Speight. — Terrace-development of Canterbury Rivers. 39 



this last appears to rne the most satisfactory explanation. No 

 doubt the erosion of its bed which the river is enabled to per- 

 form owing to the diminution of the supply of waste would tend 

 to be neutralised by the depression of the land proved on page 

 32. If the land had been low, and the former supply of waste 

 comparatively small, this depression would have been sufficient 

 to produce aggradation instead of corrasion. But the land is 

 still high, the rivers are still powerful torrents, and the supply of 

 waste fast diminishing. These factors are sufficiently great to 

 nullify the effect of depression in the higher portion of the river- 

 course ; but the rivers have now reached such a stage in their 

 development that in their lower course aggrading is now going 

 on : hence depression has made its influence apparent. This 

 is what might reasonably have been expected ; and, if depression 

 continues, this effect will become more and more marked, so 

 that the terraces will tend to disappear. However, should the 

 slight elevation which has taken place recently continue, 

 aggrading in the lower portion of the river- course will cease and 

 terracing will be resumed. 



I have been confirmed in my conclusion that the supply of 

 waste is a controlling factor in the terrace- development of our 

 rivers by observation of the history of shingle fans. In their 

 youthful stage they are built up by an aggrading stream ; in 

 their vigorous middle period they are partly channelling their 

 fans and partly building them up on their outskirts ; when they 

 reach their mature stage they become channelled and terraced 

 by the stream that runs through them. This terracing closely 

 resembles that on the plain course of our rivers. It is more 

 marked near the apex of the fan, and falls off towards the fringe 

 This may be due to the fact that the river is more confined 

 near the apex of the fan, and therefore more capable of vertical 

 corrasion. But it is also due to the fact that in former times 

 of excessive supply of waste that waste was chiefly deposited 

 just below the gorge. It may perhaps be due to increase in 

 volume of the river as it enlarges its drainage-area. However, 

 increase in volume will not explain the fact that after every 

 freshet a stream apparently terraces its fan on a diminishing 

 volume. 



In his accounts of the formation of the Canterbury Plains, 

 Captain Hutton maintained that they had been levelled by the 

 sea and subsequently raised, so that the rivers were able to 

 terrace them. If this were the case, terracing should progress 

 up - stream, should show a maximum development near the 

 sea, and not, as in this case, near the gorges. If, however, the 

 loess is not marine but of eeolian origin, as seems very probable, 

 and since it is incapable of resisting marine erosion, there can- 



