Marshall. — The Glaciatlon of Ktw Zealand. 341 



It is, however, possible to make an independent estimate of the thick- 

 ness of the ice. It is well known that the ice of a glacier decreases some- 

 what rapidly in thickness towards its lower end. This is a natural result 

 of the fact that at the low altitudes the relatively high temperature melts 

 it. The rate of melting in Switzerland is between 19-7 ft. and 26-2 ft. per 

 annum. It is probable that the rate is somewhat higher in New Zealand, 

 for the latitude is lower, and in consequence the rays of the sun are stronger 

 and the amount of summer rainfall is higher — and it is known that the 

 rainfall is perhaps the most important agent in melting the ice. The rate 

 of ablation, or of lowering of the ice-surface, mainly due to melting, is in 

 Switzerland 19 ft. to 25 ft. per annum (De Lapparent). Here it is certain 

 that the rate of ablation will not be overestimated at 20 ft. per annimi. 

 The rate of flow of the ice of the Tasman Glacier is 12 in. per annum at the 

 Malte Brun spur — say, opposite the De la Beche bivouac. This was mea- 

 sured in summer ; and in glaciology the summer rate is supposed to be 

 twice that of the winter rate. The average throughout the year at this 

 point is, then, 9 in. per day. The Mueller Glacier 40 chains from its ter- 

 minal face moves at the rate of 2-9 in. per day on an average throughout 

 the year. We shall not underestimate the rate of flow between the De la 

 Beche bivouac and the terminal face of the Tasman Glacier at 6 in. per 

 day on an average throughout the year. In other words, the ice travels 

 over its bed in this part of its course a distance of 182-5 ft^. per year. If 

 the ablation is supposed to commence to be in excess of accumulation at 

 the De la Beche bivouac, and to amount to 20 ft. per annum at the ter- 

 minal face, the average rate of ablation between these points will be 10 ft. 

 per annum. If the onward movement is 182-5 ft. per annum, and the 

 terminal face is eleven miles distant, the ice at the De la Beche bivouac 



5280 

 will take ^.- x 11, or 311-3 years, to reach the terminal face. 



During this time its thickness will decrease by 311-3 x 10 ft., or 3,113 ft. 

 In other words, this method shows us that the ice of the Tasman Glacier 

 is 3,113 ft. thick at the De la Beche bivouac. Since the surface of the ice 

 is at this point 4,782 ft. above sea-level, its lower surface must on this 

 estimate be 1,669 ft. above sea-level, or 689 ft. lower than the valley-floor 

 at the terminal face. Or, expressed differently, the Tasman ice must lie in 

 a trough-shaped basin similar to that of Lake Wakatipu. 



Whichever of these estimates of the ice-thickness is taken — 2,296 ft. or 

 3,113 ft. — it is, at any rate, evident that the thickness of the ice in the 

 Tasman Valley is very considerable ; but it is well known that it has little 

 or no effect on the climate of the surrounding country, not to mention New 

 Zealand as a whole. 



Viewed in the light of these facts of the low level and thickness of ice in 

 a valley of the present day, the statements as to the low level and thickness 

 of the ice in the Wakatipu Valley during the glacial extension becomes less 

 impressive. 



Whatever may be the origin of the Wakatipu basin, it is, at any rate, 

 evident that its form has not been changed by any faulting since the ice 

 that once filled it disappeared. In the absence of definite information we 

 are justified in assuming that the terraces round it are actually horizontal, 

 and this would almost certainly not be the case if faulting had occurred. 

 Soundings show that the floor of the lake is so destitute of irregularities 

 that no fault scarp can cross it. The reversed slope of its lower end must,, 

 then, have existed when the basin was filled with ice. 



