There is a layer of limestone near our house that is composed of what we have been calling, for want of a better name, "brain rocks." This is because their texture is like the folds or sulcus of the brain. What are they? That's what we'd like to know.
Here is one spot we find them -- south of Robinson in Brown County, Kansas, on the hill south of the point where Buttermilk Creek crosses Redbud Road -- coordinates 39° 47' 51.18" N and 95° 24' 21.05" W:
The rocks are mostly eroded out of the hill here, but are almost everywhere you look. (Click on any picture for higher resolution.)
These rocks are sometimes found in a conglomeration, and sometimes they are separate. They range from tiny, to up to 5 inches in diameter. Many are broken in half, revealing layers on the inside. Below are photos to show the texture and detail of several samples.
Update:
We received the following excellent replies to our queries -- the first from the Fort Hays State U. geology department:
I suspect that the rock you're looking at is some variety of limestone or dolostone. Without being able to do some basic field tests I can't be more certain. Limestone would react vigorously in weak acid, whereas dolostone would visibly react with acid only if powdered. Both would be scratched by steel or glass....and the second from the Kansas Geological Survey:
The Kansas Geologic Survey geological map for Brown County indicates the nearest bedrock is Pennsylvanian-aged rocks of the Wabaunsee Group. It's impossible for me to say from here exactly where you fall in the overall stratigraphy of that group but it's constituted dominantly of shale with a number of limestone formations interspersed.
[...] I hesitate to go much beyond that without seeing the rocks firsthand. I do suspect that some of those rocks on your blog post (the ones that show concentric zonations, particularly) are showing post-depositional effects of interaction with groundwater, either during the process of diagenesis (original solidifaction of the sediments) or during subsequent hydrothermal events or weathering. [...]
Sincerely,
Ron Schott
Department of Geology
Fort Hays State University
According to Bob Sawin, a geologist here who has studied this area, these are stromatolites, a kind of fossilized algae from the Pennsylvanian Period of geologic history, roughly 300 million years ago. You can learn more about them at http://en.wikipedia.org/wiki/Stromatolite.
These are actually fairly common in various Pennsylvanian and Permian rock units in eastern Kansas. Bob says these are from a rock layer called the Soldier Creek Shale, which is part of a formation called the Bern Shale. Here's a scientific reference about them (though this is a publication that would probably be pretty hard to find, and would undoutedly tell you more than you need to know).
Sawin, R.S., West, R.R., and Twiss, P.C., 1985, Stromatolite Biostrome in the Upper Carboniferous of Northeast Kansas, /in/, Dutro, J. T., Jr., and Pfefferkorn, H. W., (eds.), Compte Rendu IX International Congress of Carboniferous Stratigraphy and Geology, vol. 5, Paleontology, Paleoecology, and Paleogeography, Southern Illinois University Press, Carbondale and Edwardsville, Illinois, p. 361-372.
Rex Buchanan
Deputy Director, Outreach and Public Service
Kansas Geological Survey
