Mormon Rocks is an interesting geologic feature in the Cajon Pass. Large sandstone rocks can be seen poking out of the land if you are driving by. Originally thought to be an offset of the Devils Punchbowl Formation, the Cajon Formation (early-Miocene aged sandstone) protrudes out from the Cajon Pass area, just north of the San Andreas Fault and Highway 138, and west of Interstate 15.
Mormon Rocks were named for 19th century Mormon Settlers who used the natural caves as a resting stop for both people and animals (ex. horses) en route to San Bernardino.
The rocks are actually much older than the Devils Punchbowl Formation, based on the fossils it contains. The Cajon Formation is 18-20 million years old and formed when this part of Southern California was submerged, just after the time subduction ceased from the former Farallon Plate under the North American Plate.
Here, compression and shearing over millions of years resulted in the rocks distinctive tilt. It is thought that this area was the site of the original plate boundary before plate motion shifted slightly along the current San Andreas Fault a few miles to the southwest. A similar process likely occurred at present day Devils Punchbowl. Notice that the Squaw Peak Fault is a thrust fault, which resulted in these rocks (shown in orange above) being brought to the surface and exposed here. The Squaw Peak Fault is parallel to the San Andreas. It formed due to the local stresses asscoiated with the San Andreas at the time of its formation here about 18 million years ago.
When the San Andreas Fault was first organizing, the plate boundary was quite wide and consisted of many sub faults (inclduing the Squaw Peak Fault). Over time, many of these subfaults became less active, while a few were more active inevitably decreasing the width of the plate margin from several miles to about 1/4 miles. The more active strands became what we now know as the San Andreas Fault zone.
Works Cited:
https://digital-desert.com/mormon-rocks/
https://calisphere.org/item/4f1999004faa0af1ab5fbf210ed6d129/
https://www.outdoorproject.com/united-states/california/mormon-rocks
Chapter 11, California's Amazing Geology by Donald Prothero
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. "Mormon Rocks" 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 Mormon Rocks in the background at the posted coordinates.
3. Describe the (a) colors, (b) sizes, (c) grain sizes, and (d) fossils (if they exist) you see at Mormon Rocks.
4. Describe any erosional features AND why do you think they are occuring.
5. Estimate the angle that Mormon Rocks are tilted by.
6. What visible natural topographic features indicate an active fault is nearby?