Tag Archives: Prarie Grove member

Geo-pic of the week: Shark Tooth

Shark tooth westfork

You wouldn’t know it to look around now, but Arkansas, at times in the distant past, was teaming with sharks (and other marine fish).   Indeed,  Arkansas was in part or wholly covered by ocean many times in the past.  One such time was 250 million years ago, during the Carboniferous Period.  The fossilized Cladodus tooth pictured above belonged to a primitive shark that had sharp teeth with multiple points of varying size that it used to gig fish before gulping them down.  The long point at the middle of the tooth is broken off and displaced to the right in this picture.  

This particular specimen was found near West Fork, Arkansas.  It was collected from the Prairie Grove Member of the Hale Formation, a limey sandstone.  However, shark teeth can be found locally, throughout other parts of the state, in marine rock layers spanning hundreds of millions of years. 

Geopic of the week: Honeycomb weathering (with sleepy bats)

bat for geopic

Pictured above is a weathering style called honeycomb weathering.  Honeycomb is a type of differential weathering that produces a Swiss cheese appearance on the rock’s surface that can be quite striking.  This example is from a sandstone outcrop (Prairie Grove member) at Devil’s Den State Park in northwest Arkansas.

Honeycomb weathering has been observed in many rock types, from igneous to sedimentary, forming in wet and dry environments.  It has even been noted in man-made structures.  Despite being common, the causes of honeycomb weathering are poorly understood.  Some studies have linked its formation to exposure to salt in coastal regions, but that doesn’t explain its occurrence in north Arkansas.

In this instance, some clever bats have taken advantage of one of the pits in the rock and are using it as a sleeping shelter.

 

For more views of honeycomb weathering click here.