What kind of rock is this?
January 3, 2014 2:50 PM   Subscribe

I have seen photos of this type of rock on several Appalachian Trail blogs and galleries over the years. Two instances were in the Bland County, Virginia section.
posted by maggieb to Science & Nature (9 answers total) 4 users marked this as a favorite
 
Sort of reminds me of worm casts, but it's weird to see so much of the surrounding matrix removed.
posted by Big_B at 3:09 PM on January 3, 2014


It looks like some kind of sedimentary feature in sedimentary rock, although I have never seen anything quite like that before. The basic sedimentary rock types include shale, sandstone, and limestone. Shale typically displays very fine planes along which the rock breaks. I don't see that in the photo. It doesn't really look like a sandstone to me either. It could be limestone which often presents with that drab gray color. But as Big_B stated, it is odd to see a sedimentary feature with so much of the surrounding rock removed.
posted by Seymour Zamboni at 4:06 PM on January 3, 2014


Looks more like a mass of interconnected roots than anything mineral.

Like, but not quite like Banyan roots.
posted by jamjam at 4:20 PM on January 3, 2014


Example of worm cast. Another one.
posted by Banish Misfortune at 4:27 PM on January 3, 2014


This looks like an "ichnofossil;" i.e. a trace fossil left by the behavior of an organism; in this case, probably by burrowing since it seems to be on a singular bedding plane (boring organisms are more vertical). Often when this occurs it changes the matrix just enough, either through chemical or physical processes, that when mineralization occurs these become harder than the surrounding matrix, and the matrix either is not preserved or erodes away more quickly.

From looking at the t-junctions and y-junctions, these particular feeding and dwelling trackways would be called Thalassinoides. Thalassinoides are thought to be made by a type of crustacean.
posted by barchan at 4:29 PM on January 3, 2014 [7 favorites]


I forgot to add that it's not unusual to see features like this isolated. For example, the crustaceans may have been living in an area of sand and a storm event dumped a bunch of mud in the area. The burrowing organisms may go crazy feeding in the nutrient rich mud, and as a result, bring a bunch of sand with them (even more so if they are Fugichnia, or escape burrows) or fecal deposits, or whatever, and change the matrix, but the next time the tide came in, the rest of the mud got washed away, leaving just the more sandy burrows. (Different grain sizes get eroded differently due to the energy of the water.)
posted by barchan at 4:39 PM on January 3, 2014 [1 favorite]


There is a geolgy subreddit on reddit.com. You may have luck there if you don't have luck here.
posted by beccaj at 5:33 PM on January 3, 2014


Nah barchan has it. I've seen worm casts where the casts are a different material than the surrounding matrix, but barchan's explanation of introduction of mud (or even just silty/clayey water) over a sandy substrate could explain it. The mineralization of the tube material themselves is somehow more resistant than the sandy matrix and millions of years later, tada!

Nice find.
posted by Big_B at 5:59 PM on January 3, 2014


Thanks, all. This piece is pretty much on the trail. It has been trod upon, caressed and wondered about a lot.
posted by maggieb at 8:36 PM on January 3, 2014


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