Western San Gabriel Mountains, CA

Stop #5:  Sedimentary rocks of the Vasquez and Mint Canyon Formations

The Mint Canyon Formation consists of sedimentary rock that is mostly sandstone and conglomerate.
We will stop where it is mainly conglomerate. The sediments that formed the rock were deposited from a
river about 12 million years ago. Although 12 million years might seem old by the standards of human
longevity, 12 million years is a short time geologically. These are relatively young rocks. By studying
the sedimentary rocks, geologists have been able to determine that the streams were flowing from the east
toward the west.

The clasts (rounded rock fragments) within the conglomerate can be used to decipher geologic history.
For example, within the conglomerate are clasts of the anorthosite seen at the last stop. This tells us that
the anorthosite has been exposed at the surface since at least 12 million years ago. Because sediments
travel downhill, the presence of the anorthosite tells us that there were mountains in the area of the
present San Gabriel Mountains 12 million years ago.

Another type of clast in the Mint Canyon Formation is a volcanic rock with a type of mineral said to have
a rapakivi texture (we don’t find this type of clast where we have stopped). The only location these rocks
could have been derived is from volcanic rocks exposed in the Salton Sea area of California. This
location is about south of where we have stopped. The location of the source for the clasts is also on the
other side of the San Andreas fault (Figure 4). Geologists infer that the reason for the large distance
between the Mint Canyon conglomerates and the source rock is because of movement of the San Andreas
fault. All of this movement has occurred since the Mint Canyon rocks were deposited 12 million years
ago.

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