SNOW CRUSTS 39 



to cement them together. This carrying up of ice-vapour from below to enrich the 

 surface in ice somewhat resembles the feature of the surface caking of garden soils, if 

 left for some time undisturbed. In the latter case the caking is the result chiefly of 



Feet inches 

 I (about) Hard crust snow. 





■■' ''■'•■^V- 9'f'i'. f>. *i V-?!.^ 



8 Loosely cohennt granular ice crystals. 

 I Hard cnjst sno^ of older drift. 



2. (about). Probably loosely coherent granular 

 ice crystals. 



Surface of old sea ice . probably 3 to 4-years old 

 and about i5toi6 feet thick 



FiQ. 8. SKETCH SHOWING EFFECT OF SUBLIMATION A^'D TRAWINC UPON 

 SNOW NEAR 'PINNACLED ICE" OF McMURDO SOUND 



water, ascending by capillarity, and carrying mineral matter dissolved in it, 

 depositing that mineral matter at the surface of the ground when the water evapo- 

 rates. In the case of the Antarctic snowfields the crust is formed, not by 

 capillarity, where there is no thaw, but by ascent of relatively warmer ice-vapour 

 comincr in contact with the cold air at nio-ht above the snow surface. Where snow 

 crusts develop on snow-drifts near sea level the hardening of the surface is much 

 facilitated by actual thawing, and it is even possible then that capillarity may assist 

 in conveying thaw water up to the snow surface. On very hot days evaporation 

 proceeds so rapidly that it completely removes the harder top crust, and sets free at 

 the surface the granular ice crystals originally formed beneath the surface. 

 Shackleton states in regard to the snow surface over which the Southern party 

 travelled on the high plateau (from 7000 feet to over 10,000 feet above sea level) to 

 the south of the Beardmore Glacier : " Still further south we kept breaking through 

 a hard crust that underlay the soft surface snow, and we then sank in about 8 

 inches. This surface, which made the marching heavy, continued to the point at 

 which we planted the flag." * 



Again Shackleton describes the crust snow ou the surface of the Great Ice 

 Barrier {op. cit. 12) : " After we had passed latitude 80° S., the snow got softer day 

 by day, and the ponies would often break through the upper crust, and sink in right 



'Heart of the Antarctic," vol. ii. p. 18. 



