"^"^ol. 54.] ON THE GLACIAL GEOLOGY OE SPITSBERGEN. 203 



layers of the glacier, where not hidden by ice-talus, is so abundant 

 that we found it in places impossible to draw any sharp line -of 

 separation between the glacier and the beds on which it rests. We 

 found, in fact, a gradual passage from ice charged with morainic 

 material into subgiacial morainic material which has been saturated 

 with water and frozen hard. We soon learnt that the idea of a 

 ' ground-moraine ' as a moraine formed beneath the sole of a glacier 

 was not in accordance .with the facts of Spitsbergen geology, for 

 most of the glaciers there have no soles. But we learnt to regard 

 the whole of the debris-bearing layers of the glacier as the repre- 

 sentative of the ground-moraine, for they act as it was supposed 

 to do. 



How the lower layer of the glacier becomes charged with debris 

 was shown most clearly by sections at several of the glaciers which 

 we examined, especially the Ivory Glacier, near Agardh Bay, and 

 Booming Glacier at the head of Advent Dale. The photographs 

 reproduced in PI. XY, fig. 2, & PI. XYI, figs. 1 & 2, as also the 

 diagram on p. 204, illustrate the main stages in the process. As a 

 glacier moves most quickly where it meets with least resistance, 

 the upper layers flow forward more rapidly than those near the 

 base. Accordingly, in a glacier which ends with a cliff-like face, 

 the upper layers ride forward over those below, until they overhang 

 ats a projecting cornice. As the cornice is pushed still farther 

 forward, masses of it break away and fall to the foot of the cliff. 

 There they accumulate as an ice-talus bank, which in time becomes 

 so large as to check the advance of the lower layers of the glacier. 

 If the talus-bank is too heavy to be pushed forward, the glacier 

 is forced to ride over it. The ice, therefore, that was originally the 

 uppermost layer of the glacier will then form the base, and be in 

 turn forced to rise upward over later talus -banks that accumulate 

 in front of it. As the process is continuous, the glacier advances 

 by an ' overrolling ' motion, the top layer falling to the bottom, 

 and then working upward over other fallen masses. 



The use of the word ' overrolling ' is open to the objection that 

 it suggests a very flexible or viscous character of the ice. Some 

 of the most conspicuous features of these glaciers, such as their 

 false-bedding, their differential flow, their apparently rapid advance 

 and the readiness with which they expand into radial fans, suggest 

 that their ice is far more mobile than that of the Swiss glaciers. 

 But further acquaintance with the Spitsbergen glaciers led us to 

 change this opinion, especially in the case of the debris-laden strata of 

 ice. Closer examination showed that the false-bedding was due to 

 shearing. As the glacier presses against its talus-bar, bands of ice 

 are driven forward and ride across the lower layers along thrust- 

 planes. Thus PI. XYII, fig. 2, shows part of the upper face of 

 Booming Glacier : the face is broken into two by a narrow platform. 

 This ledge marks a band rich in debris, above which is one of 

 the thrust-planes. Smaller shearing-planes occur all through the 

 debris-laden portion of the glacier. Hence the mechanical move- 

 ment of the lower sections of the glacier appears to be due to a 



