Speight. — Terrace-development of Canterbury Rivers. 37 



particles are dropped first, and if there is not an approximately 

 equal quantity of smaller material for the river to move in place 

 of the material dropped there will be an excess of energy left 

 over for corrasion. Under ordinary circumstances there is an 

 insufficient supply, and so the river-channel is lowered. 



The supply of waste has such an important bearing on the 

 corrasive power of a river that a consideration of the circum- 

 stances which control the supply in the Canterbury mountains 

 will be relevant here. One of their most striking features is 

 the vast supply of debris supplied by their slopes exposed to 

 frost erosion. This effect is so marked that whole mountain- 

 sides are covered with angular debris, which is continually moving 

 downwards, but especially so in the case of shingle-slips. These 

 are often from 2,000 ft. to 3,000 ft. in height, and may be as 

 much as a mile wide. The reasons for this excessive supply of 

 waste are as follows : (1.) The iointed character of the rocks 

 in the drainage-basins of the rivers. (2.) Owing to intense 

 folding of the rocks, they frequently dip at very steep angles, 

 and therefore the weakest beds are exposed to the atmosphere 

 without being protected by more resistent beds. (3.) The age of 

 the folding dates back to Mesozoic times, and therefore weather- 

 ing agents have been able to exert their influence to a marked 

 extent. (4.) The range, both annual and diurnal, of the tem- 

 perature is very great. (5.) The absence of close plant-covering 

 over large areas. All these causes promote extensive disintegra- 

 tion, and any explanation of the life history of our rivers must 

 take them into account. 



One of the principal factors determining the production of 

 waste is the extent of mountain-slope not protected by a close 

 covering of vegetation. The area of most vigorous denudation 

 is between the snow-line and the upper limit of this covering. 

 The snow protects the underlying rocks to a certain extent ; 

 but, nevertheless, even here the denudation is rapid, but espe- 

 cially on those steep faces where snow cannot he. When the 

 snow is turned to ice the effect is somewhat similar. Erosion 

 will not proceed as rapidly under the ice as on the slopes at a 

 higher and lower level free from ice, but exposed to the action of 

 frost. The effect of elevation of the land mil be to make the 

 area above the snow- line greater and to expose a much greater 

 area to the influence of frost. The part affected in the Southern 

 Alps is principally that between the 3,500 ft. and the 7,000 ft. 

 contour lines. If the land were raised, the country affected 

 would be approximately that between the same levels, but the 

 area included would be very much greater ; although this would 

 be diminished by the accumulation of ice in hollows where it 

 could not melt. Large areas below the snow-line would be 



