Barbed Tributaries of the Hutt River

Living to the east of the Hutt River the Silverstream, Mangaroa, Pinehaven and Stokes Valley Streams are all peculiar in that they join the river at an acute angle which points upstream. Such confluences have been called “barbed” by Cotton (1914, p. 296) who considers that they commonly have some special structural explanation. Two explanations could apply here.

• (1) Since the regional strike of the greywackes in this area is broadly north-south the tributaries may have been adjusted to the strike direction. In contrast the Hutt River is controlled by a fault which intersects the strike of beds at a low angle and it becomes possible for barbed junctions to develop.Against this thesis, the homogeneity which elsewhere characterises the greywackes appears to apply to the eastern valley-side also and marked rock control of minor streams appears to be lacking.

• (2) Alternatively, the eastern tributaries may be controlled by splinter faults passing out from the Wellington Fault. This view is strongly supported by the mapping by Grant-Taylor (1949) of a shatter zone in the Mangaroa Valley and by the presence of dislocated surfaces along the hills to the west of the Mangaroa Stream.

The general lack of a pronounced pattern on the eastern slopes of the Hutt Valley is atributable to the gentler slopes and lighter dissection there and to the probability that the most intensely faulted part of the area east of the Wellington Fault is buried under alluvium. Those lineations which do occur parallel to the Wellington Fault become less numerous and weaker to the east.In general the shearing and jointing of the greywackes die out away from the fault, and it seems likely that splinter faults like the Mangaroa also die out in that direction.