24 Transactions. 



I select the Rakaia Gorge as typical of all the rivers, because 

 it is the simplest in form, and now consider how the terraces 

 arise here in the light of the fact that they are the remains of 

 former flood plains. 



In the Rakaia Gorge they are nearly all connected with 

 obstructions : — 



(1.) The highest ones are intimately related with the morainic 

 heaps of the old glaciers, or those heaps of morainic material 

 but roughly assorted by fluvio-glacial action. The rough angular 

 and subangular blocks were rather difficult to remove by river 

 action, and they protected portions of the original gravels, or 

 they allowed flood plains to be built up under their protection 

 either on the upstream side or on the downstream side of the 

 obstruction. The topmost terraces are nearly all associated 

 with these morainic heaps, and they form a series totally distinct 

 from the lower ones. 



(2.) The lower series of terraces have in most cases some 

 connection with the underlying hard rocks, which in the Rakaia 

 Gorge are principally volcanic. The flood-plain remnants arc 

 frequently on the downstream side of prominent bluffs of solid 

 rock. These protect flood plains which have been built up on a 

 foundation of solid rock or cut out of former river gravels. The 

 bluff causes the stream to move across towards the opposite 

 bank. Flood plains are therefore likely to form under its pro- 

 tection, as there is likely to be relatively slack water immediately 

 below it in which suspended matter is dropped. A flood plain 

 is thus rapidly formed, and when formed the bluff continues 

 to protect it, prolong its life, and thus promote terrace-develop- 

 ment. If these terraces have been formed on a floo: of solid 

 rock they will be doubly secure, owing to the influence of cause 

 No. 3, mentioned subsequently. If, however, they are terraces 

 cut out of old gravels, the bluff will still exert a protective 

 influence. The former condition explains the occurrence of 

 most of the terraces in the gorge proper ; immediately after the 

 river has passed through the gorge, the latter is the most im- 

 portant. The sheltering action of bluffs is very apparent in the 

 Ashburton and Waimakariri Valleys. 



(3.) The third condition which promotes terrace-development 

 near the gorge of the river is the occurrence of defending ledges 

 of solid rock, which the river exposes as it lowers its bed through 

 the gravels and lacustrine silts above the rock bar, or through 

 the gravels of the plains immediately below it. The influence 

 of defensive ledges was urged by Hugh Miller the Younger in 

 a paper read before the Royal Society of Edinburgh in the year 

 1882. I have not been able to see this paper, but an account 

 of Miller's theory was published in the " American Journal of 



