I'm not a geologist, I am a hiker. And after many hikes in California's Sierra Nevada and Utah's red country I figured it would be great to learn some geology and I'm just posting this question to see if I can get some clarity. Obviously, I don't know what I'm talking about and my experience is limited to reading a book, so I'd very much appreciate your opinion on what I saw on a hike today.

The hike was to strawberry peak (San Gabriel Mountains/Angeles National Forest) looking at rocks and wondering if someone can validate what I think they are :D

I was expecting to see mostly rocks like this - I think this is granite with coarse crystals - igneous rock from down deep in the earth (hand for size reference) - I believe I have this right to call this rock granite? And also that it was formed deep in the earth in a magma chamber and pushed up in whatever episode built these mountains? Is this really granite?

This also looks like granite, is that correct? What would make the layers and straight lines in this formation? The cracks I believe are from the stresses of the rock getting uplifted and surfaced but I don't know about the lines in the rock itself


What I was not expecting to see was stuff like this - which seems to me like sedimentary rock because of the layers and the fact that I could very easily pull off straight layers from it without any force at all:


Lastly there is what seems like a quartz vein inside this sedimentary(?) rock, is this just another layer of the rock or what type of process could inject quartz (or whatever this is) into the rock?

enter image description here

Thank you in advance!

  • $\begingroup$ The layers look like a foliation to me, a feature typical of metamorphic rocks. $\endgroup$ May 28, 2022 at 8:42
  • $\begingroup$ Ah interesting. I’ll have to read about that. I did see some layers that were in bent shapes so perhaps there was so metamorphosis going on $\endgroup$
    – Amperie
    May 29, 2022 at 0:02
  • $\begingroup$ Yes, these can be metamorphic granitoids, but I would rather think that these are gneisses. $\endgroup$
    – Weiss
    May 29, 2022 at 11:20

1 Answer 1


There are several types of rocks in San Gabriel mountain:

Geology of the San Gabriel Mountains is mostly Mesozoic (65 to 245 million years ago) granitic rocks, but there are also Precambrian (544 to 4,600 million years ago) igneous and metamorphic rock complexes. There are also occasional Pleistocene (57.8 to 65 million years ago) nonmarine sedimentary deposits adjacent to the riverbed.


Your first picture migth well be a Mesozoic granite, but also a Precambrian metamorphic granitoid.

The second one shows foliation, so it is a Precambrian metamorphic rock. Imposible to precise more with the picture.

From the third and fourth it is impossible to know what they are from a picture. They migth be Pleistocene nonmarine sedimentary deposits.

You migth wish to read some of the publications USGS published about the zone.

You have some more info about the geology of the zone here.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.