30 March 2011

Rusty rinds on peely rocks

Posted by Callan Bentley

Another group of interesting cobbles from the same sand and gravel pit that I described earlier in the week:

Here’s the same sample, gone all animated, for your perspectivizing pleasure:

What we’re seeing here is some fine-grained gray colored rock (siltstone, I guess?) that’s developed a pronounced weathering rind marked by a large amount of rust. This rusty rind is tougher than the original, unaltered rock, and it peels off, like an orange peel. There were a lot of these in that gravel pit for some reason…

Here’s one that split down the middle, like an apple halved by a meat cleaver:

My favorite one — as a certifiable “purty rock,” I brought it home as a little gift for Lily:

There’s a little bit of Liesegang banding going on in the lower left “corner” of this one — this suggests groundwater soaking in to the cobble and leaving a hematite deposit at the “soaking front.”