Places of the Borrego Desert: Formation of Butler Canyon, Jackass Flat, and Lower Rockhouse Canyon
Dry Wash, Butler Canyon and Jackass Flat are puzzling features on the western edge of Clark Valley shown in the following map:
See also map without lines around feature names.
Jackass Flat is a curious flattish area between upper Butler Canyon and lower Rockhouse Canyon. How did it originate?
Butler Canyon is a deeply-cut canyon that looks exactly like the part of Rockhouse Canyon above Jackass Flat, yet it drains only a small area compared to Rockhouse Canyon. How did Butler Canyon get cut so deeply?
Dry Wash currently drains into Butler Canyon, but looking at the topo map, it looks like it really "should" continue across Jackass Flat and drain directly into Rockhouse Canyon. Instead of doing that, why does it make a sudden right turn and drain into Butler Canyon, taking the "long way" to get to Clark Valley?
The answer to all of these mysteries is that these are tell-tale features of the San Jacinto Fault in this area. (There are many others, such as Buck Ridge itself, and the arrow-straight drainages of Dry Wash and Alder Canyon.) Before the San Jacinto Fault moved it, Butler Canyon used to be at the location of lower Rockhouse Canyon, and was in fact the drainage for Rockhouse Canyon and Dry Wash.
The San Jacinto Fault carried Butler Canyon to the northwest. For a while, the upper Rockhouse drainage continued to flow into Butler Canyon through Jackass Flat, which reversed the former flow of Dry Wash through Jackass Flat. Eventually, a small drainage of northwest Clark Valley worked its headwater back into the Rockhouse Canyon drainage, and captured that drainage. This left Jackass Flat high and dry. Continued erosion eventually created the 200 foot scarp at the eastern edge of Jackass Flat, as the waters of Rockhouse Canyon flowed more rapidly on its now shorter course to Clark Valley.
The following cartoons illustrate the above story. All dates are just wild guesstimates, but are probably correct within a factor of two or so.
Two million years ago, what today is called Butler Canyon was the lower part of Rockhouse Canyon:
Roughly two million years ago, the San Jacinto Fault became active, and started moving the lower part of Rockhouse Canyon to the west. Here is a guess at what the drainage pattern looked like one million years ago:
Note that what is now Butler Canyon is about halfway between its present day position and the present day position of lower Rockhouse Canyon. The drainage continued to follow Butler Canyon as its moved west.
This drainage pattern continued until geologically recently. Here's what it might have looked like 0.2 million years ago, when Butler Canyon was close to its present location, but still receiving the Rockhouse Canyon water flow:
At this point, a small tributary of lower Rockhouse Canyon had worked its way up toward Jackass Flat, driven by the steep gradient in the topography there. Once it made its way to Jackass Flat, it captured the drainage of upper Rockhouse Canyon, and became the new lower part of Rockhouse Canyon:
