Silbury, to the best of my knowlege, has never been 'chalked faced' Silbury is mostly made of calcium carbonate - from its surface to its core; it may even have glowed (fluoresced) under certain conditions.

Originally Silbury would have stood as a gleaming white structure set at the centre of a green (and sacred?) landscape. Whether or not it was then turfed over by its builders (or by later generations) or naturally acquired its grass covering (I would estimate the natural grassing of Silbury taking no more than ten to twenty years) is an interesting though irrelevant question when considering the possibilty and meaning of a spiral path to its summit.