bernardino Mountains of unknown elevation (Meisling and Weldon, 1989); (2) uplift of the range in the Quaternary Period starting about 2 m.y. ago created the elevated and eroding landscape familiar to us today (Dibblee, 1975; Sadler, 1982; Spotila and others, 1999). This uplift history was accompanied by strike-slip faulting and related deformation within the San Andreas fault zone along the south margin of the range. Interaction between tectonic agents responsible for uplift of the range and tectonic agents responsible for strike-slip tectonics in the vicinity of the San Bernardino Mountains remains a challenge to geologists.
- The modern landscape of the San Bernardino Mountains is a product of erosional dissection by streams and rivers that are gradually stripping away rock products and carrying them downstream to alluvial basins at the base of the range. The next few million years of earth history will witness a competition between erosional agents that will tend to reduce the elevation of the San Bernardino Mountains and tectonic agents that may continue to increase their elevation.
See additional selected resources about the Transverse Ranges region
Geology of the Inland Empire (includes information about the San Andreas Fault and other fault systems)
San Gabriel Mountains
San Gorgorio Pass Region
Santa Barbara Coast Plain
Return to the Geology of the Transverse Ranges
Note: Information on this website is modified from:
Matti, J.C., Morton, D.M. and Cox, B.F., 1992, The San Andreas fault system in the vicinity of the central Transverse Ranges province, southern California: U.S. Geological Survey Open-File Report 92-354 , 40 p., scale 1:250,000.