Speight. — Modification of Spur-ends by Glaciation. 49 



where the action has not been intense, owing to the thinness of the ice and 

 the shortness of the period during which the area has been covered. Also, 

 there will be a progression of phenomena, varying in intensity on moving 

 from the outskirts of the glaciated area; and phenomena characterizing 

 the areas where glaciation has been intense, inexplicable in themselves, 

 may be elucidated from the intervening regions where glaciation has been 

 intermediate in its intensity. 



The region of the South Island whence most of the instances to be 

 mentioned later are drawn had reached a submature stage in the cycle of 

 erosion before the incidence of the glaciation. Valleys had been cut in an 

 elevated area, and a well-developed stream-system had been established 

 with long spurs trailing down into the main valleys ; but the district was one 

 of alpine character, with peaks approaching in elevation, if not exceeding, 

 the present European Alps. 



A most interesting case illustrating the nature of the slight modification 

 to which spurs may be subjected on the outskirts of a glaciated area is 

 furnished by Lake Manapouri. The chief complex of spurs entering the basin 

 occupied by the lake reaches down from the north, the spurs running in a 

 north-and-south direction, whereas the direction of the chief ice-stream was 

 from the west, and in its passage eastward it cut across the long trailing 

 ridges of the pre-glacial land-surface. Erosion was most marked in the western 

 reaches of the lake, where the ice was thickest and had acted for a longer 

 time, so that a great trough or hollow was formed, with precipitous sides 

 carried far down below the present surface of the lake (depth 1,458 ft.). 

 Not all of this is to be credited to excavation by glacier-action, but some 

 portion to the damming-back of the water by the morainic bar of the combined 

 Te Anau and Manapouri glaciers. While the ice has profoundly modified 

 the western portion of the lake and removed the spuis of the pre-glacial 

 valley-system, the change in the eastern spurs has been slight, merely cutting 

 them into a series of notches placed one below the other down the backbone 

 of the ridge, all with the same characteristic profile, and continued down 

 to lake-level, where exactly the same landscape -form is reproduced in the 

 islands that dot the lake. (See Plate VII, figs. 1 and 2.) These notches form 

 a kind of stairway M 7 ith the treads inclined backward so that the level of 

 the tread is lower at tae foot of the riser than on the edge of the tread (cj. 

 glacial stairway in a valley). The spur has thus been little modified, so that 

 its original form can be restored. The slight modification suggests that the 

 ice, though deep, as is evidenced by the height up the spur to which the 

 series of notches reaches, can have exerted its action for a comparatively 

 short time or it would have produced a profounder impression. Although 

 s : gns of ice-action are found some twenty miles to the east of these spurs, 

 the period of advance must have been quite short, or the spurs would have 

 been more profoundly modified. Traces of this peculiar landscape-form 

 are to be found on all the spurs to the eastern end of Manapouri where they 

 were likely to be exposed to the full force of the ice-flood, so that it can 

 hardly have been an accidental feature. Still farther eastward the spurs 

 aie unmodified. In Plate VIII, fig. 1, which is a view of the lake looking 

 west, there are also signs of the same form, with a more developed knob 

 in the background. 



The form of the modified spurs suggests another point. Judging from 

 the shape of the islands which lie off the ends of the spurs, it is clear that 

 before the ice-advance the spurs continued down below the present level of 

 the lake. It therefore negatives the idea that the hollow in whioh the lake 



