Speight. — Terrace-development of Canterbury Bivers. 25 



Science," vol. xii. 1902, by Professor W. M. Davis when describ- 

 ing the Terraces of Westfield Kiver, Mass. The phenomena he 

 there 'describes are reproduced in our rivers : good examples 

 occur in the Kakaia, but excellent ones are to be seen in the 

 Waimakariri at Little Gorge Hill, where the railway crosses. 



It may be urged that there is no great difference between 

 cause (2) and cause (3). It is quite true that a defending ledge 

 may, under certain circumstances, become a protecting bluff ; 

 but the latter will be after the bluff has done duty as a defending 

 ledge and the river has lowered its bed considerably. However, 

 in very many cases the action is quite distinct, and some pro- 

 tecting bluffs have never been defending ledges. 



(4.) The same result is obtained also by the defending action 

 of a tributary stream which pours in a load of sediment. Ac- 

 cording to the general law of stream action, a tributary if fully 

 supplied with waste will deposit it on joining the main river 

 flowing on a gentler grade. In any case, the tributary pushes 

 the main stream over. This action is much the same as a 

 defensive ledge. The bank is defended from the destructive 

 action of the main stream by the force of the tributary. If the 

 main river can lower its bed, then we shall expect to have a 

 series of terraces ; but they are different in character from those 

 due to the previous causes. They are usually lower and broader, 

 and the sequence is more perfect ; they are extremely common, 

 and seen in almost every case when one stream joins another. 

 Thev afford the most complete record of the oscillations of a 

 river across its bed, and are more remarkable in this respect 

 than those due to cause (3). Splendid examples of such terraces 

 are to be seen at the junction of the Kowhai with the Waimaka- 

 riri, and also at the junction of Woolshed Creek with the South 

 Ashburton. 



Closely connected with the action of tributary streams is 

 that of ta!us cones. One of the causes of the partial destruction 

 of the terraces is the formation of talus cones from the high 

 shingle banks. These grow, owing to the erosion towards the 

 head of the cone, till intermittent streams flow down them. 

 Erosion then proceeds apace. In this way a portion of the 

 terrace is rapidly destroyed ; but the cone or fan on the floor of 

 the river-valley protects the remaining portion of the terrace 

 from the erosive action of the river, so that rapid destruction 

 of one portion prolongs the life of the remainder. This action 

 is to be seen in many places near the Rakaia Gorge. 



Cases of all these four modes of terrace-development are 

 to be seen in the gorge itself, or immediately after the river 

 debouches on to the plains. In the Rakaia they may be seen 

 as far down as the Curiosity Shop beds, about three miles 



