Whitcomb Ave Littleton and Harvard

I guess it is no secret that there is a stone chamber on Whitcomb Ave on the right as you drive south from Littleton. It is next to the road, across from a barn. Thing is, there is a second stone chamber less than a quarter mile to the north on the same road. It is up a gully, about 10 yards from the road, also on the right going south. If you look closely, there is also what looks like the remains of yet a third stone chamber, with its fronting removed and now part of a wall in the back yard of a house on Whitcomb Ave. All of this occurs in and around that circle I drew on the map.