View from GZ looking up canyon
Chino Canyon is one of the steepest canyons in North America. From its origin near the top of the Palm Springs tramway at an elevation of 8,500 feet, it drops to 2,700 feet in about a mile. It is popular with tourists who want to get on the tram and see some of the best natural viewpoints in Southern California. The canyon, like many in the area, was formed due to a combination of tectonics, gravity, and water.
Chino canyon is a prime example of an alluvial fan, which is common in washes originating in steep mountain ranges. Mt. San Jacinto (10,834 ft) is the second highest peak in Southern California, behind Mt. San Gorgonio (11,503 ft). The area between both peaks is considered the San Gorgonio Pass, and carries I-10 from Beaumont into Palm Springs. It is dotted with windmills due to the high winds that funnel into the pass.
The San Gorgonio Pass is bounded by the San Andreas Fault to the north side, with many strands that break way from one another. Strands of the San Andreas Fault include the Mission Creek Strand, which is considered to be the most active of the strands in this region. Additionally Garnet Hills strand of the San Andreas is also active, but to a lesser extent. The Banning Strand marks the southern boundary of the foothills of the San Bernardino Mountains. It is important to note that while these look like three separate faults, they are indeed all considered part of the larger San Andreas Fault zone. Much research has gone into which strand may rupture during the big one. Depending on which one goes and which one does not, will determine how much shaking Los Angeles gets. If the northern most strand takes the rupture along the Mission Creek Strand (which is most likely), then the "big one" may rupture past Mt. San Gorgonio and into the Inland Empire, causing a massive earthquake as large as an Mw 8.3 (Mw is short for moment magnitude, which is the most common way to measure the size and energy of an earthquake). However, if the southern strand ruptures, then it may stop as it approaches Mt. San Gorgonio due to the complicated woven matrix of faults on the south flank of the mountain. This is unfortunately not likely, but it is possible and would create a smaller earthquake and save Los Angeles from minutes of shaking.
Aerial view looking up canyon. Much of the alluvial fan has been heavily developed and is hidden under homes.
An alluvial fan is the accumulation of sediment that spread outwards from a source, typically a wash originating from a steep canyon. Alluvial fans are quite common, but what is not so common is that they are so close to populated areas. Alluvial fans form over generations of storms and rockfalls. They are easy to see from the air, but easy to miss on the ground due to their size. One common feature is that they have generally the same grade or slope throughout. Their grade is constrained by the size of clasts and the natural grade of the bedrock which underlines the fan. If the bedrock is completely horizontal, the fan will not get as large as bedrock that is tilted by a few degrees, which would give clasts an extra gravitational "push" as they roll out of the canyon. Additionally, there must be a reliable source of sediment so that thousands of years accumulation can occur and the fan can grow.
View from Palm Springs Aerial Tramway, looking at the source rock of the Chino Canyon alluvial fan. Note the small rockfall.
Let's do an example of how the fan can grow. An earthquake might occur nearby (lets assume this originates on a nearby strand of the San Andreas Fault), which causes a rockfall in the canyon. (The photo above is one I took while on the Palm Springs Aerial Tramway. While on the tram, you can get a great view up close of the source rocks that make up the allivual fan at the mouth of the canyon.) The rocks are then spread across the wash and stop a short distance later. A few months go by and a large storm event causes these rocks to be picked up by the water, and transported down canyon until the energy of the water is low enough to deposit them. This process occurs several times a year, and over thousands of years, alluvial fans can become quite large and distinctive geologic and geographic features on the landscape.
View from Palm Springs Aerial Tramway, looking at the source rock of the alluvial fan. Note the mafic (black) and felsic (white) dikes that cross cut the granite. Scale from top to bottom is roughly 1500 feet.
Clasts sizes can range anywhere from mud and clay to boulders the size of houses and everything in between. The clasts that make up this alluvial fan are granitic and are heavily diked by cross cutting both felsic (white, silica rich) and mafic (black, silica depleted) lavas. It would appear that the oldest dikes in Chino Canyon are mafic, with the younger ones being felsic. Typically, it is a mixture of sandy sediment which fills in the gaps of the larger clasts. If you would to move down canyon, clasts tend to become smaller and smaller. Therefore, if one would look at a cross section of an alluvial fan, the clasts would get smaller as you go from bottom to top in a vertical sequence. In the second image below, several storm events can be seen, due to the repeated layers of similarly sized clasts that lay on top of one another. The source of these clasts are found very close to the terminus of the alluvial fan, only at most by a few miles (see photo below).
Note how above the larger clasts in the sequence, the particle sizes are generally smaller with sand and mud infiltrating the cracks in between the larger clasts as the fan grows outward. When a flood or landslide event occurs, the largest rocks move the furthest down the canyon. These are known as a debris flow and can repeated several times within a sequence. As smaller events take place, sand and mud can fill in the cracks between these larger clasts. Large floods can repeat on the order of 50-1000 years, so it is very common to see repeated patterns in cross section. These types of sequences are called "fanglomerates" and they are distinct from conglomerates due to the angular shape of the larger clasts, signifying they were sourced locally. If the clasts were rounded, it means they had time to be rounded off during hundreds of miles of transport. That isn't the case with alluvial fans as the sediment are always locally sourced (less than a few miles).
Additionally, as fans grow, some channels near its mouth may become more active than others. In a sense, they act as a river delta where if during a flood event, channels can change direction entirely making the fan widen out with time. The picture below is from the top of the tramway looking down canyon. The terminus of the alluvial fan can be seen, about 2-3 miles from its source.
Works Cited
https://www.flickr.com/photos/38037974@N00/982674737
https://www.mercurynews.com/2021/05/17/new-san-andreas-fault-research-might-change-how-damage-shakes-out/
https://www.flickr.com/photos/tommy750/8464759234
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Chino Canyon - Alluvial Fan" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with the alluvial fan directly behind you in the background. You should be facing the visitor center with your back towards Mt. San Gorgonio.
3. Locate a few clasts/rocks that are easy to pick up and hold. Describe the (a) shape such as roundess or jaggedness, (b) colors, (c) textures, and (d) fractures.
4. Based on your answers to 3, are these clasts locally sourced (within a few miles) or do they originate further away (few hundred miles)? How can you tell? Explain.
5. From this vantage point, how far does the alluvial fan extend? Can you see where it ends?
6. Estimate the grade (or slope) of the alluvial fan from this location. Is it the same grade throughout?
7. How can an earthquake originating on the nearby San Andreas Fault effect the size of the alluvial fan? Would it make it grow or shink? Explain.
8. Based on your previous answers, is the fan shrinking or growing in size? How can you tell?