Spkioiit, Cockayne, Laixc. — Momit Arrmno/iith Disfricf. 339 



this action can be seen in the Sounds district on the sovith-west of the 

 South Island of New ZeaUmd, and those at the liead of George Sound may 

 be specially cited as furnishing excellent examples of the phenomenon. 



(/.) Efficiencif of Glaciers as Eroding Agents : Eindence furnished hi/ the 



Locality. 



A consideration of the glaciation of the area would be incomplete Mdth- 

 out some reference, however slight, to the evidence bearing on the much- 

 discussed problem of the efficiency of glaciers as erosive agents. It is ad- 

 mitted, even by those who admit least, that glaciers act as flexible rasps, 

 and remove the minor inecjualities of the land-surface. The landscapes of 

 the area under consideration give abundant evidence of this, but they are not 

 so decided on the major question of the power of glaciers to excavate the 

 beds on which they lie. It appears to the present writer that corrie glaciers, 

 instead of acting as protective agents, as suggested by some observers of 

 wide experience, do actively erode their beds, and also their side walls and 

 their heads, especially the last, much more rapidly than the streams which 

 issue from them erode their beds. Also, small valley glaciers have this 

 power as well, and enlarge the amphitheatres at their heads at a more rapid 

 rate than the rivers which they give birth to erode their valleys, so that 

 these are narrow in their lower reaches, whereas their upper portions, which 

 have till recently been filled with ice, are wide and of basinlike form. In 

 this case, however, full consideration must be given to the possible neutral- 

 ization of the erosive power of a stream overloaded with waste, as many of 

 these streams are. Still, after making full allowance for this, there appears 

 to be ample proof that ice in corrie glaciers and in the smaller valley ones 

 related to them does not act as a protecting but as a powerful erosive agent. 



There are other matters suggested by the landscape-forms of the Rakaia 

 Valley which require very careful consideration in this connection, and 

 apparently point in the opposite direction. First of all, there is a marked 

 absence of waterfalls and hanging valleys. On the northern side of the 

 main river the numerous parallel streams which rise in the main divide 

 and flow south enter at grade (Plate VI, fig. 2), and the same is true of those 

 on the southern bank, although they are few in number, and, with the 

 exception of the Lake Stream, are much smaller. At first sight it would 

 appear that the glaciers have only acted as a rasp, and modified but slightly 

 the features of the previous valley system, were their efficiency as erosive 

 agents not clearly indicated by the basin which occurs in the main valley 

 of the Rakaia below the intake of the Lake Stream. Here there is a great 

 hollow over twenty miles in length and 500 ft. deep in places, shut in at its 

 lower extremity by a rock wall, which was once occupied by a great glacier 

 and was subsequently either partially or wholly filled with silt. It was 

 finally drained by the outflowing river cutting down a narrow gorge through 

 the lip of the basin, a result hastened by the pouring-in of enormous supplies 

 of waste as the glaciers retreated up the valley. This landscape-form is re- 

 produced perfectly, but on a smaller scale, in the Upper Ashburton Valley. 

 In this case it seems impossible to attribute the rock-barred hollow^ to any 

 other cause than basal excavation by a glacier. Exactly similar features 

 can be seen in the valleys of the Rangitata and the AVaimakariri, and they 

 no doubt occur in the less-advanced condition in the valleys of the Tasman 

 and Godley, in the basin of the Waitaki, further south. Here lakes still 

 occupy basins once filled Avith ice ; but they also, like the basins further 



