Final answer:
The mountains around Brobdingnag are portrayed as impossible to cross due to their high and inaccessible features, poised rocks, and treacherous terrain that serve both as natural barriers and as strategic defenses by the inhabitants.
Step-by-step explanation:
The mountains around Brobdingnag are described as impossible to cross for a variety of reasons. The text references high and inaccessible mountains, signifying that the ruggedness and sheer scale of the geographical features present an insurmountable obstacle. Unique challenges highlighted in the excerpts include sheer cliffs, balancing rocks poised to trigger avalanches, and crumbling, cracked, rotten terrain that make it treacherous for both the mythical inhabitants and explorers alike.
In particular, one excerpt describes how the inhabitants, possibly cliff-dwellers, cleverly cut a gigantic boulder so that it precariously balanced on a fine point, ready to be pushed to start a landslide and block the outlet to Deception Pass. This not only suggests the treacherous nature of the landscape but also the inhabitants' adaptability and their defensive strategies involving the environment. Such a landscape would be difficult to traverse, justifying the sentiment in the question.