As you have seen, I've had photos of a special rock that used to be my grandmother's embedded here on my blog. There is a description of the rock and a bit of the story behind it written below. This is a special rock that once belonged to my grandmother. She lived most of her life in Oregon and northern California... The grains are silt-to sand-sized and in two colors. Both the lighter "matrix" color has the same size grains as the dark. Is it volcanic? Is it sedimentary?
I posted on a couple of geology blogs in hopes that someone had seen a similar sample somewhere, and although I've gotten some great ideas, the mystery of the rock is still a mystery. My sister even asked a blogger friend if he knew about such a rock. There are three viable ideas floating around (the infilling of mud cracks idea was shot down quickly because of the rounded corners of the red--dessication cracks are a brittle process that causes sharp edges). My favorite idea is the first one bulleted below:
- In-situ leaching along microfractures in a fine-grained rock causing the red iron minerals to get flushed out and leaving the light matrix. The rock type could be either a sandstone or fine-grained volcanic.
- An igneous rock that has been hydrothermally changed along a zone of alteration (which could also be related to the idea above since it has to do with water moving through the rock or sediment).
- A conglomerate that formed when broken up red sandstone was quickly deposited into a fine-grained, lighter color sand, then compacted and hardened.
These are great ideas, but WHERE would such a rock have been formed?
Many people did like to call the rock "fossilized giraffe skin". One thing for certain, I saw nothing like my rock when Googling images of rocks with the descriptions bulleted above.
A few people encouraged me to take my rock sample to a university and ask professors there about it. That is a good idea, but I don't have a university nearby with a geology department. Next time I am in Albuquerque on a weekday I can try to find the Geology Museum there on campus!
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6 comments:
It looks to me like the red iron oxides have been reduced (along some fractures, possibly?). I've seen a lot of red siltstones with circular spots that are the same color as your whitish areas - the circular spots form around bits of organic matter. A pattern like your looks as though water travelled through some parts of your rock more easily than others. If the water was more reducing, it could change the color like that.
Can you give me "history" of the rock from depositional environment to discovery? Lake environment? Hydrothermal? What do you think?
I was looking at your post and pretty much came up with the same idea that Kim had. Water running along tiny fractures and reducing the iron nearest the water pathway.
Thanks, Ed for posting your comment. I think that your ideas are the most likely answer to the process that made my rock what it is. I Appreciate your input!
I've seen this type of rock before. I recognized it instantly. It's a sedimentary rock- - sandstone. I grew up rock hunting California, and now live in Oregon. Can't remember where I've seen it, but I might have a piece. I'll look for it and probe my memory.
As to how it was formed, I don't know. I support the theory of one sandstone being broken up and becoming part of a newer sedimentary rock.
Your specimen is quite nice. If I recall correctly - the larger the chunk, the less concentration of red to be found. Your piece is such a great mix!
I've seen this type of rock before. I recognized it instantly. It's a sedimentary rock - sandstone. I believe I have a piece - I'll have to look for it.
I grew up hunting rocks in California, and I now live in Oregon. Can't remember where I've seen it. Yours is a fine example. If I recall correctly, the larger the chunk, the less red to be found. Yours is such a fine mix!
Not sure how it was formed. I support the theory that one rock broke up and became part of a new rock. This can easily occur with a landslide settling on the shore/floor of a body of water, or the wash of a flash flood transporting loose debris - in this case the red sandstone.
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