Posted by: Alan Pitts | May 28, 2010

Mud Cracks

The last two Wednesdays, I had the opportunity to travel out to the Valley and Ridge province of West Virginia with my buddy Aaron for some hiking, exploration and maybe a little geology along the way ( OK, actually it was quite a lot of geology).  On our way to hike Seneca Rock driving along the old WV route 55 we saw these great mud cracks:

Mud Cracks are V-shaped fractures which form as a response to dessication or drying out.  They form in both siliclastics or carbonate rocks.  You can see the pseudo-polygonal shape left as these mud cracks meet at roughly 120 degree triple junctions.  This is in the Tonoloway formation, a Silurian age (appx 370 mya) rock unit characterized by tidal carbonate deposits representing a passive phase on the future east coast of North America.

Here is another picture from the same area,  These look sort of similar columnar jointing, which is caused by the cooling of basalts. These are two different processes in different rock types, but essentially a response to the same phenomenon of contraction.

Here is a profile view:

Mud cracks are  an important primary structure, which provide a clear idea of which way was up when these rocks were deposited.  In the case of these photos, the mud-cracked surfaces of the beds are “up”.  This may seem like a pretty obvious observation….up being up.  But when looking at folded and faulted rock strata in mountain belts, up is not always necessarily up.

That being said, here is what makes this out crop and these mud cracks so cool:

In this picture Aaron is standing on the surface with the mud cracks.  In fact the whole wall is covered with them.  These beds were originally laid horizontally, and you can see by comparison with the road, they are now nearly vertical.

Here are some recent mud cracks that I spotted near where I parked my car:

These are very fresh , but formed under the same conditions as the lithified mud cracks did several hundred million years ago. Kinda cool huh?

Here are some other sweet mud cracks over at Fossiliferous Weekly.


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