The best use of road-building material in New Zealand depends to a great extet on an understanding of the country's geology and petrology. Certain areas are volcanic in origin and provide good roading materials such as basalt or, as in the case of Taranaki, andesite, a softer rock. The scoria of the Auckland area is an asset. In the centre of the North Island, around Tongariro National Park, the brown ash is very poor and fails completely when wet. The rhyolite ignimbrite and pumice of the Waikato Valley are familiar to most travellers, but these have limitations. In general, the North Island is deficient in good roading materials, and the plastic clays and volcanic ash in the Auckland Province make remedal measures most expensive. Some areas, such as Hawke's Bay, Canterbury, and Southland, are fortunate to have excellent greywacke gravels, the reslt of the denudation of the main ranges. Although these gravels are most useful for road bilding tey require special consolidation to stand up to the stresses of modern traffic. Frost and ice are special winter problems in some places. In varous parts of the country roads have had to be built on poor swamp foundations, as, for instance, near Bluff, to the north of Wellington near Paraparaumu, and in the Waikato and Bay of Plenty districts.
In both islands there are high mountain ranges which binder road communications. There are no low alpine passes suitable for road crossings on the Southern Alps between the Lewis Pass and the Haast Pass, a distance of 180 miles. In the North Island the only low crossings are the Manawatu Gorge and the Waioeka Gorge. Throughout the country ribs from these ranges or foothills set the highway engineer an exacting problem.