Speight. — Terrace-development of Canterbury Rivers. 19 



valley-walls also show signs of the truncation or partial trunca- 

 tion of the spurs. This is often attended by the formation of 

 short glacier shelves due to the erosive action of the glacier. 

 These shelves occur particularly where the glacier came over 

 the shoulder of a spur and cut down its bed in a manner analogous 

 to the action of a corroding stream. Good illustrations of this 

 are to be seen towards the head of the Waimakariri, up the 

 Bealey River, at Arthur's Pass, and in the neighbourhood of 

 the West Coast Road between the Cass and the head of Sloven's 

 Creek ; but these last cases belong to another part of the river- 

 valley. 



The Valley Path. 



The first part of the river-course grades into the second. 

 Here the valley is flatter and wider, and still shows signs of glacier 

 erosion ; glacial terraces or shelves are common in much the 

 same position as in the first part of the river-course, frequently 

 in sets of three, as noted by Captain Hutton. In some cases 

 it seems likely that terraces are formed by the erosive action 

 of tributary glaciers. These are turned round by the resistance 

 of the main glacier at the junction, and made to override the 

 projecting spurs on the downstream side of the valley. The 

 spurs are thus cut down to a marked degree, and show true 

 terraces of primary erosion. These terraces are cut out of solid 

 rock, and have a steep fall downstream— steeper than the grade 

 of the valley, and of no great length parallel to its axis. This 

 action is most probably going on now where the Ball Glacier 

 joins the Tasman ; and if we could see the side of the valley 

 underneath, it would almost certainly show these glacier terraces. 

 Good illustrations occur where a large stream, the name of which 

 is unknown to me, joins the Waimakariri on its south bank 

 about six miles above Bealey. This case is a most important 

 one, as it shows conclusively that even the smaller tributary 

 valleys were formed previous to the recent glaciation. The 

 stream enters the main river by a channel cut out of the solid 

 rock, and in the bottom of this glacial striae are plainly visible, 

 running across the bed of the stream and nearly parallel to the 

 axis of the main valley. The channel must have been eroded 

 previous to the glaciation, as it is very well marked, and depressed 

 about 50 ft. below the level of the surrounding rocks, which are 

 remarkably ice-worn as well, and form part of a truncated 

 spur entering the main valley at right angles. It appears almost 

 impossible that the channel of the stream can have been formed 

 solely by glacier erosion, and the recency of the glaciation is 

 emphasized by the perfection of the markings in a position 

 where they are very likely to be effaced. 



