An karst feature in Nippenose Valley, called a karst window.

"A karst window is a special type of sinkhole that gives us a view, or window, into the karst aquifer. A karst window has a spring on one end, a surface-flowing stream across its bottom, and a swallow hole at the other end. The stream is typically at the top of the water table. Karst windows develop by both dissolution and collapse of the bedrock. Many karst windows originated as collapse sinkholes." (from James C. Currens, Kentucky Geological Survey)