Wednesday, May 5, 2010

Seen in the Field: Fluffy Polygonal Ground and Mudcracks (Two Generations?)

Continuing on with the mud crack scheme, I ran across this small field of polygons the other day. Note the lath and 8x8 for scale. In the wide photo above, we're looking to the east, the next two photos look north and show an area mostly to the right of the darker brown area on the right side of the wide photo.
This part of the field shows the fluffy nature of some of the polygons. This photo not only has the same lath (hard to spot) and 8x8 for scale, but also some size 7 footprints that walk across from left center toward the lower center.
Here are the same large fluffy polygons enlarged, with the same footprints.
This photo is from somewhere in the central area of the first, wide photo. The larger polygons are separated from each other by mud or dirt that filled in after or during continued to repetetive formation of the polygons, possibly by both dessication and freeze-thaw action. Dessicration cracks (mud cracks) have formed mostly inside the larger polygons, but some of the cracks cut across the polygon borders.

At first I thought that the large polygons were just a large version of mud or dessication cracks — and maybe they are — but this type of patterened ground is often associated with periglacial areas — though these weren't found in a periglacial region, more like an upland steppe or desert area. These muddy areas have recently been exposed to repeated freezing and thawing (winter and spring); temperatures will reach 90°F or higher come summer.

So, do these pictures show two generations of mud cracks — large and small, early and late — or are two processes at work here? (This is not my "two generations" post; that post will probably come up on Friday, unless I get impatient and post it tomorrow.)


Anonymous said...

Any bentonite beds around there? I would guess so--it looks very much like the bentonitic shale weathering surfaces that are characteristic of badland country in Alberta. See, for example

You call the texture "fluffy"; up here we call it "popcorn texture".


Silver Fox said...

Good idea, the expanding clays - there are expanding clays as part of the hydrothermal alteration in the area, and one of the shales *could* contain expanding clays, though it's not know for that.

I like "popcorn texture" as a term. I used fluffy because it reminded me somewhat of the cryptobiotic crust found in some desert areas (which looks fluffy to me).