Here is one more email from a colleague of Norman's.
I would have to agree with M. Niquette, who stated in a response below, " When these prehistoric rock mounds are encountered it has been my experience that they occur on promontories with commanding views, high ridge fingers, and generally on mountain tops." And I will add that prehistoric cairns, almost without exception, are built above running water. The farmer stone piles that I have seen look nothing like prehistoric cairns to the trained eye. Farmer piles look like stones that have been dumped out of a wagon. They are usually low and strung out instead of neatly set with a "mound" shape. If one studies the context of the site, farmer piles are never found on top of the ridge but low on the hillside. It is usually noticeable that the surrounding land was once cleared for farming. I have never seen field clearing stones piled into neat stacks that resemble prehistoric cairns. I certainly have never seen filled circles or squares or heard about children doing this work. In this part of the state (Ozarks) I have not seen any low stone walls near an area cleared for farming. Farmers did not have the time to fool around with setting stones, you are right, it makes no sense. I cannot speak for the northern part of the state which has the majority of the farm land.