C. A. Cotton — Block Mountains in Neiv Zealand. 257 



on the resistant oldermass, the rate of further downward cut- 

 ting will become comparatively slow. Before this stage the 

 measure of the relief has been increasing progressively with 

 downward cutting and may still increase slightly if maturity of 

 dissection of the surface is still to be attained. After the 

 attainment of maturity, reduction in height of the interfluvial 

 areas of weak covering strata may go on more rapidly than 

 vertical stream corrasion on the resistant underlying rocks. 

 Even if the streams had attained grade without cutting through 

 the cover in the early stages of dissection, after the cover has 



Fig. 3. 



Fig. 3. A northwestward-sloping stripped plateau surface forming the 

 back slope of the Rough Ridge block in Central Otago, and descending 

 beneath covering strata planed by the Ida Burn. 



been largely removed from the higher part of the block they 

 will be forced to cut deeper and will eventually become super- 

 posed. With the complete removal of covering strata, all the 

 streams will be incised to some extent. Owing to the resistant 

 nature of the undermass the ravines will long remain narrow, 

 while inclined flat areas on the interfluves will survive. This 

 stage will be attained earliest at the middle parts of the slopes 

 of block surfaces. On lower slopes, undissected interfluvial 

 areas are likely to be larger, and on slightly inclined higher 

 slopes where there is little concentrated wash, remnants of the 

 cover mav be expected to survive for some time longer (see 



fig. 2, E); 



These sloping plains of denudation, almost entirely stripped 

 of their cover, and crossed by many steep-sided, generally con- 

 sequent ravines, which increase rapidly in depth as followed 

 upstream, with here and there remnants of the covering strata 

 relieving the otherwise flat interfluvial surfaces, though com- 

 mon in JVew Zealand, have received scant attention. They 

 have been noted by McKay, Bell (1907), the writer (1916 h) in 

 the Aorere district of northern Nelson, and by Thomson (1914) 

 in South Canterbury. The more level portions of similar sur. 



