The Coast Platforms.
Along parts of the coast no relics remain of elevated platforms cut by the sea during pauses in the movement of uplift. They have either been completely cut away by the waves or cut off by faulting along new lines of fracture. At other places extensive shelves remain. The most prominent begins at Tongue Point and extends some distance west-ward (see fig. 7, and Plate XVIII, fig. 3). The shoreward edge of this shelf
appears to indicate the base-level at the time when the streams of the district developed the greater part of the existing upland topography. For that reason the writer has named that erosion cycle the Tongue Point cycle.
The height of the shelf at its inner edge at Tongue Point is 240 ft. Its slope seaward is at first 10°, but rapidly decreases, and at the end of Tongue Point, where the shelf is half a mile broad, it runs gently out at an angle of 2° or 3°.
The upper surface of the shelf is covered by a veneer. 6 ft. or 8 ft. in thickness, of gravels similar to those of the present beach. They vary irregularly from beds of coarse roughly rounded gravel and boulders, material similar to what is being supplied to-day in large quantities by the smaller streams, to layers of fine flattened discs of beach-shingle varying from the size of a threepenny-piece to that of a penny. A layer of the coarser gravel is seen on the right in Plate XVIII, fig. 3.
The varying height of the outer scarp of this marine terrace as seen from the sea is clearly due mainly to the varying breadth of the portions that have withstood the action of the sea, the seaward slope of the shelf being regarded as nearly constant. At the extremity of Tongue Point it comes down to 170 ft. Beyond the next creek to the west, where there is a well-preserved but narrower remnant, the outer edge bounded by the present scarp is, as might be expected, higher. It is evidently this apparent variation in the height of the shelf that
led Park* to remark that he had satiafied himself “that it was not an uplifted marine platform of erosion.” It may be remarked that a section, even on a vertical plane through a coastal platform, parallel to the average direction of the coast must not be expected to yield a perfectly horizontal crest. It ought to show a convex crest opposite to bluffs, where the old coast approaches it, and a concave outline opposite bays, where the old coast recedes. To this initial irregularity there may be added slight variations in the amount of subsequent uplift. Remnants of this terrace extend nearly to Cape Terawhiti, and it may be seen also at Te Kaminaru Bay, on the western coast.
There exists also a higher shelf, which was examined at Tongue Point. It may be seen in fig. 7. Its height is about 450 ft., and, like the lower shelf, it is covered with a layer of water-worn pebbles. Its width at the point examined had been reduced by the cutting of the lower shelf to about 50 yards.
At Baring Head, on the coast south-eastward of Pencarrow Head, at the entrance to Port Nicholson, similar shelves occur,† and again at Cape Turakirae. They may be seen from the deck of a steamer entering Port Nicholson. The sketch, fig. 8, represents them as seen from
Fig. 8.—The Elevated Coast Platforms Between Pencarrow Head and Baring Head, as Seen From The, Signal-Station On Miramar Peninsula.
Pencarrow Head in centre, Baring Head on right.
the signal-station on Miramar Peninsula. They are cut through by the revived Wainuiomata. The writer has not examined these platforms closely, but believes they correspond in a general way to those at Tongue Point, the sunken area of Port Nicholson lying between. The highest platform at Baring Head appears to be about 500 ft. above sea-level. It has been shown above that the general outlines of the coast appear to be determined by subsidence of land blocks, but, on the other hand, it cannot be assumed that the whole of the uplift of which we here have evidence is differential uplift along these lines of fracture. At many places on the New Zealand coast marine platforms and raised beaches are known, indicating uplift of varying amount.‡ McKay has recorded Recent shells on a beach at a height of 500 ft. at Amuri Bluff, about eighty miles south-west of Wellington. If this beach can be correlated with the highest shelf at Wellington it may indicate that the stretch of land between has moved as a whole. The latest movement, which took place in 1855, and was described by
[Footnote] * Trans. N.Z. Inst., vol. 42, 1910, p. 586, and fig. 3.
[Footnote] † See Park, loc. cit., p. 585, fig. 2.
[Footnote] ‡ See Marshall, loc. cit., p. 31.
Lyell,* affected both sides of Cook Strait. It was, however, a tilt to the west, which depressed the western shore of the strait and elevated the Wellington side as a whole—that is, the area shown in fig. 1—by an amount varying from zero on the north-west to about 9 ft. on the south-east. The raised beaches of the Wellington coast which owe their elevation to that movement have been described and figured by Bell.† They may be seen also in Plate XVIII, fig. 2, and Plate XXI, fig. 2. Both views are of parts of the eastern shore of Miramar Peninsula.
There is some evidence that this tilt is a continuation of an earlier tilting movement in the same direction, the axis of the movement lying a little to the west of Wellington. On the south-east a series of very fresh raised gravel beaches at Cape Turakirae, the highest being 90 ft. above the sea. are mentioned by Aston.‡ On the north-west there appears to have been a downward movement of small amount subse-quently to the general movement of elevation the proofs of which have been given. This movement, which has drowned the lower reach of the Porirua Stream, does not appear to have been more than 30 ft. or 40 ft. The stream had previously developed a broad strip of flood-plain, and this has been drowned to a distance of about a mile and a half from the sea. At Porirua there appears to have been little or no movement either up or down in 1855. Raised rock platforms similar to those at Wellington are not found. This agrees with the accounts of eye-witnesses given in substance by Lyell.§
[Footnote] * “Principles of Geology,” 10th ed., 1868, vol. 2, p. 82.
[Footnote] † Trans. N.Z. Inst., vol. 42, 1910, p. 538, and pl. 41 and 42.
[Footnote] ‡ B. C. Aston, this volume, p. 208.
[Footnote] § Loc. cit.