i have run across these types of stone structures in my line of work off the beaten path, i was told by old civil engineers that they were bult by WPA WORKERS after the depression of the 30's, many of these workers built structures like this while working on building culverts,roads,trails and the like and they would use these to stay on the work sites for a few days to sometimes weeks.`just what i was told and no reason to doub't it. cool eh!