Why would older rocks be less resistant to erosion?
While it's always a bit of a walk on the wild side trying to guess what Faith might be thinking, I'm going to hazard a guess. My guess is that when she looks at a cross section of strata exposed in a canyon or elsewhere, she momentarily forgets that the layers have been buried for most of their history. It's then just a short step to receiving the intuitively divined certain knowledge that the really old layers should have weathered more than the younger layers, despite their not
actually having been out in the weather the whole time.