To see the original blog on skolithos trace fossils click here
Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years. Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.
In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures. These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment. The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.
In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock. This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.
To see more views of skolithos traces from Arkansas click here
Pictured above is a bluff of St. Peter Sandstone exhibiting some spectacular black staining. The bluff is exposed near the confluence of Sylamore Creek and the White River north of Mountain View, Arkansas. Bluffs with this staining are referred to as “painted” because it looks like paint has been poured over the face of the rock.
The stains, which are manganese oxide, were deposited by groundwater as it seeped from the sandstone. The St. Peter Sandstone contains a minute amount of manganese that gets picked up by water as it flows through the rock. When the groundwater flows out of the sandstone, some of it evaporates leaving the manganese behind. Over time, a coating of manganese builds up on the bluff face.
The St. Peter Sandstone is also found along certain reaches of the Buffalo National River. The “Painted Bluff” – as it is known locally to river folk – is another great example of manganese staining.
This is a picture of a paleokarst feature from the Upper Buffalo River in Newton County, Arkansas. Paleokarst features, like this one, are ancient caves or sinkholes that have been preserved in the rock record.
In this case, a sinkhole formed when bedrock was exposed above sea level and acidic rainwater dissolved a vertical pit in the bedrock. When sea level rose and covered the area again, more sediment was washed in and the sinkhole was filled with sand. Eventually the sand became sandstone and a cast of the sinkhole is preserved today (center of photo).
All of this happened about 450 million years ago. Paleokarst features are one more clue geologists use to decipher earth’s history. If you didn’t know better, you might float right by and never give it a second thought.