Central Otago, Nelson and Marlborough structural interpretations

In Central Otago, and in much of Nelson and Marlborough, the structural interpretations suggest a succession of broken, tilted blocks. Some slope "regularly" with a sharp fault-scarp face and a gentle back slope, as that between the Wairau and Awatere Rivers. In a few cases, notably the horst between the Takaka and Motueka valleys, blocks have risen relative to neighboring areas with little tipping. The greatest variety in angle and direction of dip of adjacent blocks is found in the Otago region. The schistose scarp ridges are often high, sharp, and jagged, inviting such names as "Raggedy" and "Rough Ridge". There is a structural resemblance to Nevada's basin-and-range country but Central Otago has a much larger proportion of its area in ranges; the basins are confined to rather narrow flats in the major valleys. Small as these valleys are, they had an important part in the drama of agricultural and pastoral invasion.

South of Central Otago, between it and the sea, lies the southern area of hills, valleys, and plains which Jobberns describes as "a maturely dissected upland with residual hills and valley plains." There the most extensive lowlands, and those economically most useful to the agriculturists and graziers, are adjacent to the lower courses of the Taieri, Clutha, Mataura, and Oreti Rivers. Much larger than any of these, and the principal element in the lowland east of the mountains, is the great plain of Canterbury. Its remarkably even coastline of over a hundred miles is interrupted only by Banks Peninsula. Tapering out along the sea at both northern and southern ends, it reaches its greatest breadth of forty miles just inland from the peninsula. The plain is built of coalescing gravel fans, and there is an almost imperceptible rounding up to the heights of the interfluves from the valleys of the mountain-born rivers which occupy the swales. The gravels are extremely porous and the covering soils thin. The great depth of ground water below the centers of the fans has had very important consequences in the utilization of the area.

The transition from mountain to plain is sometimes abrupt, sometimes gradual. In the latter case, the intervening areas, in the east as in the south, are termed hill-and-down country. This type of land form reaches the coast, with no intervening plain, east of the mouth of the Clutha, bordering the delta plain of the Waitaki on both sides, and again north of the outlet of the Waimakariri. In among the downs, hills, and low mountains are other plains or flats: the Cheviot, Hanmer, and Culverden basins of North Canterbury are examples. In the northeast the "plains" are discontinuous patches of river valleys and deltas between the tipped blocks. The Nelson plain proper is a narrow band around Tasman Bay, but it is often meant to include the low-lying gravelly Moutere Hills which form a small triangle southward with the true plain as its base.

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