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the point B until the outlet has been lowered. Now if 

 the mass of the glacier be reduced, say to a quarter of its 

 former size, the mass so reduced can at this stage, only 

 transport the mass of a small cross-section through its 

 corresponding centre of gravity. That is, in our ideal case, 

 the new cross-section will, from considerations of symmetry, 

 be somewhat as shown by the profile ABC, the vertical 

 and longitudinal measures of glacial strength having shrunk 

 considerably upon reduction in volume. But the centre of 

 gravity in this case for the cross-section of the basin 

 excavated below the associated base-levels will be at O', a 

 point vertically above O. That is the reduced glacier can- 

 not corrade below the profile ABC and part of the ground 

 moraine, or load, is deposited. The glacier, at this stage, 

 is said to be overriding its moraine. But increase the 

 volume again, and the centre of gravity of the cross-section 

 possible below base level descends and the moraine is 

 cleared away. Similarly one might consider the centre 

 of gravity of any cross-section of a cutting curve in a glacial 

 channel and consider the weight of the glacier in the cross- 

 section as being gathered at the centre of gravity and 

 being transported down stream. Then upon a reduction 

 of glacial volume the centre of gravity is withdrawn to 

 a greater distance from the old limits of the cutting curve, 

 (that is the lateral measure of stream strength is diminished) 

 and the old cutting curve at this stage becomes occupied by 

 moraines which descend in terrace form from the cutting 

 curve to the channel centre as the glacial diminution is 

 continued. 



Or again, the question might be approached from a 

 consideration of kinetic energy. 1 



Nevertheless, although the same geologists would have 

 been the first to admit the validity of these dynamical 



' Andrews, E. C, " Corrasion of Gravity Streams," Appendix in. 



