Topographic Expression and Historical Notes

The Wellington landscape is dominated by two landforms, one tectonic—the Wellington Fault scarp, and one erosional—the Kaukau Surface.

The Kaukau Surface (Cotton, 1912: 249; 1955: 14), a senile erosional surface of late Tertiary age, has been ruptured by Pliocene and Pleistocene faulting, and it is the dislocation of this surface which in the first place defines the Wellington Fault.

The Wellington Fault downthrows to the south-east and its scarp forms the western margin of the Port Nicholson and Hutt Valley depression (Pl. 43). The general trend of the scarp is north east-south west and the trace of the probable underlying fault is roughly marked by a very prominent line of bluffs, and at the south-west end by fault valleys which run to the southern coast of the Wellington Peninsula at the mouth of the Karori Stream.

McKay, in a map published in 1892, indicated the Wellington Fault and showed the Hutt River as being controlled to its headwaters by this fault. He correlated the North Island faults with the then known South Island faults, but correlation is still somewhat in doubt (cf. Cotton, 1954).

Bell (1910: 536) recognized the Wellington Fault but thought of the Hutt Valley as a complex graben. Cotton (1912) also described the Wellington Fault, and in 1914 recognized that the Silver Stream, Upper Kaiwarra Stream and Tinakori Stream are aligned along the Wellington Fault shatter belt. Cotton (1914) also noted that the Hutt Valley continues the line of the Wellington Fault north-eastwards, and in 1921 presented his “downwarping” theory to account for the formation of the Port Nicholson—Lower Hutt Basin. Cotton was the first to propose that the basin occupied a fault-angle depression (1921), and he (1951) was also the first to show that the Wellington Fault had an important strike-slip component. The Wellington Fault is now generally accepted as a transcurrent fault with a variable downthrow to the south-east. In the writer's opinion the fault is not simply definable, for it follows an almost mile-wide zone of intensely crushed rock. It is convenient in dis-

cussion to speak of the central part of this zone as the Wellington Fault and of the zone as the Wellington Fault Zone. Although some workers may speak of the fault plane as being exposed in given positions there is nowhere along the fault line any considerable exposure of what might be called the main fault. Instead there are many faults which are sub-parallel to the fault line, and the characters of these may reasonably be attributed to the Wellington Fault as a whole.