248 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 19 3 2 



ORIGIN OF THE ROSS SHELF ICE 



Scott believed the present shelf ice to be a relict or direct descend- 

 ant of the old and much greater structure formed during the period 

 of maximum glaciation. David elaborated this idea. He described 

 the shelf ice as being essentially a great shrunken piedmont formed 

 mainly by the confluence of the glacial streams that flow down from 

 the plateau and fan out.^ These great ice streams are, in other words, 

 ribs that flatten out away from their sources. The intervals between 

 them have been frozen over, and snow has accumulated upon this sea 

 ice until the whole has assumed a more or less common level. 



Priestley differentiates two main methods of shelf-ice formation 

 according to the relative importance of the basal element, whether sea 

 ice or land ice. He believes that the Ross Shelf Ice originated chiefly 

 upon sea ice formed in a comparatively landlocked and shoal area.® 



It is quite impossible to estimate quantitatively the relative im- 

 portance of these two sources of supply, land ice and sea ice, for 

 the ice shelf. Unquestionably the great outlet glaciers such as 

 Beardmore, Liv, and the rest are more important sources and are 

 the causes of the movements within the shelf ice itself. Yet when we 

 crossed the shelf at its widest part on our way to and from the Queen 

 Maud Mountains we failed to observe any undulations that might 

 represent the ribs, and we did get the very distinct impression that 

 the greater part might have been formed essentially by sea ice be- 

 coming permanent and snow accumulating upon it. In the vicinity 

 of the Bay of Whales we saw no evidence that glacial ice ever played 

 a part. Not even in the overturned icebergs did we find any unmis- 

 takable glacial ice. Little America w^as built upon a shelf -ice bay 30 

 feet above sea level surrounded on the inland sides by older and 

 higher shelf ice. According to Martin Ronne, who was with Amund- 

 sen, the Little America basin was a bay of open water with an ice- 

 berg in it back in 1911-12 when Amundsen was established near by 

 at Framheim. The independence of this newer piece of shelf ice is 

 evidenced by the fact that where it merges with the older and higher 

 shelf ice it is yet separated from it by planes of disjunction indi- 

 cated either by crevasses or definitely aligned haycocks. 



CHANGES IN THE SHELF FRONT 



Scott's earliest observations showed that in places masses of ice 

 as much as 35 to 40 miles in width had gone out to sea since it 

 was first charted by Ross. The whole ice front at that time ap- 

 peared to have retreated some 30 miles since Ross's visit. Neverthe- 



5 David, T. W. E., and Priestley, R. E., Geology (Sci. Results British Antarctic Expedi- 

 tion 1907-1909), vol. 1, p. 125, London, 1914. 



"Wright, C. S., and Friestlcy, R. E.. Glaciology (British {Terra Nova) Antarctic Expe- 

 dition 1910-1913), pp. 163-169 and 205-222, London, 1922. 



