Shanks was also in the salt business. The land in question probably extended down to the gut behind the Queen's Head. Probably the area in your photos. (I think '2.5 feet' in Mackenzie must be an error?) I would not be surprised if the circular stone work were more salt making pans. Here are some circular Elizabethan specimens in Cumbria.
http://www.visitcumbria.com/wc/crosscanonby-saltpans/