earthworks. The climb from the western side of the Nepean River at Menangle up to Mittagong is fairly sustained, a fact that is hard to appreciate at high speed on the modern freeway. The highway climbs non-stop over a distance of from the Pheasants Nest bridge over the Nepean River to Yerrinbool, before dropping slightly before the final climb to reach the tablelands at Aylmerton, a climb of over in . Early road construction Governor Lachlan Macquarie ordered the construction of a road, which became known as the Great South Road (the basis of the northern end of Hume Highway) in 1819 from Picton to the Goulburn Plains and he travelled to Goulburn in 1820, but it is unlikely that even a primitive road was finished at that time. The Great South Road was rebuilt and completely re-routed between Yanderra and Goulburn by Surveyor-General Thomas Mitchell in 1833. The Main Roads Management Act of June 1858 declared the Great South Road, from near Sydney through Goulburn and Gundagai to Albury, as one of the three main roads in the colony. However, its southern reaches were described as only a 'scarcely formed bullock track' as late as 1858. The road was improved in the mid-1860s with some sections near Gundagai "metalled" and all creeks bridged between Adelong Creek (approximately 10 kilometres south of Gundagai and now known as the village of Tumblong) and Albury. Mitchell's route in New South Wales, except for the current-day bypasses at Mittagong, Berrima and Marulan (dual carriageways were completed in 1986), is still largely followed by today's highway. Mitchell intended to straighten the route north of Yanderra, but was not granted funding, although his proposed route through Pheasants Nest has similarities to the freeway route opened in 1980. Mitchell's work on the Great South Road is best preserved at Towrang Creek (10 kilometres north of Goulburn), where his stone arch culvert still stands, although it was superseded in 1965 by a concrete box culvert which in turn was superseded by the current route of the highway when it was duplicated in 1972. By contrast, in Victoria there was an early and major change to Mitchell's route. Mitchell's original route between Albury and Melbourne went through Mitchellstown on the Goulburn River and took a long detour to the west of Mount Macedon. In March 1837 Charles Bonney blazed a new trail from Mitchellstown through Kilmore to Melbourne,