Cotton. — Block Moittitcthix <ind a " Fo>^xU '' Denudation Plain. 71 



plateau of the Aorere Valley, with heie and there shiall mesas of covering 

 strata — pure limestone passing downward into a quartz grit with a 

 calcareous cement, wnth a thin layer of conglomerate at the base. The 

 limestone mesas are riddled with caves, and it is obvious that the 

 removal of this lowest stratum of the cover is being effected mainly by 

 solution. As the limestone areas are forested, they show out conspicuously 

 in contrast with the rest of the plateau, which is bare of vegetation, 

 with the exception of rushes and a few tussocks of coarse grass strug- 

 trling for existence in a " sour "' and slimy soil (see figs. 7 and 8). 



There are areas many acres in extent which are quite flat and nearly 

 level; but the surface of the plateau as a whole is by no means uniform. 

 Besides a number of narrow gorges cut recently beluw the general sur- 

 face, but collectively not affecting a large area of the plateau, to which 

 reference will he made in a later section dealing Avith the drainage and 

 dissection of the " downs," the principal irregularities are such as may 

 be ascribed to deformation of the denudation plain — of course, along with 

 its cover — at the period when the larger differential movements were also 

 taking place. Owing to the presence of a system of small faults, or pos- 

 sibly flexures, generally transverse in direction to the general elongation 

 of the depression, the surface of the " doAvns " descends towards the 

 middle of the northern boundary in a series of broad irregular steps, each 

 differing in height from its neighbours by a few tens of feet. These may 

 lie seen in Plate IV, fig. 1, which is a view looking south-westward aci'oss 

 the " downs." A wider panorama from about the same point of view 

 is shown in fig. 7. The foregoing appears to be the most satisfactory 

 explanation of tlie irregularities in the floor of the depression, but it 

 must be remembered that the surface has been long subject to erosion, 

 that an unknown thickness of cover has been removed from it, and that 



right. Angle of view, from south to west-north-west. 



the initial forms of the small fault-line scarps, if such there be, have 

 been much modified. Moreover, the stream which has effected the 

 removal of the debi-is of tlie covering strata has wandered rather widely 

 over tlie area, and as a result there are some more or less definite 

 fluviatile ten-aces cut on the undermass in the lower central part of 

 the " downs." A layer of river-gravel occurs on, and proves the origin 

 of, the more definite terraces bordering for some distance the stream 

 (the head of the Big River) which now drains the " downs," and a 

 sprinkling of gravel over a much wider area — perhaps sporadically over 

 the whole floor — may liave a similar origin ; but it is probable that 

 most of the surface gravel is a residuum of the conglomerate at the base 

 of the former cover. The presence of scattered knolls of the relatively 

 weak limestone of the cover ])roves that the higher flat areas are not 



