<p>Interesting. The photo is low quality so I can not say for sure but possible candidate explanations are:</p>

<ul>
<li>a septarian concretion</li>
<li>a fulgorite</li>
<li>a dendrite</li>
<li>a differential staining based on differences in porosity</li>
</ul>

<p>To get a better answer please include some close-ups of the central area and some pictures of the other sides of the rock</p>

CC BY-SA 3.0
2015-08-21T12:56:13.693Z
My vote is differential staining or dendrite.
Earth Science Expatriate
CC BY-SA 3.0
2015-08-22T13:39:33.670Z
@GaryKindel Indeed, in particular when looking at the lower part of the rock, the dendritic concretion seems the most probable one.
furins
CC BY-SA 3.0
2015-08-04T13:05:58.987Z
2015-08-04T13:13:01.133Z
2015-08-04T13:13:01.133Z
furins
<p>I have what i think is a piece of siltstone with an interesting pattern on the face. It looks like a lichen or lightning bolt, as shown in the picture. The interior of the pattern is the same color as the stone itself, while the negative area is black. I was wondering what may have caused this pattern to form.</p>

<p><a href="https://i.sstatic.net/xnl3E.jpg" rel="noreferrer"><img src="https://i.sstatic.net/xnl3E.jpg" alt="enter image description here"></a></p>

CC BY-SA 3.0
2015-08-04T03:28:21.513Z
A creek in Des Moines, Iowa
dfghj
<geology><rocks>
What could have formed the pattern inside this piece of siltstone?
dfghj
# What could have formed the pattern inside this piece of siltstone?

(Asked by: username_0 on 2015-08-03)

I have what i think is a piece of siltstone with an interesting pattern on the face. It looks like a lichen or lightning bolt, as shown in the picture. The interior of the pattern is the same color as the stone itself, while the negative area is black. I was wondering what may have caused this pattern to form.

[![enter image description here](https://i.sstatic.net/xnl3E.jpg)](https://i.sstatic.net/xnl3E.jpg)

**Tags:** geology, rocks

> **Comments:**
> **username_0**: A creek in Des Moines, Iowa  

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## 1 answers

### Answer by username_1 on 2015-08-04. Score: 1

Interesting. The photo is low quality so I can not say for sure but possible candidate explanations are:

* a septarian concretion
* a fulgorite
* a dendrite
* a differential staining based on differences in porosity

To get a better answer please include some close-ups of the central area and some pictures of the other sides of the rock

> **Comments:**
> **username_2**: My vote is differential staining or dendrite.  
> **username_1**: @GaryKindel Indeed, in particular when looking at the lower part of the rock, the dendritic concretion seems the most probable one.  

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