200 GENERAL GEOLOGY 



Negative forms due to erosion are valleys, both of the wide U, or alb * valley 

 type, and of the trogtal type, hanging valleys, cirques, steps, treads, terraces, and 

 rock basins. In the physiography of South Victoria Land the great feature above 

 all others which arrests the eye of the observer is the extraordinarily long, straight 

 valleys occupied by the outlet glaciers. They resemble nothing so much as vast 

 I'ailway cuttings 50 to 100 miles in length and 5 to 20 miles in width, their smooth 

 sides rising from 3000 to 5000 and 6000 feet above the present level of the glaciers. 



Plate LVIII. illustrates a typical glacier-sculptured valley, the Ferrar Glacier 

 Valley, showing the remarkably smooth spurless walls. 



Evidence has already been quoted to show the existence of alb and trogtal 

 valleys at Granite Harbour, at the Mawson Glacier, and at the Beardmore Glacier, 

 while there is probable evidence of a similar structure at the Ferrar Glacier. On 

 a larger scale than typical alb valleys are the ancient spillways, through which, 

 during the maximum glaciation, the inland ice overflowed across the great horst to 

 Ross Sea. These are as much as 40 miles in diameter near the Drygalski Ice 

 Barrier Tongue. The great horst has sagged downwards between Mount Nansen 

 and the Ferrar Glacier Valley, thus allowing a vast sea of ice to overflow it 

 bodily during the maximum glaciation. 



The rocks of the horst being capped by an almost horizontally bedded formation 

 (the Beacon Sandstone) to a thickness of at least 2000 feet, facilities for observing 

 the amount of ei'osion are particularly good. 



The lowest portions of these gaps are still occupied by glaciers which have now 

 sunk into their trogtaler. The erosion of the rocks of the horst has been carried on 

 to far below sea-level, at all events in the neighbourhood of the Drygalski Ice 

 Barrier Tongue. It is of course possible to argue that the ice there, which is some 

 2000 feet in thickness, has simply occupied a pre-existing deep bay in the sea coast 

 formed by tectonic subsidence. Until far more soundings than are at present 

 available have been taken, one must admit that actual scientific measurement on 

 this subject is somewhat lacking, but as far as the evidence goes, it shows that 

 in the neighbourhood of the present glacier snouts the sea floor is over-deejiened, 

 shallower soundings being obtained farther out to sea than close inshore. (See 

 sections illustrating Chapter III.) Obviously this fact is in favour of the rocks of 

 the horst having been cut down far below sea-level by ice erosion. 



Another possible view of this phenomenon is that under the weight of ice, 

 during the maximum glaciation, the whole of the land surface, including the shore- 

 line, sank from 2000 to 3000 feet, and that the land glaciers were gradually 

 submerged as the result of this subsidence. At present there is no proof whatever 

 of such a subsidence having taken place, whereas there is evidence of the gi-eat 

 horst having risen vertically, probably for fully 50 feet, in recent geological time. 



* Mr. T. Griffith Taylor uses the terui kar terrace for the flattish floor of what we have termed the 

 alb valley. 



