Cotton. — Block Mountains and a " Fossil " Denudation Plain. 69 



Dissection . 



The sloping plateau is crossed, as noted above, by a number of young 

 gorges. These are evidently superposed consequent ravines, for they 

 descend the slope of the tilted plateau, and the courses of the streams 

 which cut them were evidently guided by the slope of the surface of the 

 former cover. They are quite indifferent to both dip and strike of the 

 undermass rocks upon which they now flow. These gorges are, in general, 

 narrow-floored and steep-walled, and some distance back from their 

 debouchures the larger of them are incised to a dejoth of several hundred 

 feet below the sloping plateau. The larger streams head in a range of 

 mountains to the south-east, some peaks of which rise to heights of 

 2,000 ft. and more above the plane of the sloping plateau produced in 

 that direction. These mountains, the Haupiri Range, may be satisfac- 

 torily explained as the dissected remains of a higher block separated 

 from the sloping plateau block by a fault or flexure. The streams rising 

 in the Haupiri Mountains have already considerable volume when they 

 begin to cross the sloping plateau in extended, superposed consequent 

 courses, and it is to the action of these vigorous streams that the some- 

 what advanced dissection of the plateau is to be ascribed. The diagram, 

 fig. 5, represents the dissection of a sloping block surface such as that 

 of the south-east side of the Aorere Valley by streams from a higher 

 block behind it. A moderate area of the stripped plateau is represented 

 as surviving, and also a few remnants of the cover. On the sloping 

 plateau of the Aorere Valley a few such residuals, small limestone mesas 

 and buttes, remain (Bell, pp. 24—25). ^ 



The Actual Valley of the Lower Aorere. 



In the foregoing sections the name " Aorere Valley " has been used 

 with reference to a tectonic depression (w-hich, as will have been 

 gathered from the description, is strictly neither a Grahen nor a fault- 

 angle depression, but partakes of the nature of both), modified as it is 

 at the present day by erosion. Nothing has been said of the Aorere 

 River beyond the fact that it occupies the depression. 



It must be noted, however, that the present open valley is largely 

 the work of the river, the amount of waste which it has transported 

 seaward since the episode of differential movements initiating the depres- 

 sion being truly enormous, and consisting of almost the whole of the 

 covering strata (of unknown thickness, but probably several hundred feet 

 at least), as well as a very large contribution from the dissection of the 

 undermass by its tributaries and its own upper course. The actual 

 erosive work performed by the river itself has been considerable, as is 

 evidenced by the occurrence of terrace after terrace up to a height of 

 many hundred feet on the sloping plateau. As these terraces occur on 

 the lower portion only of the sloping plateau, and as the form of the 

 latter has been modified by them only to an inconsiderable extent, they 

 have not been mentioned in the account of the plateau given on earlier 

 pages. When, however, the valley-side is examined closely, the terraces 

 and their thick covering of gravel are readily recognizable. As the 

 gravel is auriferous, it has been largely excavated, and the workings 

 reveal some former channels of the river refilled when from time to time 

 degradation gave place to aggradation. 



The terraces occur up to a height of perhaps 600 ft.* above the 

 pi'esent level of the river, and there are also residuals of gravel-covered 



* Accurate data are not available. 



