22 TraJisactions. 



as much as 500 ft. in the case of the^Rakaia— and the hypothesis 

 of the axis of elevation seems hardly competent to explain 

 this remarkable occurrence in the valleys of all the principal 

 rivers. 



The deepened portion of the valley has been filled with 

 glacial silt and angular debris from the hills ; traces of old sub- 

 lacustrine fans or deltas are to be seen in many places. These 

 sedimentary beds are now being eroded by the rivers as they 

 cut down through the bars of solid rock that form the main 

 floor of the gorges by which the rivers issue on to the plains. 

 The present shape of the river-valleys is due, therefore, to the 

 modifying action of glaciers and other agencies on a previous 

 matured stream system, the rough features of which were ante- 

 cedent to the glacier extension. With this explanation it will 

 be possible to consider the third division of the rivers' course 

 as regards their terrace-development. 



The Gorge Path. 



In this paper I apply the term " gorge path " to that part 

 of the river-course from its first meeting the lacustrine beds 

 above the bar of rock till it has freed itself from all rock obstruc- 

 tions in the upper portion of the plains. It is only the middle 

 section of this which shows the true character of a river gorge ; 

 but it is most convenient to consider the more extended length 

 with regard to the terraces. 



The three principal rivers of northern Canterbury burst 

 through the outer range of mountains by gorges of a similar 

 type. The Ashburton Gorge was formed under peculiar con- 

 ditions, owing to the great changes in the directions of drainage 

 caused by the extension of the glaciers. If we take the Rakaia 

 as a typical case, we have a river flowing through a bed of glacial 

 silt which partially filled the old Rakaia Lake, and then coming 

 to a winding gorge cut out of the solid rock which forms the 

 floor of a wide valley. This valley is nearly three miles in width, 

 tolerably flat, but covered with heaps of morainic and fluvio- 

 morainic matter. The river flows in meanders at a depth of 

 nearly 500 ft. below the main floor. This winding trench was 

 begun immediately after the ice began to retreat, no doubt 

 while the lake was in existence above the solid bar of rock. 

 Owing to increased power of corrasion, the river has deepened 

 its meanders far below the upper floor of the gorge, and is now 

 actively removing the projecting spurs between them. Several 

 cases of nearly demolished spurs and of islets in the river-bed 

 which are now quite cut off are to be seen in the Waimakariri 

 as well us in the Rakaia. 



