Frank S,
About 15 - 20 years ago I read about a marathon runner who went to a Peruvian mountain range to train and apparently got the shock of his life. His training buddy was a 70 year old who ran in bare feet and the writer could not keep up with him.
Considering that a reasonably athletic fit person would have little difficulty navigating this terrain if lightly encumbered but any large animal especially a team harnessed and pulling wagon would be stopped toot sweet you have a credible theory.
BTW when a host encounters adverse conditions they are broken to smaller divisions and given cart blanche to progress.