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Theoretical Discussion of the Growth of a Coastal Lowland under Conditions of Fluctuating Waste-supply.
Where a coast, perhaps originally a fault coast—though this is not essential—has been cut back to the mature stage, like the ancient coast of the old land of south-western Wellington (fig. 2, A), and a change to progradation takes place, a strand-plain, generally dune-covered, is developed (fig. 2, B). The material is mainly “imported” sand, if it is assumed that the excess of material has been brought from another section of the coast, but there will be mixed with it some gravel of local origin near the mouths of streams. As the streams grow in length seaward, however, with the growth of the prograded strip, they are constrained to aggrade so as to build up channels with sufficient slope to maintain their flow, and a proportion, perhaps the whole, of their gravel is thus used up, and accumulates as fans along the base of the cliffs of the old land (fig. 2, B), Clearly, if these streams are somewhat closely spaced and bring down much gravel their fans will become confluent, forming a piedmont alluvial plain; but, on the other hand, if their supply of gravel is smaller in proportion to the amount of sand being thrown up along the shore-line they will remain separate, and on the spaces between them dune sand may accumulate to a considerable thickness.
There is thus developed a coastal lowland of general seaward slope, with a somewhat irregular surface, and possibly with a width of many miles.
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If now the landward dunes have become fixed by vegetation and somewhat consolidated, they will in time have a normal drainage system developed on them, and their irregularity of surface will be reduced by the filling of the hollows and the wearing-down of the initial hills, so that their surface will become a peneplain (fig. 2, C).
Fig. 2.—Diagram of successive stages in the growth of a composite coastal lowland during two periods of progradation separated by one of retrogradation.
If now after a period of slow progradation, perhaps followed by a period of stationary shore-line, retrogradation begins again, the seaward-sloping coastal lowland, as its toe is cut back to a line of cliffs of growing height, will be dissected owing to the shortening of its streams. Since the dunesand areas lie between the larger streams from the old land, they will be traversed only by systems of small streams arising on them or heading on the cliffs behind. The energy of such streams is slight, but they work on very weak material and so erode quite rapidly (fig. 2, D). The larger streams from the old land will now trench the fans they formerly built, and they may destroy the fansurface altogether or reduce it to fragmentary terraces, developing streameroded plains at lower levels, which may in places be widened so as to plane down parts of the dune-sand areas.
The lowland will now resemble in a general way a dissected coastal plain of subaqueous origin, but there will be differences in detail, for the initial form in this case is a subaerially graded surface, and when the streams entrenched below it have again become mature the slopes of their valley-plains will approximate to that of the interfluves, and also such of the interfluvial areas as are underlain by dune sandstone will not be quite flat, but will retain the small relief of a peneplain.
There may be pauses in the retrogradation of the coast, and even reversals from time to time, and thus several series of terraces or valley-in-valley forms may be developed (fig. 2, E).
It may be postulated that after the coastal lowland has been cut back by
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the sea to perhaps half, or less than half, its former width progradation again sets in, and a new strand-plain, dune-covered like its predecessor, grows seaward from the recently cut cliffs. This will give rise to various modifications in the form of the dissected lowland. It will cause vigorous aggradation in all the gravel-bearing streams from the old land, and some of these may completely fill the valleys they have previously excavated in the lowland. Thus the fans are reconstructed (fig. 2, F). Intercalated short periods of retrogradation or other causes may lead to channelling of the fans from time to time, followed by renewed aggradation, and similar results may be produced by fluctuation of level. Thus there may be a considerable amount of complexity in the structure of fans.
Along the border of the inter-fan areas the even seaward slope of the lowland is not so readily restored. Some aggradation will take place, however, along the lines of the small dissecting streams, but irregularly, the greatest effects being seen where the channels, perhaps kept open for a time across the newer strand-plain, becomes blocked by sand-dunes, forming lakes partly within and partly beyond the border of the older dissected lowland (fig. 2, F). These, when filled, become swamps with arms extending up the floors of the tributary valleys, which are in process of aggradation with fine silt. Similar swampy flats will occur as normal features also among the dunes on the newer strand-plain.
Meanwhile the cliffs along the toe of the older lowland will be reduced to gentler slopes by subaerial erosion, which goes on quickly in this soft material, and reduction of the surface will still continue. Parts of it are by now maturely dissected, and there may be close interfingering between the spurs of the older and the dunes of the newer lowland.
In a coastal lowland developed as outlined above four quite distinct physiographic types of surface can be recognized: (1) The older dunesandstone areas of the dissected older lowland, with peneplained tops, mature topography towards the margins, and more or less dissected terraces in the valleys; (2) the gravel fans, which may still be confined between low banks of the dune sandstone or may overlap its peneplained surface; (3) the newer sand-dunes, which still exhibit the forms due to accumulation; and (4) swampy flats, which have accumulated in lakes due to ponding among the newer dunes or between these and the toe of the older lowland, or are the result of aggradation in the silt-bearing small streams trenching the older lowland. In the coastal lowland of south-western Wellington all four of these physiographic elements are important.