17 April 2014


Posted by Callan Bentley

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…