The problem with the the grading , if used as a support for a roof is it leaves a lot of wasted space at the lower end due to the boulders being quite small . It is however a sensible approach to allow the rain to drain away , which of course would create a very obvious impression easily observed by excavation , I can't think of one example of this although we do have plenty from habitation excavations.
Funnily enough sheep do leave a similar type of patina , and sometimes a trace of dye too (not that that would last many years ) just not seen at Findowie though.