Alternatively use of a local seam could indicate that in prehistoric times there existed mineral resources that have left no trace in the modern geology. But my feeling is that if confirmed it would go to show that they had a vastly different conception of what was and was not workable mineralogy to that presently used to rule ore deposits in or out of consideration for prehistoric usability (after all, archaeologists now believe that the Ring of Brodgar includes stones from communities off Orkney Mainland).