Adkin. — The H orowhenua Coastal Plain. 



117 



in this district. Bordering the present Horowhenua coast-line, a belt of 

 dimes, the aeolian origin of which has never been questioned, covers a strip 

 of country from three to six miles in width. The dunes along the inner 

 margin of the belt are the oldest, and these have reached a state of very 

 considerable stability. These old dunes are generally covered with a thick 

 la'yer of humus derived from the growth and decay of manuka and other 

 vegetation, and are also fixed by a turf of natives grasses. In spite of 

 these characteristics no trace of a drainage-system is being or has been 

 incised upon them. Rain descending upon the dimes sinks immediately 

 into their substance, to the complete exclusion of any run-off whatever. 

 The great difference between the dunes and the sandstone is that in the 

 one a cementing medium is entirely lacking — hence their great porosity 

 and immunity from dissection by surface waters ; whereas in the other 

 the hydration of certain ferrous constituents, together with the presence 

 of colloidal matter,* has cemented the sand-grains into a fairly compact 

 and somewhat impervious mass — sufficient, indeed, to permit of dissection 

 by stream-action. 



The River -terraces. 



In support of his theory of occasional phases of retrogradation of the 

 shore-line of a strand-plain. Cotton claims that, in addition to the line of 

 cliffs cut by the sea in the toe of the plain, terraces or valley-in-valley forms 

 resulted, and furnish evidence of the retrogradation. The only true terraces 



Fig. 3. — Generalized diagrammatic section of the Ohau fan. showing 



actual form of terraces. 



of the Horowhenua lowland are those that fringe the sides of the trench- 

 like valleys cut in the large gravel fans. It can be shown by two lines of 

 evidence that the existing terraces do not furnish evidence of coastal retro- 

 gradation : (1.) If, as Cotton himself points out, the rivers rebuilt their 

 fans during a second (or some later) period of progradation, they must have 

 filled and obliterated the terraces of the trenches incised during the preceding 



* E. C. Barton, The Work of Colloids in Sandbank and Delta Formation, Geog. 

 Journ., vol. 51, pp. 100-15, 1918. 



