Very interesting. Reminds me of "pop rocks" (NOT the candy). Perhaps someone on this thread can tell me about them.
In the red clay and sand here in West Tennessee, there are often erosions we call "gullies". Growing up, my cousins and I had all kinds of fun playing in the gullies, and we'd often find what my grandfather called "pop rocks". He said if you put them in a fire, they would burst and make a loud popping noise, because they were hollow. We never tried that, but we almost always broke them open. In the cavity, there was usually some really pretty purple sand. Sometimes it was more red than purple.
Do these have a name? How are they formed?
I always heard those referred to as "Indian Paint" as a kid...the story they told us was that they'd crack them open and use the sand for war paint. I'm in northern GA and it's red clay here also.