The Hutt Valley.
A brief note on the Hutt Valley is here offered, because the Hutt River continues the line of the Wellington fault to the north-east, just as the, fault-line valleys already described continue it to the south-west. I am-
able at present, however, merely to hint at a few points. The physiography of the Hutt Valley is a subject that deserves careful investigation and thorough treatment.
The lower portion of the valley is undoubtedly a part of the Port Nicholson depression, resulting from the inbreak along the Wellington fault, that has been filled by a delta built forward by the Hutt River. This is proved by the continuation of the fault-scarp into the lower valley without break.
There appear to be good reasons for believing that the valley of the Hutt River to its very source is determined by a line of faulting which continues the line of the Wellington fault; but it has yet to be shown how far the valley is a fault valley, and how far a fault-line valley.
The predominant, longitudinal, topographic features, which I. regard as guided by the strike of the strata, trend rather more north easterly just north of Wellington than they do in the south of the peninsula: but within the basin of the Hutt they are again nearly meridional, trending about N. 10° E. Everywhere the valley of the Hutt meets these longitudinal features at an acute angle (seefig. 1). The Hutt River, which flows S. 60° W., receives tributaries flowing on the one side S. 10° W. and on the other N. 10° E. In the field this is most conspicuous, though the map shows it well only in the case of large tributaries like the Mungaroa and Akatarawa.
Eight miles from the mouth the broad plain of the lower Hutt Valley, which is probably all delta, ends, and the valley, narrowing almost to a gorge, is clearly the work of erosion. Farther up, the valley opens out again, and for five miles has a flat floor, in places a mile in width. The valley here is strongly asymmetrical. The south-east side is formed by long, sprawling, gently tapering spurs, the lower ends of which seem to be buried in an aggraded plain. The spur-ends are not truncated, and thus there is no evidence that the valley has been widened by the river swinging against this side. The north-west side is, on the other hand, formed by
high facets, all in line, which strongly suggest a fault-scarp, but may be a fault-line scarp. It is, of course, true that a vigorous stream, in the process of excavation of a broad-floored mature valley, might itself cut back both its valley-sides to this form, but the absence of bluffs on the south-east side of this portion of the Hutt Valley negatives this explanation.
This portion of the valley may perhaps be a Graben, bounded, as the northern end of the Port Nicholson depression is perhaps also bounded, by a fault on one side and a flexure on the remaining sides. Fig. 2 shows two trough subsidences formed in this way along a single fault, and bearing to each other a relation similar to that between the lower and upper plains of the Hutt Valley. The sea may be supposed to have access to the nearer of the two troughs, and to fill it up to the broken line. The broken line in the farther trough represents the level of a lake occupying the trough, and overflowing at the lowest point (the south-west end) along the faultscarp. If a river were already in existence flowing south-westward along the fault-line, and if the subsidence took place rather slowly, the farther trough might be filled with waste as rapidly as it was formed, a [ unclear: ] graded gorge being at the same time cut along the fault-line between the two troughs. When later a delta had been built out into the nearer trough the system would bear a good deal of resemblance to the Hutt Valley.
In the actual case the pre-faulting surface was of strong relief, and unless the Hutt River already existed it is difficult, if not impossible, to account for the present large drainage-basin of the river, for the amount of the fault-movement seems insufficient to effect profound changes in drainage.
Thus we have several reasons for believing that before the Wellington fault movement took place the Hutt River existed as a fault-line river guided by the same old fault as the Upper Kaiwarra.
Another indication of the previous existence of the Hutt is found in the presence of a broad terrace along the north-west side of the river near the mouth, evidently a remnant of the flood-plain of the Tongue Point cycle, the front of which is formed by the fault-scarp.
The continuous scarp that bounds the Hutt Valley on the north-west side seems to be—at least, in part—a fault-scarp. If it is true, however, that the recent break occurred along the floor of an older valley, the fault-
scarp can be of no great height, and the higher facets, like those in the Upper Kaiwarra and Silver Stream Valleys, probably form part of a faultline scarp.