Art. XLI.—On a Shingle-spit in Lake Coleridge.

By R. Speight, M.Sc., F.G.S.

I referred briefly in my note on the Lake Heron spits to their peculiar form, which appeared to be a fairly common feature, if one can judge from the frequency with which spits of similar shape are figured in publications dealing with the physiography of lake-shores. This I attributed directly to wave-action, for when watching the waves coming down Lake Heron before the strong north-westerly wind I noticed that when they broke on the spit they were invariably parallel to its shore, the change in the direction of the wave-front being due to the retardation produced by the pronounced friction of the bottom in the shallower water near the beach as compared with its comparatively slight effect in the deeper water off shore. The form of the spit, with its general convex curve lakewards, is therefore that of the involute of the different wave-fronts as they break, so that it must have an intimate mathematical relation to the circumstances determining the formation and speed of waves in the immediate vicinity. Unfortunately, I have not sufficient knowledge of mathematics to tackle such a difficult problem, and I cannot demonstrate the exact curve which should result from its solution, although I am confident that it should be obtainable by mathematical methods, and the result would confirm the inferences drawn from observations in the field.

The present note is intended to draw attention to a hooked spit occurring in Lake Coleridge whose formation appears to me to be entirely due to wave-action.

Sketch-plan of the Shingle-spits in Lake Coleridge. (Scale, 10 chains to an inch, approximately.)

Situation and General Surroundings

The spit to which I wish to refer particularly is situated in the southwest corner of Lake Coleridge, about a mile from the Lake Coleridge homestead. At its proximal end, where it is joined on to the solid land, there is a roche moutonnée with distinctly striated and fluted surfaces, and on its extreme point grows the rata-tree made historical by references in Lady Barker's “Station Life in New Zealand.” It is satisfactory to note that this tree is still flourishing, although it is the only one growing on that part of the lake-shore. From this tree stretches first of all a true beach, and then the free portion of the spit, about 20 chains in length. The spit varies in width up to about 3 chains. About half-way along it divides into two, the outer part being of recent formation and separated from the parent

spit by a strip of shallow water. The water is comparatively deep within the loop of the spit, and between its end and the shore is as much as 20 ft.

Lake Coleridge is a long narrow lake, some eleven miles in length, and a mile and a half wide at its widest part, narrowing to three-quarters of a mile about half-way up, where a peninsula juts out from the eastern side. It is very deep, with steep under-surface slopes. Its length lies directly in the line of the northerly winds which sweep down from the Southern Alps, and at times raise a nasty and dangerous sea. The spit is placed at a spot where these waves would in all probability reach their maximum height and have a maximum effect. No other wind is deserving of serious consideration as far as this spit is concerned. There can be no current in the lake in the vicinity of the spit, since what small discharge there is takes place from the other end of the lake, eleven miles away.

Origin of the Material of which the Spit is built.

The spit is formed entirely of the greywacke which is the principal rock occurring in all the mountain district of Canterbury. The pebbles attain a diameter of about 3 in., but included among them is a large quantity of small material. Finer detritus mixed with peaty matter has formed a fringe of swamp along the inner margin of the hook, especially towards its proximal end, where there is a considerable area of boggy ground. The great bulk of the material of which the spit is formed has come from farther up the lake, where there is a supply of pebbles of all sizes in the cliffs of glacial boulder-clay fringing its shore. These cliffs commence about 10 chains away from the point on which the rata-tree grows, and extend with intermissions of solid rock for nearly three miles up the western side of the lake. It is remarkable, however, that pebbles can be transported along a shore which deepens so rapidly. I have been able to secure a tracing of the minute and accurate survey made by Mr. F. T. Kissel, the Resident Engineer for the power scheme, and I find that in places along this stretch of shore the 60 ft. subsurface contour-line is at times within 2 chains of the margin of the lake, although in others it is as much as 6 chains. It would seem that even these stretches of steep slope are not thoroughly efficient in preventing the transport of beach-shingle. That it does so travel is certain, for at the intake of the tunnel for the power-supply, which is about a mile and a half from the spit, a peculiar form of hard slaty greywacke was encountered. The spoil from the tunnel is thrown into the lake, and this is carried down and forms small beaches between it and the spit, and is, indeed, transported across the water, 20 ft. in depth, which separates the distal end of the spit from a beach at the southern end of the lake. This beach is formed of fine shingle, some of which is undoubtedly derived from the tunnel, and its presence serves to emphasize the importance of waves as transporting agents even when there are no appreciable tides or currents to aid them. Mr. Kissel has pointed out to me that the strongest waves occurring in the lake tend to destroy the small beaches round its shore, because the loose material is carried outwards to greater depths by the stronger undertow and dropped down the steep subsurface slopes into deep water, and is thus removed from the belt along shore where waves exert influence. Moderate waves are, therefore, in this case more potent beach-formers, because they do not move the shingle so far out. This factor is perhaps of relatively small importance in the neighbourhood of the spit, since the water deepens off shore gradually, and shingle is not likely to be carried out into deep water by the undertow. No doubt this

