Speight.— Terrace-development of Canterbury Rivers. 21 



from the south-east, and it is therefore likely that it affected 

 the rocks further south-west. If this axis extends into mid- 

 Canterbury, it might account for the slight break in the grade 

 of the rvers which occurs at their gorges. They have a flat 

 grade above and a steeper grade below, as the following table 

 taken from Haast's " Geology of Canterbury " will show : — 



Distance Fall of 

 in Rivers, 



Rangitata— Miles. per Mile. 



From junction of Havelock and Clyde to 



beginning of plains . . 29 35 



From beginning of plains to railway . . 23£ 39f 



From railway-crossing to sea . . . . 8| 29 



Ashburton — 



From Clearwater Creek to beginning of 



plains near Two Brothers . . . . 11^ 37* 



From Two Brothers to railway . . 25 44 



From railway to sea . . . . . . 10| 28| 



Rakaia — 



From junction of Wilberforce River to 



Gorge Island . . . . . . 19 25* 



From Gorge Island to railway . . 21* 23* 



From railway to sea . . 16 234; 



Waimakariri — 



From junction of Bealev River to junction 



of the Esk River.. .. ..21 24 



From junction of Esk River to junction of 



Kowhai River . . . . . . 17 33 



From junction of Kowhai to White's old 

 accommodation - house at height of 

 605 ft. .. .. .. ..15 26| 



From White's accommodation-house to 



tidal boundary . . . . . . 22 27£ 



The flat grade followed by a steeper grade is apparent in 

 the Rangitata, Ashburton, and Waimakariri ; but in the case 

 of the Rakaia there is no marked break : its bed is almost uni- 

 formly graded for a long part of its course. This difference in 

 grade may be due to crustal movements, but I think it is more 

 probably due to glacier erosion ; however, there is no impossi- 

 bility that it may be due to both causes. The Canterbury 

 valleys are of very ancient date, and were developed to a mature 

 stage before the recent glacier extension. The glaciers hollowed 

 out their middle portion, particularly where two valleys join, 

 but left across the mouth a bar of solid rock, owing to the falling - 

 off in erosive power near the terminal face. Behind this bar 

 there is always a deep depression or basin in the solid rock — 



