<p>It's pretty hard to say anything about this kind of weathering without actually being there and having a closer look.</p>

<p>Usually, the simplest explanation for reddish or brownish colours in otherwise grey rocks is the presence of iron oxides. You can see that the brown area is on a fault or joint face. This leads me to think that maybe this was a conduit for groundwater that oxidised the rock. When the road cut (or whatever it is) was made, this part was exposed. Notice that you have some brown elsewhere - it could be that those are faults or joints cut perpendicular to the face, so that's why you only see a brown streak instead of a surface.</p>

CC BY-SA 3.0
2015-10-11T01:24:22.727Z
2015-10-11T01:24:22.727Z
Gimelist
<p>Below is a roadcut in central Marin County, California, in an area maps show to have Franciscan melange.  I assume this is graywacke, but as you can see it varies from gray on the sides to a buff color in the center.  Is the buff section due to some chert content, or did that section weather differently (and if so why)?  Thanks!</p>

<p><a href="https://i.sstatic.net/Q0nzA.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Q0nzA.jpg" alt="enter image description here"></a></p>

<petrology><california>
Why does the color of this hillside rock vary?
ed94133
# Why does the color of this hillside rock vary?

(Asked by: username_0 on 2015-10-10)

Below is a roadcut in central Marin County, California, in an area maps show to have Franciscan melange. I assume this is graywacke, but as you can see it varies from gray on the sides to a buff color in the center. Is the buff section due to some chert content, or did that section weather differently (and if so why)? Thanks!

[![enter image description here](https://i.sstatic.net/Q0nzA.jpg)](https://i.sstatic.net/Q0nzA.jpg)

**Tags:** petrology, california


---

## 1 answers

### Answer by username_1 on 2015-10-11. Score: 3 | Accepted answer

It's pretty hard to say anything about this kind of weathering without actually being there and having a closer look.

Usually, the simplest explanation for reddish or brownish colours in otherwise grey rocks is the presence of iron oxides. You can see that the brown area is on a fault or joint face. This leads me to think that maybe this was a conduit for groundwater that oxidised the rock. When the road cut (or whatever it is) was made, this part was exposed. Notice that you have some brown elsewhere - it could be that those are faults or joints cut perpendicular to the face, so that's why you only see a brown streak instead of a surface.


---

