Cotton. — Block Mountain >< and a " Foxxil " Deniuhition Plain. 67 



This sloping plateau is certainly, as McKay noted, a plain of marine 

 denudation — a plain of denudation, that is to say, to the formation 

 of which the finishing touches at least were t!iven by marine erosion as 

 the sea advanced over it and deposited upon it the covering beds since 

 lareelv removed. The occurrence, however, of terrestrial formations at 

 the base of the covering strata in neighbouring ai'eas, notably on the 

 north-western and northern parts of the Wakamarama Range, is perhaps 

 an indication that the surface of the undermass had there been reduced 

 to small relief by subaerial agencies in the period preceding submergence. 

 It is, therefore, reasonable to suppose that here also a peneplain was in 

 existence, and that the plain of marine erosion is one of those produced 

 by the stijDping and removal of the waste mantle fi'om a peneplain, 

 accompanied by a minimum of erosion of the fresh underlying rock. 

 The large extent of the planed surface — which, indeed, probably extended 

 formerly over a much larger area than that in whicli the plain is now 

 preserved — thus receives a simple and probably correct explanation. 



^] ,-v)C^;v'S^-~^- ^-^^^^^Xr^ 



Fig. 6. — View looking south-east up the sloping plateau of the Aorere Valley. 

 After a photograph by the Geological Survey. 



TTie tilted plateau of the Aorere Valley has been revealed owing to the 

 removal from its surface of the covering strata, a few residual areas 

 of which testify to their former wide extension. Where seen the floor 

 beneath the covering strata proved to be a cleanly eroded surface of 

 fresh rock, and the basal beds in this locality have always been described 

 as conglomerate of marine origin, generally a few feet in thickness and 

 thoroughly sorted, only the hardest of the materials of the undermass, as 

 a rule, surviving. Within a few feet vertically this quartz conglomerate 

 passes into pvire limestone, the presence of which indicates prevailing 

 open water, the shore-line being some distance away and any neighbour- 

 ing land being of small relief and supplying a negligible quantity of 

 mechanical waste. 



Bell regards the formation of the denudation plain as having taken 

 place in a strait necessarily initially narrow and afterwards opened by 

 marine erosion to the width of the sloping plateau of the present day, 

 the shores of which strait constituted an " old land " of mature moun- 

 tains. There are, however, sevei'al rather serious objections to this 

 explanation. 



First, if the land-forms of the region prior to submergence were 

 mature mountains, and if submergence of such a region of strong relief 



3* 



