This had me thinking so I researched a little and discovered..
"A detail from one of the walls of the Sacsayhuaman complex. Many stones show strange impressions or scrape marks, as if the surface has been soft during tooling." by Professor Davidovits from the Geopolymer Institute in France.
Now if you read one of my earlier posts I questioned similar markings on the walls at Rock Gate.
magnetic current ? acid ? what do you think?