Vol. 54.] THE GLACIAL GEOLOGY OF SPITSBEKGEN. 205 



series of shearing-planes, owing to the pressure of the ice behind, 

 rather than to a viscous yielding or to the action of continuous 

 fracture and regelation. 



The overrolling advance of the glaciers affords a simple explana- 

 tion of the origin of the debris-bands and intraglacial material with 

 which the lower part of the glacier is so richly charged. Let us 

 consider the case of a glacier passing over loose materials, such as a 

 raised beach. The ice-blocks which form the talus are generally 

 irregular in shape, and as they fall with some force their projecting 

 corners are driven into the ground. The blocks are further 

 hammered in by the fall of others upon them. When the glacier 

 flows over the talus, the latter is probably first crushed and then 

 re-solidified, during which some or the underlying earth must get 

 frozen in with the ice. When this basal layer of ice works upward, 

 it carries with it the material which it has picked up from the 

 valley-floor. This process being continually repeated, the whole of 

 the lower part of the glacier becomes charged with debris, which, 

 if no other factor came into play, would at length be distributed in 

 layers throughout the whole thickness of the glacier. 



Such debris-bands have been described by Chamberlin from the 

 glaciers of jSforthern Greenland ; he, however, records them only 

 from quite the lower part of the glacier. His observations agree 

 with the view that has been often expressed, to the effect that the 

 intraglacial drift of the ice-sheets of North America and North- 

 western Europe was entirely limited to the basal layers. In some of 

 the Spitsbergen glaciers, however, the intraglacial material, though 

 most abundant in the lower part, is scattered throughout nearly the 

 whole thickness, as in the Plough Glacier (PI. XIII, fig. 1). 



That the intraglacial drift is picked up from off the valley-floor 

 and raised through the glacier was shown in one case by the 

 nature of the material. The terminal lobe of the Ivory Glacier, 

 which occupies part of Agardh Dale, flows over a plain of recently - 

 upraised sea-floor, whose surface is thickly covered with water- 

 worn pebbles and mud. Scattered throughout this deposit occur 

 numerous shells, with driftwood and fragments of whalebone. We 

 found fragments of the shells of Saxicava rugosa, Mya truncata^ 

 etc. in some abundance in the lower debris-bands of the glacier, and 

 on the lower slopes of the terminal moraine ; at higher levels, the 

 shells are scarcer and more fragmentary. We found one fragment 

 at the height of 400 feet, and that we did not find any still higher 

 was probably due to the hurried nature of our search. 



It may be suggested that these marine remains found in the 

 moraine were derived from some old raised beach, which is 

 situated at a level higher than that at which we found them. The 

 highest raised beach that we could find in the traverse from Sassen 

 Bay to Agardh Bay was not more than 200 feet above sea-level. We 

 found no trace of any raised beach in the upper Pulmar Valley between 

 the great morainic bar and the northern face of the Ivory Glacier. 

 In the lower Fulmar Valley we found traces of raised beaches, 

 but we doubt whether they reached the 300-foot level (see fig. 3, 

 p. 206). The highest terraces that we could see on the sides of 



