Vol. 54.] 



THE GLACIAL GEOLOGY OF SPITSBBRGEX. . 



207 



a=Ordinary granular ice. 



6= Ice-breccia. ^= Laminated ice. 



c = Black massiye ice, s=Seracs. 



then this band of ice-breccia would cut right across the lines 



of flow. The stratifi-cation of the glacier at this point is clearly 



due to deposition, for the breccia has certainly not been formed 



by crushing along a plane 



within the glacier. The Fig. 4. — Part of the eardern side of 



section shows therefore that Booming Glacier. 



the movement of the ice is 



oblique to the main axis of 



the glacier. 



The differential move- 

 ment in the Spitsbergen 

 glaciers that we have 

 been previously considering 

 must be too limited in 

 lateral range to account for 

 any commingling of boul- 

 ders from different sources. 

 The differential movements 

 caused by the deformation 



of the ice during its flow may be considerable in amount both 

 vertically and longitudinally. But the lateral movements thus 

 produced must be small. Glacial geologists long ago explained ^ 

 the intermixture of boulders from various localities by the assump- 

 tion that the ice at different levels below the same point on a 

 glacier may be moving in different directions. That this supposed 

 action does take place, and on a large scale, is, we think, proved by 

 the following case. It is further of interest as an illustration of 

 the spasmodic nature of the alternate advance and retreat of 

 Spitsbergen glaciers. 



The terminal face of Booming Glacier is at present advancing, 

 whereas nearer its source the glacier is diminishing, apparently 

 owing to a diminution in the snowfall at its head (see Pis. XYIII 

 & XIX). The upper surface of the glacier is saucer-shaped, being 

 higher at the margin than in the middle, so that it appears as though 

 the ice, in flowing forward, were climbing upward. But we doubt 

 whether this be the true explanation : it seems more probable 

 that the central depression has been formed by subsidence, owing 

 to the melting and solution of the lower layers of the ice. It is, 

 of course, possible that the ice has actually been forced to rise up 

 a slope of moraine, much as water is heaped up round the margin 

 of a lock into which a powerful current is flowing. Chamberlin ^ 

 has described cases among the North Greenland glaciers in which 

 the downward slope is not uniform in direction, but is interrupted 

 by depressions sometimes so great as to lead to a reversal in the 

 flow of the supraglacial streams. These reversed slopes, however, 

 compared with the ice-fields on which they occur, form only minor 

 undulations, and the glacier may be riding over obstructions in the 



^ See, for example, J. G. Goodchild, ' The Glacial Phenomena of the Eden 

 Valley,' Quart. Journ. Gaol. See. vol. xxxi (1875) p. 69. 



■^ T. C. Chamberlin, ' Glacial Studies in Greenland,' pt. x, Journ. Geol. 

 vol. V (1897) p. 231. 



