GeoPic of the Week: Polygonal cracking in sandstone, Arkansas River Valley

Polygonal cracking in sandstone, Arkansas River Valley

Many of the sandstone formations in Arkansas develop a distinct weathered surface like the picture above. This pattern consists of polygonal plates that die out within a few centimeters depth and is a form of tessellation or commonly called ‘elephant skin weathering’. The plates may be flat or convex upward and usually follow the curvature of the outcrop. It is thought that a crust of minerals such as silica (quartz), iron, and manganese develops on the surface of the sandstone. This crust or skin then cracks due to changing surface stresses much like a glaze develops cracks on a ceramic surface. This explanation also applies to the development of “turtle rocks” at Petit Jean State Park and “40 acre rock” in Russellville. The majority of these features are present in the Hartshorne Sandstone in the Arkansas River Valley.

Statemap Field Blog, Sept. 3-5, 2013

Hello all!

Well, it warmed up again this week, but at least the humidity stayed lower.  Definitely getting a more golden quality to the sunlight in the evenings now, and the mornings are almost pleasant—maybe fall is just around the corner! 

Spent this week exploring lower Tick Creek which is one of the larger tributaries on the north side of the Middle Fork north of Shirley.  I can attest that this creek is aptly named.  It seemed that every plant above ankle high must be covered in them.  Danny and I mapped the upper end of this creek in the 2011-12 field season on the Fox quad.  It was all Cane Hill there with a cap of Witts Springs near the top.  This week we found Imo in the main channel a couple miles from its confluence with the Middle Fork, but haven’t found the contact with the Cane Hill yet.  Will look closer to the north edge of the map next week. 

According to local landowners, there was a 6-inch deluge at the end of May that washed out many of the access roads.  We could corroborate this after seeing all the large trees and other debris that had been piled up in the creek bed by the flood.  It looks like the entire valley may have been filled with water during that event.  Another good reason not to build anything in the floodplain of these steep-sided, narrow hollows in the Ozark uplands.  Where Still Hollow empties into Tick Creek, a small delta had developed during the flood event. 

There was also fresh cutting of the alluvial deposits along the stream bank which plainly showed a typical fining upward sequence.

Stream cut in alluvium along creek bank

Closer view of fining upward sequence in creek bank

Even closer view of fining upward sequence in alluvium

 

We walked up quite a few side drainages along Middle Fork and Tick Creek to see if we could catch a glimpse of the Imo as it reaches the bottom of the Middle Fork valley, but the breakdown of the Cane Hill and Witts Springs above has covered it almost entirely.  That left us with thin-bedded Cane Hill sandstone as the lowest unit that produced a decent outcrop in these smaller branches. 

Maybe we’ll get lucky when we hike up the sides of the Middle Fork north of this area.  We did see a large slide block that had fallen into the creek bed and was weathering away virtually intact.  If we had encountered that rock in isolation without the adjacent outcrop, we may have thought there was a fault close by due to the extreme inclination.  As it was, it’s just an interesting footnote to be filed away then used to cast aspersions on future structural theories based on similarly dipping rocks. 

Deformed Imo sand body in creek bed

Coalified calamites print in Imo shale

 

What we did see of the Imo upstream in Tick Creek has a lot of interesting lithology (rounded siltstone concretions and coalified wood prints in shale) and bedding structures (soft sediment deformation).  Can’t wait to see what we’ll find upstream!

Snake count: 1

Tick attacks: severe