Cotton. — Geomorphology of South-western Well in (/'on . 215 



of waste, leading to a fluctuation in the ratio of wave-energy to load, for, 

 when the supply of waste is small, waves attack a coast vigorously, cut it 

 back, and draw much of the waste produced in this process back into the 

 deeper water off shore, where it comes to rest ; whereas when there is a 

 large supply of gravel or sand, either brought in by local rivers or trans- 

 ported along-shore by the activity of waves and currents from a more 

 distant source, the energy of the waves is used up in maintaining a graded 

 off-shore profile of the bottom as the abundant waste accumulates at all 

 depths, and some of the material is thrown up on the beach, so that the 

 shore-line advances seawards, leaving a prograded strip of new land. 



In the case in question it is probably the supply of sand, which comes 

 from rivers farther to the north-east, that has fluctuated, rather than that 

 of gravel brought down by local streams. The cause of the fluctuation is 

 not apparent. Changes of level of small amount would have an effect, no 

 doubt, by disturbing the graded profile of the neighbouring sand-covered 

 sea-bottom, and would perhaps produce alternate overloading and under- 

 loading of the waves at the shore-line. The fluctuation in the supply of 

 sand is too great, however, to be attributed to that cause alone. I have 

 not recognized the concomitant effects of such small movements on the 

 coastal topography, nor succeeded in distinguishing them from the effects of 

 advance and retreat of the shore-line, and so they are necessarily ignored 

 in this account.* 



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 de- 

 veloped (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. 



* Adkin reports that a trial well-boring on the lowland passed through a swampy 

 layer (i.e., a land surface) far below present sea-level (Trans. N.Z. Inst., vol. 43, p. 498, 

 1911). I have not been able to obtain official records of any of the bores that have been 

 put down ; but if this interpretation is correct it indicates that the lowland, during 

 an early period of outward growth, advanced, or was at least maintained, in spite 

 of considerable subsidence (as in the case of the Canterbury Plain). To make this 

 possible the supply of waste at that stage must have been very abundant. 



