254 DISCUSSIONS IN CLIMATOLOGY. 



particles will cause the mass to widen out laterally, and 

 of course as the mass widens out it will grow thinner 

 and thinner if it does not receive fresh acquisition on 

 its surface. In the case of a glacier lying in a valley, 

 motion, however, will only take place in one direction. 

 The sides of the valley prevent the glacier from widen- 

 ing; and as gravitation opposes the motion of the ice 

 up, and favours its motion down the valley, the path of 

 least resistance to the pressure produced by regelation 

 will always be down the slope, and consequently in 

 this direction displacement will take place. Molecular 

 pressure will therefore produce motion in the same 

 direction as that of gravity. In other words, it will 

 tend to cause the glacier to descend the valley. 



The lateral expansion of the ice from internal 

 pressure explains in a clear and satisfactory manner 

 how rock-basins may be excavated by means of land- 

 ice. It also removes the difficulties which have been 

 felt in accounting for the ascent of ice up a steep slope. 

 The main difficulty besetting the theory of the excava- 

 tion of rock -basins by ice is to explain how the ice 

 after entering the basin manages to get out again — 

 how the ice at the bottom is made to ascend the sloping 

 sides of the basin. Pressure acting from behind, it has 

 been argued by some, will simply cause the ice lying 

 above the level of the basin to move forward over the 

 surface of the mass filling it. This conclusion is, how- 

 ever, incorrect. The ice filling the basin and the 

 glacier overlying it are united in one solid mass, so 

 that the latter cannot move over the former without 

 shearing ; and although the resistance to motion offered 

 by the sloping sides of the basin may be much greater 

 than the resistance to shear, still the ice will be slowly 

 dragged out of the basin. However, in order to obviate 

 this objection to which I refer, the advocates of the 



