Pictured above is approximately 450 million year old (Ordovician) St. Peter Sandstone photographed near the White River, north of Mountain View, Arkansas. At the center of the picture is a feature not uncommon in this formation: a “quicksand structure”. Though not well understood, this kind of structure is believed to be a preserved conduit through which ground water once flowed to the surface in a boiling spring.
A boiling spring is where ground water rises up through unconsolidated sediment such as sand, keeping the sediment in continuous motion so that it appears to be boiling. Because ground water contains dissolved minerals, the tunnel through which the water is rising can eventually become hardened by precipitated minerals. Today, as the sandstone that formed from that sediment is being eroded away, the conduit stands in relief because it’s a little harder than the surrounding sandstone due to that mineralization.
Clues like these “quicksand structures” offer us a glimpse into what the environment was like when the rock was being deposited.
Office of the State Geologist Blog 