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.