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2021 - Trip Report - Prospecting Holes Near Cajon Summit

Prospecting Holes Near Cajon Summit

By Bill Neill

In late February, while taking geology photos of the San Gabriel Mountains and vicinity, I discovered Forest Route 3N21 that heads west from Cajon Summit, along the ridge between the Mojave Desert and Cajon Amphitheater, starting at the brake check area on Interstate 15 southbound. At its west end, this easy but winding 4WD route terminates at Highway 138 about 5 miles northwest of Cajon Junction. I drove only about a mile west of I-15, for impressive views of the Cajon basin and San Gabriels, with snow-covered Mt. Baldy about 14 miles to the southwest.

The ridge west of Cajon Summit is underlain by relict alluvial fan sediments containing cobbles and boulders of granite and metamorphic rocks, plus chunks of white vein quartz that are scattered about on the surface. Shown in the photo below, I placed several angular clasts of vein quartz on a gray slab of Pelona Schist, derived from mountains to the south.

Aligned along the ridge crest, I was surprised to see dozens of closely spaced prospector holes that I presume were dug in search of the source of vein quartz that can contain gold mineralization. Although I don’t know much about primitive gold mining methods, I presume that the holes were dug for prospecting and not dry-washing due to their fairly uniform size, depth and close spacing.

The photo below shows a straight line of diggings extending from the lower left corner past and behind my truck. The middle photo is a close-up of the same hill behind my parked truck. The lower photo is another location, with diggings extending up the hill and then turning to the left.

After returning home, I searched the Internet for the area’s prospecting history and found this posting from June 2012 on the website by someone named “MrLee”: 

Cajon Summit Mormon Diggings

I was out on the GPAA Crystal Claim 2 this afternoon and was surprised as I started to descend from the top of the hill there were layers of river rock in the cliff sides. I’m really wanting to know more about this area and how the hell there are river sediments on the tops of the hills with no mountains connected to them. Does anyone know more about this area? There are also boulders bigger than a suitcase and rounded by river tumbling all over the place. Very interesting area.

Answers to these questions might surprise “MrLee” and certainly would have surprised his Mormon predecessors, if that’s who they were.  The granitic and metamorphic rocks forming the San Gabriel Mountains are thought to have originated where the Salton Sea is now located, and have been displaced by fault movement along the San Andreas Fault, as coastal southern California shifted by earthquakes about 150 miles to the northwest over the past 5 million years, since the Gulf of California opened and separated Baja California from mainland Mexico.  More recently, the mountain range shed coarse sediments onto the Mojave Desert, before moving to its present location, which “beheaded” the alluvial fan complex and allowed Cajon Wash to erode the basin north of the fault.

The photo below was taken from near the summit of the paved road between Cajon  Junction and Lone Pine Canyon, showing the northwest portion of the Cajon Amphitheater.  The light-colored rock outcrop is made of thick sandstone layers that form Mormon Rocks; and on the skyline are steep cliffs called the Inface Bluffs, formed of relict alluvial fan sediments which dip gently northward toward the desert.  The San Andreas Fault is a half-mile behind the camera in Lone Pine Canyon, and alluvial fans sourced from the mountains once extended from the fault to the skyline ridge, but have been beheaded by erosion of the basin as the mountain range moved northwestward along the fault.

At the Cajon Summit diggings, we can only guess whether the “Mormon” prospectors were looking for placer gold or a mother lode of quartz veins.  Because the diggings follow the ridge crests, I assume that the prospectors thought the ridge line followed harder rock underneath that sourced the quartz fragments.  If so, they were hopelessly wrong; but in their defense, I’m only guessing.   ~ Bill