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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
![Thumbnail: [Volume 51, 1919 page 118]](/journals/rsnz/images/rsnz_51/thumbnails/rsnz_51_00_0156_0118_ac_02.gif)
period of retrogradation. Thus the present terraced trenches of the fanbuilding rivers cannot be relics of the retrogradational phase, and must therefore be due to some other cause and of a later date. (2.) The profile of the existing terraces is not such as would have resulted from shore-recession.
Fig. 4.—Diagrammatic section of a river-fan, showing form of terraces developed during a period of retrogradation following the normal progradation of the shore-line by a copious supply of waste.
This is clearly shown by a comparison of fig. 3 and fig. 4; the former shows the form and arrangement of the terraces of the Ohau Valley, and the latter, terraces developed in a fan truncated and trenched as a result of retrogradation of the shore-line by sea erosion.