Cotton. — Geomorphology of South-western Wellington. 217 



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, how- 

 ever, 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 dis- 

 sected 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 dune- 

 sandstone areas of the dissected older lowland, with peneplained tops, 

 mature topography towards the margins, and more or less dissected ter- 

 races 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 



An Alternative Explanation. 



An alternative explanation which would account for the existing forms 

 in the broader part of the lowland fairly well would make the " older low- 

 land " of the foregoing a coastal plain of subaqueous sands, which was 

 subjected to subaerial erosion with the shore-line stationary for a period 

 long enough to allow of peneplanation, and afterwards cliffed at the margin 

 and dissected. The remaining events would be the same as those outlined 

 in the previous explanation. An objection to this explanation is found in 

 the nature of the material of the older dissected lowland, which will be 

 referred to on a later page ; and an argument in favour of the explanation 

 previously given is found in the clearly decipherable history of the lowland 

 at its extreme south-western end (at Paekakariki), where there is evidence 

 of alternating progradation and retrogradation on a scale sufficiently large 

 to warrant the assumptions made. 



