Speicht, Cockaynk, Laing. — Moinil Arroirs/ziif/i Disfrirf. 335 



would appear plentifully, since the rocks are hard and resistant enough to 

 retain the finest nuirkings. Roches moutonnees of small size are very in- 

 frequent, and the larger ones grade into the general ice-shorn slopes and 

 truncated spurs. 



(e.) Truncated and Semi-truncated Spurs. 



These are exhibited in all stages of development in the Rakaia Valley. 

 Its sides have been straightened so that their alignment is almost perfect, 

 and the spurs exhibit the triangular facets which i^esult from the shearing- 

 off of their ends. I have shown elsewhere that such a land-form is 

 stable, and persists after other features resulting from glaciation have 

 been destroyed by denuding agents. Even in the Ashburton Gorge these 

 faceted spurs occur in an almost perfect condition. In the earlier stages, 

 if the valley is not very deep, the mode of truncation of the spur appears 

 to be that a series of strips is planed oft" the ends from above doAvnwards. 

 Shelves left in an unfinished condition suggest that this is a common pro- 

 cedure, although erosion at the base of the spur and along its whole face 

 must occur as well. These processes heighten the steep slope that the 

 end of the spur presents to the main valley, and it must become so high 

 in time that the glacier cannot overtop it, so that the valley can be 

 widened only by scouring away the end and by eroding the base. 



The angle of slope of the end of the spur is related to the width of the 

 valley and the amount of ice passing through a particular cross-section. 

 Where the valley is wider in its pre-glacial stage and the supply of ice is 

 small the walls flare more and the angle of slope is not so steep. Where 

 the valley is narrow and the supply of ice is great the angle approaches the 

 perpendicular. An exceptional instance of this action is seen on the north 

 side of Milford Sound, where the Lion Rock is vertical for hundreds, even 

 thousands, of feet. The height of the face of the spur is increased by the 

 deepening of the valley as the ice erodes its bed, and the slope tends to 

 become more and more steep as the valley is more deeply cut and the stream 

 of ice more confined, a result quite analogous to that produced by a river 

 when actively eroding its bed. In some cases the spurs are only partially 

 truncated, and specially so where a strongly marked ridge is overridden 

 by ice coming in from a tributary or round a pronounced bend in the valley. 

 Both Mein's Knob and Jim's Knob, at the head of the Rakaia River, owe 

 their form to this action, and in their case the result has been intensified 

 by the mutual interference of the Lyell and Ramsay Glaciers as they forced 

 their way together through the somewhat narrow valley just below their 

 junction. The ice has thus been crowded up on the slopes of the spurs at 

 the sides, and they both show the well-defined notch cut close to the valley- 

 wall by the more active erosion of the ice as it swings round a corner. If 

 this process is carried further it produces a rounded knob separated from the 

 main portion of the spur. Prospect Hill (Plate V, fig. 2), at the junction of 

 the Lake Stream and the Rakaia, owes its form to a part of the Rakaia 

 Glacier turning out of the main valley in the direction of Lake Heron and 

 cutting oft the projecting spur on the western side. It is a splendid example 

 of a semi-detached knob, and the downs behind it are the remnant of the 

 spur which has been reduced, but still remains at a height of 600 ft. above 

 the present floor of the main valley. In a former plateau country in which 

 the valleys are well developed the ice frequently crosses saddles and cuts 

 them down to a greater or less extent. In this way ridges are often partially 

 or completely separated from the mountain mass which they were originally 



