The Geology of the PCT
C2 Cottonwood trailhead to Whitewater Canyon
Fig. 1. A Google Earth view of the area of stops 1 to 6 on the first few miles of the PCT beyond the Cottonwood trailhead; see text. Click on the picture to get a larger version without the labels.
The geology of this area is intriguing with active faults juxtaposing different rock types, and creating other interesting geomorphological features. Narratives are given at the following stops , which are shown on this map, with their GPS locations given in Table 1, from a 14 January 2015 field survey by RT Hawke.
Table 1. Locations of the Geology Stops (WGS84, decimal degrees)
Stop # Latitude Longitude Elevation (feet) 1 33.94692 -116.69243 1842 2 33.94802 -116.69075 1865 3 33.9487 -116.68902 1891 4 33.94976 -116.68236 2010 5 33.94992 -116.67054 2216 6 33.95073 -116.66699 2308 7 33.97063 -116.67357 3208 8 33.97468 -116.67404 2984
- At this waypoint, you are standing on the Cottonwood alluvial fan (see Fig. 1), composed of rocks transported from Cottonwood Canyon and Gold Canyon. This alluvial fan is young geologically and probably eroded through the uplifted alluvial fan found on both sides of it. The small Cottonwood Creek is now dry most of the year; therefore, this alluvial fan was created in a much wetter time. Looking south across to the San Jacinto Mountains, the large Snow Creek alluvial fan is quite dramatic.
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Fig. 2. The author at stop #3. See text. Click on the picture to get a larger version.
- This is the beginning of the linear valley of Gold Canyon (Figs. 1 and 2). Different rock types on opposite sides of the canyon and the shattered diorite bedrock on the north side of the canyon (Fig. 3) are all evidence of a fault; and what a fault it is! The PCT now follows the south fork of the San Andreas Fault. It is unusual direction for the fault, which, here, runs nearly east/west directionally. Typically, the San Andreas Fault system runs northwest/southeast. The plutonic clasts (rock fragments) in the uplifted alluvium on the south side of the canyon do not match the diorite bedrock on the north side canyon. The uplifted alluvium was therefore displaced from some other granitic source southeast of here, possibly from Whitewater canyon, Joshua tree National Park or maybe even further southeast. Besides the right-lateral displacement of this alluvium, there has also been vertical uplift with thrusting to the north. This same uplifted alluvium unit is also found just east of Whitewater Canyon and also continues west from here for a few miles.
Fig. 3. The shattered diorite just north of stop #3. See text. Click on the picture to get a larger version.
- The uplifted alluvium on the south side reveals its clast size from sand to rounded boulders 3-4 feet across, with some clasts the size of cars. The roundedness of the boulders gives evidence that they were transported some distance. The river that transported these boulders would be the size of a major river of the Sierra’s, or at least something of that magnitude in a flash flood. Bedding in the alluvium doesn’t appear to be evident, so maybe they are flash flood deposits. The rocks found in the alluvium are rich in quartz and feldspar from a granitic source. Looking to the north, a thicket of mesquite also gives evidence of the fault. The ground up rock, called gouge, is clay-like and impedes ground water flow and forces the water toward the surface.
- A small alluvial fan, which is now incising into small gullies. This also is evidence of uplifting in the area, where deposition changed to erosion. This small alluvial fan formed from the adjacent diorite. The diorite rocks, however, are orange instead of their typical Dalmatian-like look. This surface weathering is due to iron minerals, either from the rock itself or transported from some other source. The locals refer to it as “Palm Springs Gold’ and use it for landscaping. It appears to occur in a number of scattered spots in this part of the range; maybe fault related or old hydrothermal areas.
- The PCT starts to bend north and goes on top of this alluvial fan. The San Andreas Fault, however, continues east through the hills covered with windmills. It is very noticeable on the other side of the hill where the fault crosses Whitewater Canyon. The small community of Bonnie Bell exists due to the ground water being forced to the surface. The lush vegetation is a dramatic contrast to the streambed below. The east side of Whitewater Canyon, where the fault runs, is a mirror image of Gold Canyon with alluvium on the south of the fault and a large outcrop of “Palm Springs gold” on the north side of the fault.
- This could be an unmapped fault. The shattered granite at this saddle, the linear nature of Gold canyon to the south, and a linear valley to the north align at 240 degrees. This northwest/southeast direction is the usual alignment of the San Andreas Fault system. It is interesting to note that all the canyons from Cottonwood Canyon to Whitewater Canyon all have this same orientation.
Fig. 4. The view north at stop #7. See text. Click on the picture to get a larger version.
The view to the north (Fig. 4) is a dramatic change of scenery, with old terraces of alluvial fans that have been deeply incised. The top gradient of these old terraced fans seems to match the historical alluvial fill of Whitewater Canyon, which appears to be at least 300 feet deep. Whitewater Canyon then once again incised through this alluvium. This unconsolidated alluvium looks the same and may be contemporary in age with the alluvium south of Gold canyon.
- Here, the trail follows this deeply incised canyon, which runs east/west and may also be fault related. The old alluvium fill of Whitewater Canyon is evident. A veneer of it was left plastered on the bedrock walls when the canyon was eroded once again. The view across the canyon reveals alluvium of a much older age. This now conglomerate rock contains a variety of rock types with clasts that are anywhere from round to angular. This rock is strongly lithified with the matrix being stronger than the clasts. Some of the larger clasts, however, do weather out intact giving the conglomerate a Swiss cheese appearance. Another alluvial fan deposit is apparent on top of the reddish/yellow Swiss cheese conglomerate. It is a much grayer color, which may have arisen from the erosion of the basalt bedrock that is found near this unit.
Fig. 5. The view east from stop #9 down one of the incised canyons draining to Whitewater Canyon. See text. Click on the picture to get a larger version.
Fig. 6. The orientation of four canyons in this area, in degrees east of north. The red line shows this section of the PCT. Click on the picture to get a larger version.
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We thank Max Duggins and Natacha Walters for their help with editing of the text.
Copyright © 2015 by RT Hawke (text) and Tom Chester (photographs)
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 17 January 2015.