COASTAL PLAIN 9 



the steep foothills of the plateau. As It is for the most part covered by the ice of 

 the piedmont aground, one can do little more than guess at its structure. A study 

 of its surface, where it outci'ops from beneath the ice in the coastal cliiFs, shows that 

 for a considerable extent it is formed of an intensely glaciated surface of ancient 

 crystalline rocks with the hollows filled with a thin covering of moraine material. The 

 deepest hollows, being below the sea level, could not be examined. No true boulder 

 clay (till) was observed. It has been suggested that this coastal plain represents a 

 down-faulted segment, but, at present, evidence of such ftiulting is wanting. Mr. 

 E. C. Andrews, of the Geological Survey of New South Wales, suggests that it is an 

 old plain of marine erosion, and it may be so. It is also possible that it may represent 

 an abrasion shelf notched back by the Ross Barrier at the time of maximum glaciation. 

 To this last suggestion it may be objected that, during the maximum glaciation, the 

 pressure of the local glaciers of the Western Mountains would suffice to repel, 

 or shoulder away, encroachments on the coast by the Ross Barrier. The Barrier may, 

 however, have shouldered them around northwards and commandeered them, so to 

 speak, for the work of eroding the terrace. It seems a gigantic work for ice to have 

 accomplished, but a mass of moving ice (and we know from the evidence of the 

 erratics that it was certainly moving) 500 miles (805 kilometres) wide, and some 

 3000 to- 4000 feet thick (914-1219 metres), represents a vast erosive force. 



It might also be suggested that this coastal plain represents in part an old plain 

 of aggradation, that is formed from deposition of continental waste, resting on an 

 ancient plain of marine erosion, somewhat analogous to the coastal platform of the 

 Malaspina Glacier.* 



As far as our observations extend, there is no great thickness of alluvial material 

 resting on the glaciated surface of crystalline rock. 



Unfortunately we have only a few soundings, too few to admit of the construction 

 of a continuous curve from where this plain ends at the coast down to the greater 

 depths of Ross Sea. The few soundings that were taken indicate depths of from 

 350 to 450 fathoms within 12 miles of the coast, deepening to over 650 fathoms in 

 the neighbourhood of a large glacier, like that which forms the Drygalski Ice Barrier 

 Tongue. The following is a section across this coastal platform from Mount Davidson 

 to Mount Erebus : 



It will be noticed that after the soundings, taken from W. to E., deepen to 350 

 fathoms at 12 miles E. of the coast platform, they shoal to 110 fathoms in the middle 

 of McMurdo Sound, then deepen again to 460 fathoms at only three miles off the west 

 coast of Ross Island. 



This shallowing towards the centre of the Sound is probably due to the vast amount 

 of morainic material carried northward by the former Great Ice Barrier, when at its 

 maximum, from the neighbourhood of Mount Discovery, or possibly it may be due to a 

 submerged chain of small volcanoes. Probably the former is the correct explanation. 



* I. C. Rus.=;ell, 1899 ; R. S. Tarr. 1907 ; L. Martin, 1909. 



B 



