266 C. A. Cotton — Block Mountains in Wew Zealand. 



(1905 h) and well illustrated in some ao^^raded depressions in 

 California described by Lawson (1906, p. 455). When such 

 spillino' over occurs, the main stream draining a basin may take 

 a spill-over course, become fixed in it during a period of de- 

 gradation succeeding one of aggradation, and abandon its 

 former outlet. 



Upon the surface of the alluvium filHng an intermont basin 

 the aggrading streams will flow in braided, ever-changing 

 channels. In the case of a trough opening to the sea the waste 

 will form a delta or a piedmont alluvial plain. Similar forms 

 will also result along initial fault coasts facing the open ocean. 



During degradation the filling from some troughs may be 

 removed and the underlying rock exposed. One of the 

 largest intermont basins in ]New Zealand, the Upper Taieri and 

 Maniototo plain in Central Otago, has reached the stage of 

 dissection at which little alluvial filling remains, and there is 

 little evidence to show whether it has ever experienced exten- 

 sive aggradation. The planed surface of the covering strata 

 of the low-lying blocks forming the floor of the basin is covered 

 generally by a layer of flood-plain gravel. The planed surfaces 

 form several terraces whose slope indicates that they were 

 formed during intermittent regional uplifts by the same small 

 streams which have since dissected them. In places, portions 

 of the undermass project above the lowland surface. A similar 

 stage of reduction has been reached by most of the Otago 

 intermont basins. 



In ]^orth Canterbury the large Waiau-Hurunui basin, which 

 Speight describes as having "an origin in deformational move- 

 ments either of folding or faulting" (1915, p. 31:8), is floored 

 almost entirely by alluvium. A few "islands" of eroded 

 covering strata project above the basin plain. The alluvial 

 filling has been trenched to some extent as a result of late 

 movements of uplift, but this aflords no evidence that the 

 period of maximum waste supply from the mountains to the 

 northwest has been passed. The transverse course of the 

 Waiau and Hurunui rivers across this basin and the adjacent 

 uplifted block indicates an antecedent course (Cotton, 1913). 

 These streams are possibly anteconsequents, for the Hurunui 

 outlet is situated at a sag in the crest of the range. The basin 

 of the Hanmer Plain in [N'orth Canterbury, which is enclosed 

 by high blocks, is completely floored with alluvium, and the 

 covering strata tliat probably exist are completely buried. 



With long-continued stillstand, piedmont alluvial plains, like 

 basin plains, must be subject to dissection after the upland 

 blocks have been reduced by erosion. The writer can not point 

 with certainty to any example of it in Xew Zealand. The dis- 

 section of the Canterbury Plains by the Waimakariri, Pakaia, 



