My father and I are working on a narrative project about the local history of our family property. This isn't for school (we're both significantly out of that age range), just something we're both interested in, something we can work on together.

We found some old aerial photos in the state archives and noticed some strange white stones in a field. It's also possible they're not stones, but we don't know what they are and, as far as we can tell without actually digging, they're gone now (or buried under 75 years of silt and mud from flooding). Here is the gallery I made: http://imgur.com/a/2bjws

We're just curious about the practice as neither of us have encountered it.

The photos are from July of 1939. The images in the gallery are cropped from a larger photo. The photo is of the floodplain along the Illinois River in IL (there is more than one Illinois River, I found out when I lived in Southern Oregon). PM for specifics.

There are two sites represented in the photos, but more exist. I just found the two most prominent sites. According to the google maps measuring tool, they are about 1.6 miles apart as the crow flies.

Thanks in advance.