Changes In Drainage Of The Karori-Khandallah Or Long Valley.
This old valley might be called the Karori-Khandallah Valley, from the names of two important settlements in it. For the sake of brevity, it is here called the “Long Valley.” Its line is now followed by the Silver Stream, the Kaiwarra and its tributaries, the upper Ngahauranga, and the Porirua. In fig. 2 the line of the old valley is indicated as a double broken line, and farther north by the line of the Manawatu Railway. Starting at the head of the valley and following it northward, we may note the changes that have taken place. At the head of the Silver Stream, which occupies the southern end of the valley, the divide is now 1,000 ft. above the sea, and the old valley appears to have continued still farther southward, the divide now being rapidly pushed northward by the activity of torrents of the south coast. Two miles and a half from its source the Silver Stream turns very sharply
to the west, and finds its way to the sea as a tributary of the Karori, having thus a roundabout course eight miles in length. As indicated in fig. 2, the capture of the Silver Stream by the Karori is a double one, two branch ravines of the Karori tributary having successively tapped the coarse of the Silver Stream. The floor of the old Long Valley here stands about 840 ft. above sea-level. The deepening of the captured stream at the elbow of capture is 400 ft. or 500 ft. Northward from this divide the Kaiwarra, which here occupies the Long Valley, descends somewhat rapidly in a trench incised in an older mature valley-floor. At the upper reservoir (U.R. in fig. 2) it follows entrenched meanders of small radius, and a portion of the old flood-plain on which the meanders originated remains as a bench far above the present stream and at a height of 660 ft. above the sea. At this point a mature dry valley on a level with the old flood-plain bench, evidently the old stream-course, swings off to the north, while the course of the Kaiwarra, flowing north-east, is a young gorge. The sketch, fig. 12, shows the old valley and the young gorge of the Kaiwarra.
Following the old dry valley mentioned above, we find ourselves in the broad mature valley occupied by the settlement of Karori. It has been invaded by the head of the Karori Stream from the south-west, as well
Fig. 12.—Capture Of The “Long Valley” Stream By The Kaiwarra.
Upper reservoir on the left; young gorge of the Kaiwarra below the upper-reservoir dam on the right.
as by the Kaiwarra from the north-east. The north-eastward continuation of the now broad and mature Long Valley through Ngaio and Khandallah is evident, but between Karori and Ngaio the floor of it has been almost completely gouged out by the numerous young deep-gorged tributaries of the middle Kaiwarra. Overlooking the Kaiwarra there are, however, abundant stream-deposits in Karori, and a bed of gravel on the western slope of the Tinakori hills at a height of 600 ft.
The lower Kaiwarra leaves the Long Valley by a steep-walled gorge, and crosses the scarp of the Wellington fault. The north-eastward continuation of the valley is occupied next by a short obsequent stream, a tributary of the Kaiwarra. Farther on, at Khandallah, it is crossed by a stream which joins the Ngahauranga near its mouth. Still farther to the northeast the valley has been invaded by the Ngahauranga, a stream which, owing its activity to its position on the fault-scarp, has worked back in a profound gorge along a nearly straight course at right angles with the
fault-line until reaching the Long Valley. It has reversed the drainage of the Long V [ unclear: ] alley for a mile and a half. It follows a winding course, but the tapering shape of the spurs on the concave sides of the meander-curves indicates that the winding character is due, at least in part, to lateral cutting that has accompanied the deepening of the gorge. At one point a narrowed and almost cut-off spur is a conspicuous feature in the Ngahauranga Valley. Plate XXI, fig. 1, is a view looking north-east across this spur and up the valley. The height of the narrowed neck above the stream on the downstream side is 200 ft., and on the up-stream side 90 ft. Its breadth is about 100 yards, while the distance roundabout by the course of the stream is three-quarters of a mile. Beyond the divide, 500 ft. above sea-level, at the head of the obsequent Ngahauranga, is the head of the Porirua Stream, which, robbed of two-thirds of its ancient length, still occupies the northern end of the Long Valley. Probably this was the outlet at the close of the Tongue Point cycle.
The cause of most of the captures in the Long Valley is, as has already been indicated, the subsidence of the Port Nicholson block, particularly along the line of the Wellington fault, giving a short descent to sea-level. With regard to the Silver Stream, it seems remarkable that its capture had not taken place earlier and in a less roundabout way than the present outlet to the Karori Stream. A reasonable explanation seems to be that in earlier times, when streams followed the Long Valley and the other main longitudinal valleys of the Wellington Peninsula, the peninsula formed part of a land-area extending to the north-west and to the south far beyond its present limits. Reasons have already been given for the writer's belief that the present coast was determined by fractures after the main lines of the present drainage were established.