Although not especially high, the Blue Mountains were sufficiently rugged to prevent European explorers and settlers from penetrating the inland of New South Wales from Sydney for some time. Although Aborigines were able to negotiate the mountain trails, it took Europeans from 1788 - when Sydney was first settled - until 1813 (25 years) to discover a way over. Even then, it was never by following the valleys, but only by traversing the mountain ridges, that a road over the mountains was made possible. http://wikitravel.org/en/Blue Mountains