part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxi 



Eskimo, which have been formed on the spot simply by the fall of 

 snow on the frozen sea. 



It is acknowledged that the erosive power of moving ice will be 

 greatest where the superincumbent mass is thickest; and the con- 

 sequent differential effect is likely to have been strongly operative 

 in the broad ice-sheets of our sea-basins moving over comparatively 

 smooth floors of weak rocks. The thickest sheets of lowland drift 

 were laid down where the ice from the basins thinned off towards 

 its periphery, and where also its motion was checked by the rising 

 slope of the ground. The existence of these rising slopes fringing 

 the areas of glaciation in the plains is, I believe, the chief condition 

 which has determined the. extent and surface-form of our lowdand 

 drift-deposits. Where the spreading of the ice-sheet was checked 

 through advancing upon an inclined plane, its burden of trans- 

 ported material became gradually concentrated in a broad marginal 

 belt, in which a large part of the material was left as boulder-clay 

 on the dissolution of the ice, while a smaller part was redistributed 

 by the thaw- waters, as outwash gravels where there was free outward 

 drainage, and as laminated clays and silts alternating with gravels 

 where the waters were ponded in lakelets between the ice and the 

 land. It was at one time supposed that every great ice-sheet must 

 of necessity have had a moundy terminal moraine, and efforts were 

 constantly made to find these limiting moraines, or alternatively 

 to account for ' extra-morainic ' drifts ; but it is, I think, generally 

 recognized now that the heaped-up terminal moraine, even in the 

 case of glaciers of Alpine t} r pe, is only a mark of exceptionally 

 long stability of the ice-front, and is rarely in evidence where 

 there has been rapid retreat or advance of the glacier, while in 

 some of the broader ice-fields now existing in Arctic regions it is 

 absent altogether. The idea that boulder-clays are produced only 

 as moraine profonde also requires modification, as it seems 

 certain that they have sometimes, perhaps often, been formed 

 from the decay of detritus-charged stagnant ice, in the manner 

 suggested many years ago by J. Of. Groodchild in his description of 

 the drifts of the Eden Valley. 1 



The process was well seen in operation in Spitsbergen in 1910 

 by those of lis who had the good fortune to participate in the 

 excursion from the Stockholm Geological Congress. We were 



1 'The Glacial Phenomena of the Eden Valley, &c.' Q. J. G. S. vol. xxxi 

 (1875) pp. 55-99, and ' Ice-work in Edenside, &c.' Trans. Cumberland & West- 

 morland Assoc. No. 11 (1886-87) pp. 111-67. 



