Monthly Archives: September 2013

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

Polygonal cracking in sandstone, Arkansas River ValleyPolygonal 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.

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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! 

Kinder Slough on Middle ForkSpent 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. 

Flood debris in lower Tick CreekAccording 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.Alluvial fan in Tick Creek 

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

 

Typical Cane Hill in small drainage to Middle ForkWe 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. 

Slide block in creek bedMaybe 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. 

 

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

GeoPic of the Week: Tilted rock layers at Cossatot Falls, Cossatot River State Park – Natural Area

Tilted Rock at Cossatot Falls

Tilted rock layers at Cossatot Falls, Cossatot River State Park – Natural Area

The Cossatot River is located in the Ouachita Mountains physiographic province in south-western Arkansas.  Notice the tilted rock layers.  Geologists informally use the term dipping to describe these rock layers.  The majority of rocks in the Ouachita Mountains are dipping and sometimes almost vertical.  Why?  The Ouachita Mountains Region contains sedimentary rocks that were originally deposited as flat-lying layers.  Later on, these rocks were uplifted and compressed northward due to a major mountain building process called the Ouachita Orogeny.  This caused the rock layers to be tilted.  The rocks exposed at Cossatot Falls are sandstones in the Mississippian Stanley Formation.

STATEMAP Field Blog August 26-28, 2013

Hello all!

This week was a hot one, so we did some creek work, but also did some road work.  Went up a section of lower Lost Creek on Monday.  Narrowed down the Cane Hill/Imo to a fairly small area.Danny on Cane Hill in Lost CreekSand on shale in the Imo on Lost Creek

Saw more nice trace fossils in the Imo.

Trace fossils in ImoTrace fossils in the Imo

Got a few points on the Middle Fork where it leaves the Shirley quad and enters Greers Ferry Lake.

Thin-bedded sandstone in Middle Fork Little Red RiverThin-bedded sandstone in Middle Fork Little Red River

Quarry near Fairfield BayAlso, took points on a 4 different shale pits or quarries.

Green persimmons
The persimmons aren’t quite ripe yet!

Deformation bands in massive sandstoneFound these deformation bands right by Highway 16 near Fairfield Bay.  These bands are associated with structural features like faults and folds.  They form in more coarsely grained sandstone when the stress crushes the individual sand grains along a plane, then recrystallizes to form a slightly more resistant lithology, that when weatherd, stand in slight relief.   We will keep looking in this area for more clues and hopefully figure out what the rocks are trying to tell us!

Snake count: 2

Tick attacks: severe

GeoPic of The Week: Ancient Plant Preserved In The Arkansas River Valley

 

PlantFossil1

Ancient plant preserved in the Arkansas River Valley

(stylus approximately 4 inches long)

Plant fossils in Arkansas are mostly preserved in rocks of Mississippian and Pennsylvanian age.  Most plants grew in swampy areas, eventually died, and accumulated to ultimately form coal. Because vast coal deposits are recognized worldwide, the Mississippian and Pennsylvanian systems are referred to as the Carboniferous Period or the coal measures.  This ancient tree-sized plant called Calamites was more abundant on higher ground along levees and floodplains of rivers than in swamps.  This picture shows a branch that would have been positioned on a segmented trunk at intervals.  This fossil was discovered in the McAlester Formation.