U-shaped structures - from the Ozarks

From reader Gary:I’m seeking ideas concerning the age and purpose of some U-shaped arrangements of piled stones I know of in the Ozark Mountains. One U is about 20 feet across, another about ten feet, and there are at least two smaller ones. All seem to be connected. (Diagram attached.) There may be others hidden under vegetation or soil.

Each U has an opening facing the southwest. The rocks are piled about a foot or so high. The topsoil is now about four inches above the level at which the stones were originally placed. The exposed portions of the stones, many of them limestone, have darkened. (They seem darker than stones in an unrelated stone fence a half mile away put up by a farmer around 1910.) Today, the site of the U’s is overgrown with vegetation including old sugarberry trees, one of which has grown up through one of the stone walls.

The U's are on a point of land at the intersection of two small dry creeks. The land is about four feet above the creek level. The creeks have water only during good rains. Can the U's be dated by the amount of the topsoil that has formed along the rocks? The soil was probably not brought in by flooding; the creeks are short and I've seen water over this land only once in 30 years. I believe the soil was created mainly by decaying vegetation. Or can the U's be dated by the darkness of their exposed surfaces? The unexposed parts are quite pale in comparison.

What would have these U's been used for? Osage Indians hunted this area circa 1550 to 1815. Cherokees used the area c.1800 to 1825. White frontiersmen began hunting in the area c.1800, but these formations look more Native American. Could they be remains of hunting blinds? Bison, elk and deer would have traveled these creek bottoms. Could the smaller U's be fireplaces? I've also heard some nomadic hunters used a base of piled stones for their hide wickiups or wigwams? Any ideas?