As you stand at the posted coordinates you will be surrounded by a beautiful rushing creek, lush trees, and majestic mountains. The tranquil setting, however, conceals a geologic history and also an ever-present threat: landslides. Landslides and landslide deposits are very abundant in the eastern San Gabriel Mountains. This earthcache will discuss an ancient landslide; active landsliding, however, is still very present in the area.
The rugged slopes of the San Gabriel Mountains are subject to extensive rock falls, many likely produced by earthquakes. Many moons ago, something catastrophic happened here. Originating high on the east side of San Antonio Canyon near the crest of Ontario ridge, a massive landslide took place. It could have been caused by a little animal dislodging a rock which hit another rock and so forth until the whole mountainside was sliding. Most likely however, there was an earthquake which violently caused rocks to dislodge and start the landslide. Tons upon tons of rocks thundered down the mountain, rolling and falling and knocking more and more loose. The massive landslide completely blocked the ancient Cow Canyon, forever changing the San Gabriel Mountains.
It is probable that before the Cow Canyon Landslide took place, water that drained from San Antonio Canyon above the Baldy Village flowed into present day Cow Canyon which would then flow into the San Gabriel River. That would mean that San Antonio Canyon above the Cow Canyon landslide was formerly the head of Cow Canyon. After the landslide took place, however, headward erosion of the lower San Antonio Canyon segmented the landslide and captured the drainage that formerly flowed west down Cow Canyon, creating the San Antonio Canyon drainage we know today.
*An adventure pass or interagency pass is required for parking.
Logging requirements:
Send me an email with the following :
-
The text "The Landslide that Changed San Antonio Canyon" on the first line.
-
The number of people in your group including their usernames.
-
Across the street from the posted coordinates you will find a road cut. Answer the following questions from where you are, do not cross the road:
- What has the roadcut exposed to you? Describe what you see.
- How can you tell that the rocks (on the top part of the cut) were part of a landslide and not of an ancient riverbed?
The above information was compiled from the following sources:
- http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5207093.pdf
- http://pubs.usgs.gov/of/2003/of03-293/SB100K_pamphlet.pdf
- http://geology.cpp.edu/janourse/ArticlesAbstracts/SCGS%20ESGMRoadLogPDF.pdf
