Continuing with our previous theme “Sharkansas”, this week’s geo-pic is on Arkansas corals. Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.
The picture above is of a tabulate coral: a now-extinct variety of colonial coral. Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water. The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock. It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.
Other varieties of coral are found in the rocks of Arkansas. For more views of Arkansas corals click here
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.
Pictured above is what geologists refer to as an olistolith. The name comes from the Greek olistomai – to slide, and lithos – rock. Olitstoliths are basically the geologic record of an ancient landslide.
The mass of rock at the base of the outcrop broke free from where it formed and slid downhill to this location. After sliding (or tumbling) to its new location, sediment accumulated around the olistolith. Eventually, it and the sediment became a new rock . That’s what we’re looking at in this picture.
This olistolith is located on the north side of highway 412, west of Springdale, Arkansas. It was deposited about 350 million years ago when that area was a gently sloping ocean shelf.