Tag Archives: dip

Geo-pic of the week: Dardanelle Rock

dardanelle rock from river

Pictured above is Dardanelle Rock located on the south side of the Arkansas River between the towns of Dardanelle and Russellville. The white truck in the lower right corner shows the scale of this outcrop. It was designated a Natural Area by the Arkansas Natural Heritage Commission in 1976.

The Arkansas River Valley is north of the Ouachita Mountains and is characterized by gently folded sedimentary rock that was subject, to a lesser extent, to the stress that folded the Ouachita Mountains.  The rock pictured here is the south limb of a broad syncline, or down-warped fold.  The north limb is about two miles to the northeast.  The bedrock dips to the north (toward the white truck), goes sub-surface beneath the Arkansas River, then reverses dip direction and rises back to the surface just southwest of Russellville.  If you could see a cross-section of the folded rock, it would look like giant a smiley face with the middle of the smile underground and the corners sticking up in opposite directions, two miles apart.

This picture gives perspective to the colossal size of geologic features geologists study.  Folds like this one, which can trap upward-migrating fluid, are sometimes rich oil and gas reservoirs.

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.