256 



The turbid water issuing from glaciers is an evidence of their degrading 

 power. The waters of the Dart and Matukituki — both glacial-fed rivers — are 

 of a milky colour. Dr. Hochstetter, in his Neio Zealand, states that the 

 Tekapo Lake is always turbid, from the fact of its supply coming from one of 

 the great glaciers of Mount Cook. Forbes explains that this turbid appear- 

 ance, constantly the same from age to age, is due to the impalpably fine flour 

 of rocks ground in the ponderous mill betwixt rock and ice. 



It may elucidate the subject if we suppose the formation of a lake basin 

 about to begin. Let a glacier descend a mountain slope to a valley, then it 

 must either penetrate through the floor of the valley on the line of the 

 mountain slope, or, in its endeavour to do so, be either bent or broken to the 

 slope of the valley. But this efl;"ort of the glacier to continue in its initial 

 slope, mu-st necessarily cause great friction, and where there is friction, there 

 must also be degradation of surface. The friction due to a change from a 

 greater to a less slope is in excess of what may be termed the sliding friction, 

 arising simply from the motion of the glacier over a uniform slope. Thus, at 

 the points of greatest friction, there will be a scooping-out process at work. 

 Nor is this an intermittent operation, but will continue as long as the glacier 

 exists. As the process proceeds, the part of the valley first operated on will 

 have been scooped out, and the valley slope assimilated in part to the mountain 

 slope. The condition of excessive friction due to diSerence of slope will then 

 apply to a more advanced part of the valley, and so on, till temperature arrests 

 the process by melting the ice. 



According to this view of the subject, we ought, keeping all modifying 

 causes out of sight, to find that the lake bottoms are a succession of slopes, 

 steepest next the glacier mountains, and gradually less and less as we proceed 

 from them. The bottom of the Wakatipu Lake complies with these conditions. 

 Taking the soundings from the map in the Otago Museum,* and interpolating the 

 soundings taken ofi^ Collins Bay, there will result for the first two miles from 

 the head of the lake a fall of 180 feet per mile, for the next four miles a fall of 

 70 feet per mile, for the next five miles a fall of 50 feet per mile, for the next 

 seven miles a fall of 40 feet per mile, for the next six miles a fall of 6 feet per 

 mile, for the next six miles a fall of 14 feet per mile, for the next three miles 

 a fall of 30 feet per mile, for the next seven miles a rise of 1 6 feet per mile, for 

 the next nine miles a rise of 96 feet per mile. 



It must be mentioned that the slope of 180 feet per mile for the first two 

 miles may be considered greater than the true glacier or lake bottom, on 

 account of the river deposits of the Dart and Bees. 



In regard to the ascending slope of the lake bottom for the last sixteen 



* Map showing Surface Features of N. W. District of Otago, by Dr. Hector, 1864. 

 Partly reduced, and accompanied by a Section to elucidate this subject, in Plate 19., 

 Trans. N. Z. Inst., Vol. ii. 



