The Alpine Fault running north-east – south-west divides Westland into two distinct parts. To the east stand the high schist and greywacke ranges of the Southern Alps; westward at lower level lie extensive areas of Pleistocene rocks, underlain by economically important areas of Cretaceo-Tertiary rocks (mainly to the north) which cover ancient folded basement rocks of possible Precambrian age broken by plutonic intrusions.

The Alpine Fault itself is a major topographic as well as geological feature of the region. The scarp forms a natural western boundary to the Alps and its course is marked in many places by a prominent fault-line valley. In places the precipitous western downthrow side falls 5,000 ft, forming a massive natural rampart. Both lateral and vertical movements have occurred along the fault; rocks to the west have been displaced toward the north-east and it has been suggested that a total horizontal shift of 300 miles has occurred, probably mainly in the late Jurassic-early Cretaceous Rangitata Orogeny (see Nelson district above). Horizontal movements continued on the fault in the Kaikoura Orogeny, but upward displacement of over 11 miles occurred simultaneously, forming the Southern Alps as they are known today.

Past movements along the fault have produced bands of cataclastic rocks, that is, rocks that have been deformed by the severe mechanical stress associated with the faulting. In these rocks individual minerals have sometimes been rolled and crushed so that frequently the entire rock has been milled to form a mylonite. More recent movements are marked by fault pugs, zones of greenish or purplish clayey, or gritty comminuted rock. In many instances the faulting movements have pushed the older schists or granite over Pleistocene morainic material. This is seen very clearly at Gaunt Creek, Waitangitaona River, near Whataroa. Such faulting and displacement of young terraces show that movement is still continuing.

The mountainous terrain east of the fault includes the highest ranges of the country, 8,000–12,000 ft; the rugged Mount Cook region is the most impressive as it is the centre of the main present-day glaciers. The highest part of the axial range is composed of intensely folded greywackes and argillites of Triassic and perhaps Permian age in this region. These grade westwards into their metamorphosed equivalents, schists. The metamorphism produced new minerals, first chlorite and then, where it was more intense, garnet and oligoclase. Rocks with these minerals were probably formed at great depths and have since been exposed by uplift and erosion near to the Alpine Fault.

Nephrite, the commonest type of greenstone used by the Maoris, is an ultrabasic rock found in some West Coast rivers as boulders carried downstream from bands occurring in the Alpine schists, notably those of the Griffin Range east of Hokitika. In south Westland, near the Moeraki River, mica has been mined from pegmatite veins in the schist.

West of the fault lie folded ancient greywacke and argillite (Greenland and Waiuta Groups) of possible Precambrian age and plutonic intrusions (mainly alkali-granites of late Paleozoic or Mesozoic age). Over much of Westland these are covered by younger sediments but are exposed in some areas, as, for instance, in the Paparoa Range and south of Ross. The Greenland and Waiuta Groups are unfossiliferous and have been intensely folded, contorted, and fractured. Some of the alluvial gold that has been won from the sands and gravels of Westland may have come originally from reefs in the Greenland greywacke, but much probably came from thin gold-bearing veins in the schists.

Section D (diagram 6), drawn through the Haast area, shows the Alpine Fault separating the Greenland and Waiuta Groups (with their associated plutonic intrusions) from the schists of Otago.

Towards the end of Mesozoic times the older rocks of Westland became greatly reduced by erosion; breccias and conglomerates accumulated locally on the eroded surface, in places reaching a thickness of several thousand feet. Deposition of coal-bearing rocks in local basins followed in late Cretaceous and early Tertiary times, the Paparoa Coal Measures immediately after the breccias, and the Quartzose or Brunner Coal Measures slightly later. In both groups of coal measures the seams are extensive and of good bituminous grade in north Westland. In south Westland only thin discontinuous seams are known.

Section I shows coal beds and late Tertiary sediments lying on the Precambrian basement rocks. To the east is the Grey-Inangahua Depression (which continues into south-west Nelson) filled by a great thickness of late Tertiary and early Pleistocene sediments, with thin coal beds at considerable depth. Late Tertiary and early Pleistocene deformation has faulted both the basement rocks and covering strata.

In the Tertiary (seediagram 7) subsidence allowed the sea to transgress, and thick sequences of marine sedimentary rocks were laid down on top of the coal measures. The mid-Tertiary limestones deposited at this time are quarried locally for agricultural lime. Marine erosion of a thinly bedded type has formed the Pancake Rocks at Punakaiki. Following the limestone, thick siltstone and fine sandstone were deposited, these being extensively preserved beneath the glacial gravels north-east from Ross to the northern boundary of Westland. The movements of the Kaikoura Orogeny folded the Tertiary rocks and at the same time raised the main ranges from which erosion produced vast quantities of gravels. The folding has produced structures which in north Westland may contain oil, the oil seepage at Kotuku being encouraging. Uplift of the ranges coincided with the Pleistocene ice age: as the mountains were uplifted they were several times subjected to attack by glaciers. Erosion by ice produced enormous quantities of glacial moraine and outwash gravel, which form a prominent feature of the country between the Alps and the sea and obscure many of the older rocks. What is believed to be the most ancient glacial deposit in New Zealand may be seen at Ross, after which the earliest glacial period in the country is named. Close to the moraines, as at Kumara and Kaniere, the outwash gravels proved to be a rich source of alluvial gold, and the action of the sea also concentrated gold in rich leads, not only along the present beaches, but also along old beaches now uplifted hundreds of feet.