Cotton. — Warped Land-surface at Port Nicholson . 141 



Wainui-o-mata (Plate XXXII. fig. 1) is a good example of a backward-tilted 

 valley. It is occupied by an aggraded plain almost to the divide at its head, 

 as shown by the contoured map (Plate XXXIII, fig. 1), but the aggradation 

 does not extend far down the main valley beyond the junction, where there is 

 merely a flood-plain in the valley-bottom. Another branch of the Wainui- 

 o-mata, Moore's Valley, is aggraded also, but -not to its head, for this does 

 not turn so far westward. 



Very striking topographic features resulting from aggradation which must 

 be the result of head ward tilting occur in a tributary entering from the west 

 the Mangaroa Eiver, itself a tributary of the Hutt (Plate XXXIII, fig. 2). 

 The Mangaroa is one of the apparently subsequent, north-north-eastward- 

 flowing streams, and so is approximately parallel to the hinge-line of the 

 most prominent tilting. The tributary referred to, or, more correctly, the 

 several small streams which join to form it, flow at right angles to this 

 direction — i.e., east-south-east, or directly against the slope of the land- 

 surface which a little farther on descends below the alluvium in the Hutt 

 Valley (a north-easterly extension of the Port Nicholson depression). It is 

 clear that, before they were tilted head ward by an earth-movement, these 

 small streams flowed in courses approximately the same as those they now 

 follow across a maturely dissected surface not very different from that 

 now existing, and the present toiDographic features indicate that the tilting 

 was so sharp that these steep headwater streams, with declivities as steep as 

 several hundred feet per mile, were caused to aggrade vigorously, so as to 

 spread an extensive sheet of alluvium. (At some stage they may have been 

 ponded, though any evidence of such ponding is now buried beneath 

 alluvium.) The valley-floors are now broad fan-like slopes of alluvium, the 

 greater part of which lies just below the 800 ft. contour (Plate XXXIII, fig. 2). 

 The fans are steep at the valley-heads, where they extend almost to the 

 divide, and lower down are swampy except where they have been drained 

 artificially. They spread out widely in the middle parts of the tilted 

 valleys and taper away towards the junction with the Mangaroa Stream. 

 To such an extent are the valleys filled about the middle of their slope 

 that the fans in two of them have become confluent across the neck of a 

 spur, the end of which now stands as an island surrounded by alluvium 

 (Plate XXXIII, fig. 2, and Plate XXXII, fig. 2). 



A few miles north-eastward, at Wallaceville, the Mangaroa Valley is 

 itself aggraded. Some aggradation is to be expected as the axis of the Hutt 

 Valley is approached, for the latter, as mentioned above, is in some places 

 deeply filled with alluvium, and from it aggraded plains extend some little 

 way up tributary valleys, converting them into embayments. Such aggra- 

 dation does not, however, extend far up small valleys that are strongly 

 tilted down-stream. Above the partial filling due to alluviation outside 

 their debouchures streams entering the depression from the slope of the 

 tilted surface ought, in general, to be rejuvenated. In spite of its belonging 

 to this class of streams the valley of the Mangaroa is aggraded for several miles ; 

 for the lower part of the valley, which crosses the tilted block-surface very 

 obliquely, appears to have suffered headward tilting owing to its crossing the 

 transverse corrugation which causes the Hutt Valley to expand so as to become 

 a basin-plain at Trentham and Upper Hutt (Cotton, 1914). This introduces 

 into the tilting for some distance a south-westward component, which 

 appears to be the cause of the aggradation in the Mangaroa, and more 

 especially in Black Creek, a small tributary coming in from the south-west. 

 The valley of the latter is filled in to form a swampy plain a mile wide and 

 three miles in length, which shows up very conspicuously on the contoured 



