I found this interesting looking slab of gray limestone last summer in the Bridger Range of Montana, in one of the talus slopes on the north side of Sacagawea Cirque.
The high-contrast pattern reminded me of something, but I couldn’t say quite what. Then I realized: it looks like one of those indigenous pictographs, where the artist puts their hand up to the rock and spits paint all over it, coloring the non-hand space and leaving the space covered by the hand free of color. The color effect is sharp-edged and darkest right at the contact with the margin of the hand, and fades off in all directions away from that edge.
This rock shows the same pattern, except nature made it. It’s strikingly similar in form and boldness to writing or hieroglyphics. Lovely, but I’m not sure how exactly it formed. Let me speculate…
I think what was happening here is that there were certain preferred fluid-flow pathways in the rock. Some were cylindrical / linear, piercing the bedding plane as a point: burrows, perhaps?. Others were tabular / planar, intersecting the bedding plane in a line: fractures? bedding-parallel feeding/crawling traces? Either way, fluids were percolated through, and these fluids interacted with local consituents to produce a reddish staining (oxidation of iron, presumably). This effect falls off with distance away from the “plumbing.” Later, perhaps, a second batch of fluids, with a different composition, pumped through the same system. This time, the local effect was to bleach the rock back to its original color (so the fluids would have been “reducing” in their chemical activity). And then, a third batch, to make the deep red “cores” to the lines and points??
Anyone want to offer a different take on this? I’m sure I can learn from your insights…
This looks like relict redox features in soil. Is this a sedimentary rock? It could be that the narrow reddish lines are old root channels. If there were roots growing there (I don’t know if the geologic time frame is right), the area surrounding the roots may have become depleted of iron, causing the grey color. The area surrounding that, where the red color looks to have bled into the surrounding material would be the zone of concentration. If the plants died and the roots decayed, a fresh source of oxidized iron-rich material may have filled the voids. This is just speculation, but may be another, more biologically-induced explanation.
Oh, sorry – did I not mention it was a limestone? Yes: a marine sedimentary rock.
I tend to concur with Lorene’s interpretation. They look alot like rhizohalos. We see similar redox features from rhizoliths in limestones around Kansas City. Continental margin with fluctuating sea level perhaps? Good rock!