Art. VI.—On the Effect of Wind-driven Sand as a Cutting Agent.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 5, 1872, Page 105

Art. VI.—On the Effect of Wind-driven Sand as a Cutting Agent. By Edwin Stowe. [Read before the Wellington Philosophical Society, 25th September, 1872.] It may be interesting to those who are acquainted with the sand-worn stones of Lyall Bay, either from personal observation, or from Plate XVII., in the second volume of the Transactions, and the description there given by Mr. W. T. L. Travers, to know that on another part of our coast effects similar to those thus far observed are being produced by somewhat similar action. The southern bank of the Waikato river for the last few miles of its course is formed by a range of sandstone, for the most part still covered by the natural vegetation of the district. But northwards from the point where this range, meeting the sea, forms a line of abrupt cliff making away to the south, there extends for a distance of about a mile, up to the actual river mouth, a long bank of considerable elevation, composed of fine sea-sand. This is entirely devoid of the vegetation usually found upon sand-dunes, and lies fully exposed to the action of the S. W. gales, which blow with such force on this part of the coast. The landward, or eastern slope of this sandy elevation, is for the most part exceedingly abrupt—as abrupt as it is possible for loose sand to be—so much so, indeed, that at a distance it appears to be a

precipitous cliff-like wall. Its base rests on an irregular undulating tract, for the most part also covered with light moveable sand, that occupies the angle between the long sand-dune and the ranges before mentioned. On this undulating tract may be found banks or beds, ordinarily about the width of a common roadway, composed of blocks of pumice that have been brought down by the waters of the river from the interior of the country. Deposited in their present position, in the shape of rounded boulders, they have been subjected to the cutting action of drift-sand till the upper portion of each mass has been cut away, and the whole bed offers a uniformly level surface, slightly depressed centrally. In this condition the beds present the appearance of paved roadways, or rather of inlaid pavements. That the blocks of pumice, of which they are composed, must have been originally deposited as rounded boulders, is sufficiently clear from the fact that the pumice, freshly thrown up by the ocean on the open beach, as well as that thrown up either by the tide within the river or by the river itself beyond the limits of the tide, is invariably devoid of angular form. The specimens removed from one of these beds will best show how sharp an edge has been produced where the original curved surface sunk in the sand is met by the intersecting line of the newly-formed plane. Wherever the pumice has been exposed along the external margin of these beds other forms may be observed, many of the blocks showing signs of the tendency that sand has to cut a sharp ridge on stones so placed. In isolated situations it is by no means difficult to find examples very similar to those already referred to as having been found at Lyall Bay. On these a sharp ridge is to be noticed—doubtless the effect of the alternate action of two currents—one, probably the stronger, being the one setting in from the sea, the other blowing down the river in an opposite direction. This effect was very noticeable in the case of a large isolated block, about the size of an ordinary milestone, that had become firmly imbedded in the sand, and which had been cut on two faces—the apex viewed laterally presenting the appearance of a sharp point. But the specimens brought away, though on quite a small scale, are sufficient to show the general form that is produced in this locality under the influence of the action of drift-sand, subject to alternate currents of wind.