did really occur in former times, before this part of the lake was made shallower by detrital accumulations.

Form of the Spit.

The form of this spit is by no means as perfect as those in Lake Heron. It is modified to some extent by the doubling of the loop, but both show a decided convexity to the lake. Gilbert insists that lake-spits usually exhibit a concavity towards the open water, but this feature is absent in all lake-spits that I have seen. If spits are formed in the way I suggest, then they must necessarily, in their initial states at least, be convex to the open water, as the evolute of a number of wave-fronts will have its outward curve turned in that direction. There is, however, a process in operation in some spits, and certainly in those in Lake Coleridge, which will turn a convex curve into a concave one. All along the distal portion of this spit there are signs that it is being moved bodily backward towards the shore by the action of the waves. During strong winds, and at times when the level of the lake is above normal, the beach is driven landwards and built up in the form of a levée, and this in spite of the stability which has been given to it by the covering of scrub and other vegetation. This spit is covered with low forest containing the following trees: Manuka, kowhai, the cabbage-tree, and the scrub-formers such as Discaria and Suttonia, together with flax and other swamp-plants. These are being rapidly uprooted along the outer margin, and the shingle beach is piled among those which are still standing. Growing flax which has been dislodged can also be seen beneath the water of the lake. During especially high winds the waves break over this levée, and form small washouts on the inner side, which continue the landward movement of the spit. This process is continued, though in a slightly less intense form, by the percolation of water through the bank when the waves are not so high. Small washouts are thus formed in a manner analogous to that described by Professor Oliver in the case of the Chesil Bank, on page 88 of his paper. The resemblance seems to be exact, except that the phenomena in Lake Coleridge are on a smaller scale. However, in both cases the net result is to move the spit landwards. I am not fully aware of all the circumstances of those spits referred to as concave by Gilbert, but I would suggest that this process will make even a convex spit sag landwards eventually. The movement may continue till the proximal portion of the spit becomes a true shore beach. This has occurred in the Lake Coleridge spit. It is also true in one of the most notable spits in New Zealand—viz., that near Nelson, locally called the Boulder-bank. This is some thirteen miles in length, and where tied on to the land is distinctly concave; but its distal portion is convex, and its end has the typical form of a true incurved spit.

In his paper Professor Oliver evidently accepts Gilbert's statement that lake-spits are concave to the open water, and, as he notes that marine spits are frequently convex, he concludes that there is some inherent difference between the formation of lake and sea spits. It appears to me that this distinction is, not based on a solid foundation. Some lake-formed spits are undoubtedly convex, and some marine ones are concave. The difference in form appears to be due to the relative importance of wave-action and current-action during the process of building. If the former is predominant, then in their initial stages, at all events, spits will tend to be convex. They may become concave owing to sagging shorewards at a later date, a result accelerated by a slight depression of the land or rise in the water of the lake. The shape of current-formed spits, or those in which currentaction is predominant, will naturally be determined by the direction of the current. This will always tend to sag shorewards as it passes from point to point—the usual position of a spit—and therefore spits formed in this way will naturally present a concavity to the open water.