Speight. — Terrace-development of Canterbury Hirers. 17 



Introductory. 



The rivers of Canterbury which will be considered in this 

 ^paper are those of the middle district — viz., the Waimakariri, 

 Rakaia, Ashburton, and Rangitata. They closely resemble each 

 ■other as regards the conditions under which the valleys were 

 formed with the partial exception of the Ashburton, so that 

 statements made about one generally apply to all. They all 

 rise in the main range of the Southern Alps, or close to it, and 

 flow in a south-easterly direction till they reach the sea, the 

 first half of their course being through the mountainous region 

 ■of western Canterbury, and the second half across the plains 

 which fringe this region on the south-east. The rocks of the 

 first portion consist principally of folded slates, sandstones, 

 greywackes, and allied sedimentaries chiefly of Lower Mesozoic 

 age. Palaeozoic rocks doubtless occur on the eastern and western 

 margins of the mountain region, but the general absence of fossil 

 evidence renders their true age difficult to determine. 



The folding of these rocks occurred most probably in Upper 

 Jurassic times, but traces of an earlier folding are also found. 

 They are distinguished throughout the whole area by excessive 

 jointing, which has rendered them particularly susceptible to 

 -the disintegrating action of frost, and has caused them to split 

 readily into more or less rectangular and prismatic blocks. 

 This effect is so marked that many of the mountains are, for 

 several thousand feet in altitude, covered with a coating of 

 debris so thoroughly that solid rock is scarcely visible. This is 

 constantly moving down to lower levels under the influence 

 of the transporting agents which operate in mountain tracts, 

 but principally owing to the torrents formed by melting snows. 



The rocks of which the plains have been formed consist 

 chiefly of gravels, more or less perfectly rounded, and of sands, 

 silts, and mud. The last predominates in the outer margin 

 of the plains. There is in some cases an admixture of volcanic 

 material and limestone, but these are of relatively minor im- 

 portance. 



The western mountain area formed at one time part of a 

 great peneplain, and this has now been thoroughly dissected. 

 The paths of the rivers are generally at right angles to the strike 

 of the beds, so that the main streams may be called consequent, 

 while the tributaries are generally parallel to the strike, and are 

 therefore subsequent ; but, owing to the age of the river-valleys 

 and the influence of other disturbing agencies, marked departures 

 from this rule frequently occur. A recent severe glaciation, 

 after the valleys had reached a mature stage, exerted great 

 influence on them, and its effects are still plainly evident. The 

 rivers are all perfectly graded at the present time, but it is highly 



