The Blue Mountains, west of Sydney, proved a formidable barrier for the early white settlers in New South Wales, and it took 25 years to find a practicable way across. The main road road and railway today follow the originally discovered route along ridges, and there is only one other road across. It's not that they are particularly high: from the coastal plain, they appear as gentle hills. But they are a dissected sandstone plateau, and the earliest explorers, following the valleys inland, came to vertical cliffs. A good part of the plateau is World Heritage listed wildereness.