Speight. — Modification of Spur-ends by Glaciation. 47 



Art. V. — The Modification of Spur-ends by Glaciation. 



By R. Speight, M.A., M.Sc, F.G.S., F. N.Z.Inst,, Curator of the Canter- 

 bury Museum. 



[Read before the Philosophical Institute of Canterbury, 6th October, 1920 ; received by 

 Editor, 31st December, 1920 ; issued separately, 27th June, 1921.] 



Plates VII-XT. 



The subject of the changes which glaciers exert on the form of stream- 

 valleys is such an interesting one that special aspects are worthy of detailed 

 consideration. It has not, however, been fully considered so far as this 

 country is concerned, although Andrews in his classic paper on the glaciation 

 of south-western New Zealand (1905) has drawn attention to certain forms, 

 such as the total truncation of spurs, and the development of sitting-lion 

 and titan-beehive shapes, as well as the formation of a double slope on 

 the valley-sides and especially on the spur-ends. The present author has 

 pointed out certain other features (1907 and 1911), but observations made 

 during the past few years in the alpine region of the South Island of 

 New Zealand have suggested that still other forms exist. The faceting 

 of spur-ends as a general result of the overdeepening of glaciated valleys 

 and the formation of tributary hanging valleys has been dealt with in 

 various places by W M. Davis, G. K. Gilbert, de Martonne, and others : 

 but apart from this, judging by the literature at my disposal, little 

 has been written. Davis has, however, insisted that the detached knobs 

 on the floors of valleys, either separated from or in close proximity to 

 the valley-walls, are remnants of a pre-glacial land-surface which have 

 escaped destruction. He says (1900, p. 274), " On entering the glaciated 

 valley of the Rhue it is found that the regularly descending spurs of 

 the non-glaciated valleys are represented by irregular knobs and mounds, 

 scoured on their up-stream side and plucked on the down-stream side ; 

 and that the cliffs formed where the spurs are cut off are sometimes 

 fully as strong as those which stand on the opposite side of the valley. 

 The spurs generally remain in sufficient strength to require the river 

 to follow its pre-glacial serpentine course around them, but they are 

 sometimes so far destroyed as to allow the river to take a shorter course 

 through what was once the neck of a spur." Again, on page 276 he says, 

 " It is seen that just before the complete obliteration of the spurs some of 

 the remnant knobs may be isolated from the uplands whence these pre- 

 glacial spurs descended. It is out of the cpiestion to regard the ruggedness 

 of such knobs as an indication of small change from their pre-glacial form, 

 as has been done by some observers. The ruggedness is really an indication 

 of the manner in which a glacier reduces a larger mass to smaller dimensions 

 by plucking on the down-stream side as well as by scouring on the up-stream 

 side. It is possible that knobs in other glaciated valleys than that of the 

 Rhue mav be of this origin ; thev should then be regarded not as standing 

 almost unchanged and testifying to the incapacity of glacial erosion, but as 

 surviving remnants of much larger masses, standing, like monadnocks above 

 a peneplain, as monuments of the departed greater forms." 



