26 Transactions. 



below the Gorge Bridge, where harder sandstones and limestones 

 underlie the shingle. Here there is a good example of the com- 

 bined effects of the above-mentioned agencies. A protecting 

 bluff determines the commencement of a terrace on its down- 

 stream side, and also protects the bank so as to cause one on 

 its upstream side. The bluff also acts as a defensive ledge 

 to the higher terraces, which were at first dependent on the 

 larger and more resistent blocks of the terminal moraine of the 

 Old Rakaia glacier. A little above the bluff are excellent 

 examples of the protective action of talus cones, and ou the 

 opposite side of the river, a little higher up, of the action of a 

 tributary stream developed from a talus cone. 



All the above-mentioned causes are in operation in the Ash- 

 burton and Waimakariri Rivers, but it must be noted that all 

 terrace remnants cannot be assigned to them. A number of 

 smaller remnants are not related in any way to obstructions — 

 that is, as far as can be detected at present. It is possible 

 that some of the terraces above the gorge, where the river is 

 cutting out the silt and gravels filling the old lake, rnay^be the 

 remains of old lacustrine beaches. 



The Plain Path. 



The fourth division of the river-course is that across the 

 plains, when the river no longer meets solid obstructions in its 

 bed. The terraces here are simple and continuous in character; 

 the sequence is not so complete, as the remains of flood plain* 

 are, as a rule, fewer and higher. The river-bank sometimes 

 drops from the level of the plain to that of the water, a distance 

 of as much as 400 ft. in a single face. These terraces are caused 

 by the river moving across its bed lowering its channel as it 

 does so, making and again destroying its flood plains. One 

 reason why the terraces are so high is that the lower ones. 

 being composed of loose and incoherent materials, are readilv 

 removed, and the river is able to 3wing, in some cases, the whole 

 width of its former highest channel. The high terraces are 

 formed by the river planing off a strip every time it swings 

 across its bed, and swinging to the full width possible a large 

 proportion of the times. There is thus a tendency to produce 

 high and simple terraces. These are higher, however, in the 

 upper part of the plains, and gel lower as the river approaches 1 lie 

 sea; in fact, it is certain that the Waimakariri is rapidly raising 

 its bed in its lower portion — so much so that it threatens danger 

 to Christchurch, and demands the erection of costly protective 

 works as a defence in flood -time. On one occasion, in the year 

 1868, the river burst through, flooded the neighbouring country, 

 took a course by an old river-bed, and ran in a considerable 



