The west side, which borders on Cumberland, is indeed bounded by a chain of almost unpassable Mountains, which in the language of the country are called Fells....
"Climbing in The British Isles. Vol. 1 - England"
W. P. Haskett Smith
The unpassable walk itself was an avenue of approach.
"A Matter of Proportion"
Anne Walker
Last of all, Carbo himself sent down several troops of horse against him by the river Arsis, which Pompey assailed with the same courage and success as before; and having routed and put them to flight, he forced them in the pursuit into difficult ground, unpassable for horse, where seeing no hopes of escape, they yielded themselves with their horses and armor, all to his mercy.
"Plutarch-Lives-of-the-noble-Grecians-and-Romans"
Clough, Arthur Hugh