Fold mountains
Why do the Applachians have to be pre-Cretaceous? Good question. I believe
that there are undisturbed Cretaceous sediments actually impinging on the
mountains in the South, but that would only date the mountains in Georgia and
Alabama. There's a (very) little Late Eocene in Vermont that seems to postdate
the Green Mountains. I guess the key is the Mesozoic Red Beds to the East of
the Applachians. The question is — how disturbed are they? Could they
possibly have been in place when the Applachians were raised? I don't know the
answer to that.