While you are on the subject, please see what "National Geographic" articles from the late 1940s, the entire 1950s, or very early 1960s you can find that address the states along the Mississippi River (IL, MO, AR, TN, MS, LA). I seem to recall this photo and caption were inside of an article about a geographic area such as the deep South, or a certain state. My poor memory cells also tell me the cave in question is near the Mississippi River.
Now I am curious to see how much of the above turns out to be correct. I am way outside of my areas of research on this one, but that photograph made a distinct impression upon me years ago.