C. A, Cotton— Block Mountains in New Zealand. 265 



5. Trougli-filling. 



An enormous annount of waste results from the stripping of 

 back slopes and dissection of faulted and folded fronts. Ex- 

 ceptionally, such waste may be all removed as it is supplied 

 but in most places deep aggradation of troughs will take place 

 progressively with deformation and with the degradation and 

 dissection of the higher blocks. 



Fig. 10. 



Fig. 10. Submaturely dissected fold surface towards the northern end 

 of the Raggedy-Blackstone block in Central Otago. View looking northward. 



Where initial depressions are open towards the sea and por- 

 tions of low-lying blocks are submerged, or where an area un- 

 affected by movement borders a region of uplifted blocks, the 

 new deposits will overlie the sediments of the predeforma- 

 tional period without stratigraphical break but with abrupt 

 change in the nature of the detritus. Conglomerate may over- 

 lie mudstone and pass upward into fanglomerate, as in Marl- 

 borough (Cotton, 1914). The passage of fine-grained sediments 

 upward to conglomerate in the upper Tertiary i^ocks of many 

 parts of the South Island of New Zealand is another example. 

 In marginal areas of shallow water, however, the predeforma- 

 tional cover will be eroded by wave action, while unsubmerged 

 areas will be eroded by subaerial agencies, so that when fans 

 and deltas of waste from neighboring high blocks are built 

 forward these will generally rest unconformably upon a 

 denuded surface. 



When the phase of maximum aggradation is reached, inter- 

 mont basins will be occupied by alluvial plains through which 

 the eroded summits of small, isolated blocks may project. At 

 this stage, portions of the divides between basins may be buried, 

 the waste spilling over from one basin to another in the man- 

 ner described by Davis in his discussion of the arid cycle 



Am. Jour. Sci.— Fourth Series, Vol. XLIV, No. 262-October, 1917. 

 19 



