Geology of Temecula Canyon
Tom Chester
Temecula Canyon is one of the most impressive features of the Fallbrook / Temecula area, where
the erosional forces of river water have dug deep into the granite of the rising Santa Ana
Mountains. The Gorge has steep immediate walls ~235 feet above the River, and has a total
relief of ~828 feet in a distance of only 0.25 miles. The geological story of its formation is
equally fascinating.
The story begins about 225 million years ago, when all the continents formed the single land
mass of Pangaea. The Temecula Canyon region was then underneath the ocean just offshore.
Mud and sand accumulated on the ocean floor, which sank as the material piled up. As much as
30,000 feet of sediments eventually accumulated! This formation is now the reddish Bedford
Canyon Formation found on the north side of Temecula Gorge.
About 200 million years ago, Pangaea began to break apart, and the nucleus of North America
began to override the oceanic crust which contained the Bedford Canyon Formation. As a result,
the mud and sand of the Bedford Canyon Formation were transformed into shale and sandstone
as they got progressively buried, due to the increased heat at lower depths and from the pressure
of the material above them.
About 150 million years ago, the oceanic crust from further west began to be forced underneath
the Bedford Canyon Formation, a process called subduction, and began to melt. Some of this
molten rock erupted to produce a chain of volcanoes like the volcanoes found in island arcs such
as the present-day Aleutian and Japanese Islands. The erupted material is now the Santiago
Peak volcanics, which formed mostly underwater and are as much as 1,000 feet thick.
For the next 75 million years, the subduction continued, which produced vast pools of molten
rock several miles below the surface and a string of volcanoes much like the present-day Mt. St.
Helens, Mt. Shasta and Mt. Lassen. This molten rock was mostly beneath the Bedford Canyon
Formation, except where it forced its way to the surface in volcanoes. The heat from the molten
rock metamorphosed some of the Bedford Canyon Formation. That is, some of the Bedford
Canyon rocks partially recrystallized and others were slowly deformed, changing the initially flat
layers to sometimes contorted shapes. The Bedford Canyon Formation was only weakly
transformed, and mostly retains evidence of the layers of the original deposition. However, those
layers are no longer always horizontal - in at least one place in Temecula Canyon, the initially
horizontal layers are now vertical!
Finally, 75 million years ago, the angle of the subducting oceanic crust changed to a more
shallow angle, which caused the area of molten rock to move east into Arizona and New Mexico.
The volcanoes of the Temecula Canyon region became dormant, and the molten rock began to
solidify, producing the granitic rocks of the batholith that underlie the entire Peninsular Ranges.
Those granitic rocks are locally called the Woodson Mountain granodiorites, and are found on
the south side of the gorge. These are the rocks we know and love in San Diego County which
form the big beige boulders on the hillsides, whereas the Bedford Canyon Formation weathers
more as a unit into reddish soils.
For 70 million years, nothing much exciting happened to the Temecula Canyon area except the
slow process of erosion. The high volcanic mountains were worn away, and the area became a
very flat coastal plain. It is possible that the Santa Margarita River got its start during this time at
approximately its present-day location. That location may have been determined by this local
junction between the Bedford Canyon Formation and the Woodson Mountain Formation, which
may have produced a local zone of weaker rock.
A last hiccup of volcanism occurred 9-10 million years ago, when the Santa Rosa Basalt
erupted on the surface and deposited a ~100 feet thick layer of dark red / brown basalt, topped by
a bit of white tuff, over an area of ~200 square miles that probably ranged from roughly just
north of Murrieta to the San Luis Rey River in Bonsall.
Then, 5.5 million years ago, the boundary between the Pacific and North American plates moved
to the location of the present-day Gulf of California, and the area became interesting again
geologically. An oceanic spreading center tore apart Baja California from mainland Mexico, and
the San Andreas Fault, and its host of accompanying faults, became active. The Santa Ana
Mountains began to form by uplift along their eastern end at the Elsinore Fault.
As the mountains were uplifted, it is likely that the Santa Margarita River was able to wear away
the rock as fast as it was uplifted, staying in the same place at the same height as the rocks rose
around it. The river is thus an antecedent river, existing before the mountain range surrounding
it. Otherwise, it would be very difficult to explain how a river formed in the lowland area of
Temecula suddenly cuts across a high mountain range!
The uplift removed the layer of the Santa Rosa Basalt and the Santiago Volcanics everywhere in
Temecula Canyon. The Santa Rosa Basalt is still present just a few miles away on the Santa Rosa
Plateau and near Vail Lake. The Santiago Volcanics are still found to the north around Santiago
Peak, to the west in the Santa Margarita Mountains, and to the south in Otay Mountain. Much of
the Bedford Canyon Formation was removed everywhere near Temecula Canyon, exposing the
Woodson Mountain Granodiorite on the south side of the Canyon, but the very lowermost part of
the Bedford Canyon Formation survived on the north side. (The Woodson Mountain
Granodiorite probably originally penetrated a bit higher on the south side of the Temecula
Canyon, and is probably found only a short distance below ground on the north side.)
Although the Santa Rosa Basalt has been removed from the rocks in Temecula Canyon, cobbles
of it can be found in the riverbed which have been eroded from the upper drainage basin of the
Santa Margarita River.
From the Gorge overlook, or down in the Gorge, take a look to the north and south and see the
quite apparent differences between the Bedford Canyon Formation and the Woodson Mountain
granodiorite. In addition, John Hogan and Claudia Luke have examples of these rocks on display
elsewhere today, including a sample of tufa from the Santa Rosa Basalt found in the riverbed.
Sources:

The Rise and Fall of San Diego, Patrick L. Abbott, 1999, Sunbelt.

Geology of San Diego County, Diane M. Burns, ed., 1997, Sunbelt.

Recency and Character of Faulting Along the Elsinore Fault Zone in Southern Riverside
County, 1977, Special Report 131, California Division of Mines and Geology, by
Michael P. Kennedy.

A Field Guide to Coastal Southern California, Robert P. Sharp, 1978, Kendall-Hunt.