Art. XIV—High-water Rock-platforms: A Phase of Shore-line Erosion.
[Read before the Geological Section of the Wellington Philosophical Society, 20th October, 1915.]
To most geologists, and certainly to all visitors to the historic Bay of Islands, the “Old Hat” needs no introduction. It has been carved from an emergent knob on a drowned spur, and, as the illustration (Plate XIII) shows, is most aptly named. It was first brought into scientific prominence by Professor J. D. Dana,‡ who visited the district in 1840 as a member of the United States Exploring Expedition of 1838–42.
[Footnote] ‡ J. D. Dana, Unit. States Explor. Exped. 1838–42, vol. 10, Geology, p. 109, 1849.
This note deals with the origin of the rock-platform constituting the rim of the “Old Hat.” Similar platforms are greatly in evidence in many parts of the coast around Auckland and in the more northerly harbours; they are barely covered by mean high tides, and vary in width from a few feet to 30 yards or more. From their seaward margins there is a steep descent for a few feet. Their surface is essentially horizontal but for very minor irregularities, so that they disturb the normal shore profile, which, according to Fenneman, both in building and cutting coasts “is a compound curve, which is concave near the shore, passing through a line of little or no curvature to a convex front.”* They are not slightly tilted and uplifted subaqueous shore-terraces, for they are covered at high water.
Dr. von Hochstetter† did not fail to observe these characteristic benches on his visit to Auckland, but neither he nor Professor Dana appears to have realized that they are not normal submarine platforms. Dana's idea of
their nature and origin is interesting, particularly as he adduces the “Old Hat” in support of it. He says, “There is, therefore, a level of greatest wear, which is a little above half-tide, and another of no wear, which is just above low tide.”‡
It seems essential to the formation of these high-water rock-platforms that wave-attack be not over-vigorous; that the rock in which they originate be moderately resistant to erosion, uniform in texture, and subject to comparatively ready decomposition; and that the coasts have not reached maturity of outline. Given these conditions, they will originate on undestroyed headlands and stacks quite irrespective of the structure of the constituent rocks, for they are particularly well developed at Waiheke Island and at the Bay of Islands, although the shores there are formed by highly disordered sediments
[Footnote] * N. M. Fenneman, “Development of the Profile of Equilibrium of the Subaqueous Shore-terrace,” “Journal of Geology,” vol. 10, pp. 1–32, 1902.
[Footnote] † F. von Hochstetter, Reise d. o. F. Novara u. d. Erde, Geolog., vol. 1, p. 5, 1864.
[Footnote] ‡ J. D. Dana, “Manual of Geology,” 3rd ed., p. 677, 1880.
The origin of such platforms is to be explained in great part by the well-known fact that sheets of water protect rocks beneath them from active chemical decomposition.
Surface waters charged with atmospheric gases cannot percolate effectively below sea-level, which must be a level of permanent water-saturation; the rocks below sea-level are thus relatively strong as compared with those above, which have been subjected to subaerial decay, and will resist weak wave-action, which is, however, competent to remove the upper weathered layer.
This fact, apparently, was recognized by Dana, for he says, “The existence of this platform is owing to the protection of the sea from wear and decomposition. Above, the material has disintegrated and been washed away by the action of streamlets and the waves; but beneath the water these effects do not take place.”*
But this action alone will not explain all the facts, or so it appears to the writer. He has noted, for example, the occurrence of unweathered rock at a small depth below the surface within a very short distance of well-developed platforms. He would suggest, therefore, that subaerial weathering at the shore-line is progressive, and that, as wave transport removes loosened spoil, fresh impetus is given to weathering, the water-table at the new shore-line is lowered to the level of high water, and the zone of decomposition retrogresses cliffwards. The gist of the hypothesis, then, is that the platforms are due not so much to wave-attack upon a definite zone of weathered rock as to the destruction of the cliff-faces by subaerial erosion and the removal of the resulting waste by weak wave-action. It is evident that the carving must occupy considerable time, and if the wave-action is vigorous the normal shore profile may be established and the sea-cliffs notched at their base in the well-known manner wherever the character of the rocks is favourable.
The high-water platforms of Auckland and North Auckland are invariably surmounted by cliffs of moderate height, a fact that is a necessary corollary of the hypothesis of origin. The districts in question are areas of drowned, maturely sculptured topography, with a climate favouring rapid rock-decay, so that the zone of weathering must have reached a considerable depth before depression gave rise to the present embayed coastlines. So long as wave-attack is directed wholly against the soft material situated in the zone of weathering of the interrupted earlier cycle, the normal shore profile obviously will result, and not before the rock of this original zone has been removed at the level of wave-attack can the high-tide platforms be initiated. At this stage cliffs with their height equivalent to the depth of weathering will front the sea, and, after this, as they retreat they will increase in height little by little towards the maximum when the slopes of the spurs in which they are cut become reversed. The section diagram illustrates these stages. A represents the sea-cliff at the initiation of the platforms and B is a similar cliff at a much later stage, whilst the dotted line indicates the hypothetical limit of the inwards-advancing zone of decomposition at this stage.
In conclusion, the writer wishes to thank Dr C. A. Cotton, of Victoria University College, for obtaining references to literature inaccessible in Auckland, and for friendly criticism.
[Footnote] * J. D. Dana, Unit. States Explor. Exped. 1838–42, vol. 10, Geology, p. 442, 1849.