The West Coast from Abut Head to Milford Sound lies to the north-west of the highest part of the Southern Alps. Although the Alps are only from twenty to thirty miles from the sea, they do not slope evenly towards it, but are terminated by a major fault, the Alpine Fault. No extensive areas of high land comparable to the Alps occur on the north-west of this fault and the mountains, which rarely rise above 2,000 ft., are separated by wide areas of low relief.
It is convenient to consider the Alpine Fault as forming the boundary of the Alps proper and the coastal strip which lies between this fault and the sea. The Mahitahi, Waita, and Arawata Rivers flow across this coastal strip and divide it into four districts, each with distinct geomorphic features.
The most northerly of these districts extends northwards beyond the area described herein, nearly to the township of Ross. In it, morainic hills form most of the high land and older rocks rarely outcrop. Except for the lakes, of which Mapourika and Wahapo are the largest, the low land between the moraines is covered with recent gravel plains which slope seaward from the foot of the mountains. The Wataroa, Waitangi, Waiho, Cook, Karangarua, Makawhio and Mahitahi Rivers issue from glaciated valleys in the mountains and flow over the surface of the gravel plains to the sea. The largest of these rivers, the Wataroa, Waiho, Cook and Karangarua, are particularly heavily loaded with detritus and for most of the year they are discoloured by rock flour produced by the active glaciers in their headwaters. All the rivers mentioned above are aggrading their beds and spread widely over the gravel plains in numerous branches which often change in position and size after heavy floods. The rivers are not confined by definite banks, and, except in floods, are only a few feet deep, their rate of flow depending on the grade of the gravel plains which are steepest opposite the highest part of the Southern Alps, the more southerly rivers, the Mahitahi and Makawhio, being less rapid. Two smaller rivers, the Omoeroa and the glacier fed Waikukupa, both have headwaters in the Alps and flow in more confined courses between steep morainic hills. It is along the coast of this northern district that the morainic bluffs are best exposed. These bluffs are connected by even beaches behind which many of the rivers flow for some distance parallel to the coast before entering the sea.
In the next district to the south, that between the Waita and Mahitahi Rivers, the morainic hills are replaced by higher land composed of older rocks through which the Paringa and Moeraki (or Blue) Rivers flow in wide glaciated valleys. Lake Paringa lies between these rivers and occupies the bed of an old glacier which once flowed on both sides of Fish Hill to rejoin before reaching the coast at Abbey Rocks. The lake is now drained by the Hall River, a tributary of the Paringa River, the old glacial course to the sea being filled with alluvium. The coast between Waita and Paringa Rivers is very different from that to the north, the beaches being smaller and cliffs often extending to low water mark; islands are common off the coast and show that Tertiary rocks once extended further seaward. A sea cut bench about fifty feet above sea level extends for much of the distance between Paringa and Blue Rivers and is represented further south by a similar feature, Sardine Terrace. Such high level, marine cut benches have not been observed in the morainic bluffs or in the glacially modified Tertiary rocks further south. This coast line resembles the geologically analogous stretch from Martin Bay to Yate Point described by Healy (1938).
Low land extends for much of the twenty miles between Waita River and the northern boundary of the next high block south of Arawata River. This low-lying area of alluvial flats between the sea and the Alpine Fault is little above sea level and has an average width of four miles. The flats are not unbroken, but instead of the morainic hills which interrupt the gravel plains of the northern part of the coastal strip, we have here isolated, rounded, granitic hills, very similar in shape, but varying in height from a hundred or so to a maximum, in Mount McLean and Mosquito Hill, of over 2,000 feet. These isolated, rounded hills and other similar hills further north are enumerated later when discussing granite outcrops. Their origin is uncertain, and it is not known if they represent granite intrusions from which the surrounding country rock has been eroded or if they are the characteristic weathering of a granitic belt which extends under the gravel surrounding these isolated hills. It is certain, however, that they have been modified by the alpine glaciers which issued from the mountains only a few miles to the east. Rivers draining ninety miles of alpine slopes converge on this district and flow across it to the sea. The largest of these, the Haast and Arawata Rivers, are similar to those of the northern part of the coastal strip and have wide beds with ill-defined banks. The three smaller rivers, the Okuru, Turnbull, and Waitoto, are less rapid and leave their glaciated alpine valleys to cross the coastal strip by slightly entrenched, open meanders. Large areas of swamp lie between the raised banks of these rivers. From Waita River to Jackson Bay the coast is low lying and, except for a small hillock of greywacke at Mussel Point, composed of sand and gravel.
The hills which extend along the southern part of the coastal strip from Arawata River to Yate Point are broken at Cascade, Awarua and Hollyford Rivers by wide alluvial flats. At Awarua River the flats extend from the head of Big Bay along the valley of Pyke River to Hollyford River and separate the Sara Hills from
the other high land. The only other large river is the Gorge which flows in a narrow rocky valley cut through both Greenland and Tertiary rocks.
Benson, Bartrum and King (1934) and Benson (1935) considered that the sloping moraine-covered Cascade Plateau is quite comparable with the coastal plateau described at Preservation Inlet, and later Benson and Holloway (1940) published two photographs, one of which shows part of the Sara Hills and the other a part of the coastal strip near the lower Gorge River. They considered that these two districts also represent part of the coastal plateau.
In a later part of this paper it will be shown that the Cascade plateau is not only covered by moraine as was suggested by Turner (1930), but that Turner's Conglomerate Series is also moraine and cannot be separated from the overlying material.
Healy (1938, p. 83b, Fig. 4) published a photograph of the Sara Hills showing what appears to be an even sloping surface forming the seaward part of these hills. This apparent surface appears to end near the centre of the photograph, whereas actually it extends landward and merges into the northern face of the Sara Hills. It is not at all certain that there are any flattish areas on top of these seaward sloping points, for the apparent flatness may be the result of viewing a convex ridge in profile. They may be as convex in cross-section as the similar looking moraines are known to be further north. In the same way, as on the south side of Big Bay the seaward-sloping Awarua Point is continued east by a bench cut along the north side of Big Bay.
We consider that all these features have been caused by glacial erosion and deposition and differ from similar features exhibited by the northern moraines only in that the morainic covering is thinner and more of the underlying rock is exposed.