Speight. — Terrace-development of Canterbury Rivers. 27 



body of water right through the city. The danger of this re- 

 curring is all the more serious as the river is now showing a 

 decided tendency to wear away its southern bank. The Rangi- 

 tata and Rakaia are also aggrading their lower portions in all 

 probability. Mr. Edward Dobson informs me — and examina- 

 tion of the railway-levels confirms his statement — that the river 

 is running at a higher level at the Rakaia railway-bridge than 

 its old bed immediately north of it. The old bed is about three 

 miles and a half wide, and bounded on the north by a high 

 terrace, which the railway descends near the Bankside Station. 

 At the foot of this terrace the bed is 317 ft. above sea-level ; but 

 at about two miles and a half from it, going south, the level 

 of the bed rises to 339 ft., and falls in the next mile to 337 ft., 

 which is the level of the water at the bridge. This last height 

 , is subject to slight variation, depending on the position and size 

 of the anastomosing streams. These facts seem to show that 

 in former times the river ran 20 ft. below its present level, and 

 that in all probability it is now filling up the broad, flat trench 

 which it once eroded out of the tolerably level plains. It is 

 thus showing the characters of a stream on a fan, though in this 

 case the fan is confined by the old river terraces. 



The section across the Rakaia river-bed at the railway is 

 most instructive. The railway surveys show that, in most cases, 

 the low terraces within the main terrace are not absolutely 

 flat, but have a slight inclination away from the river, being 

 higher at the edge than they are some distance back. They thus 

 exhibit a form which resembles in some degree the scarp slope 

 and dip slope of sedimentary rocks. The scarp corresponds to 

 the terrace, and the dip slope to the gently backward-falling 

 surface of the terrace. This resemblance is merely one of form, 

 and not of structure, and it is exactly what might have been 

 expected in a case where terraces are partly due to erosion, and 

 partly to building up a flood plain, the latter process being 

 the more important. It is unfortunate that this interesting 

 section r cannot be reproduced. 



Part II. 



From the foregoing description of the mode of occurrence 

 of the terraces, it is evident that there must be some cause 

 or causes of exceptional character which have contributed to 

 their formation. In order that a river may form terraces on 

 the scale that occurs here, it must have considerable power of 

 corrasion, both vertical and lateral, and in order to form high 

 terraces the former must be relatively more important. I will 

 therefore consider the circumstances that affect the power of 



