Mystery Solved....Or Is It?

Once upon a time sand was deposited on the shore of a warm inland sea. This sand was rich in iron so was a lovely shade of red. As the sea receded, the sand dried out and hardened. Eventually, the sand turned into red sandstone.  

Over time, water returned to the area and carved its way through the rocky area. The red sandstone was exposed for a long while as the water lapped around it. 

As the temperature changed, the water began to evaporate and become more concentrated. As this happened, the water began to bleach out the stone in areas where small cracks wound through it, causing light colored stripes throughout the red.  

The sandstone spent many centuries exposed to the elements, weathering time.  

Eventually, the rock broke apart and tumbled to the ground, pieces landing in a talus pile below.  

The broken chunk of rock, with its intricate striping design, laid within the pile of rubble until a human came by and took it home to be in their rock collection.

That is one possibility, anyway!

~~~~~~~~~
Neat

The Mystery of the Rock is Still a Mystery

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!