Vol. 65.] GI.A.CIAL EROSION" In'noRTH WAXES. 319 



valley directly as the volumes of the lateral and main glaciers. 

 Hence neighbouring lateral hanging valleys might open at different 

 heights above the main valley-floor ; and all the lateral valleys of 

 a well-glaciated main valley should mouth or ' hang ' at one height 

 or another above the main valley-floor, (t) Non-hanging junctions 

 should be expected only where two valleys occupied by glaciers of 

 equal size come together ; and here each valley might be less deep 

 than the united valley farther down-stream, (j) The valley-floor 

 rock-steps should not correspond in number, in altitude or in spacing, 

 along neighbouring valley-systems. 



(k) After the final disappearance of the ice from its chaunels, 

 water-streams will cascade in full view on the ungraded rock-steps 

 in the channel-beds and on the hanging valley-mouths. (I) The 

 hollows of the channel-beds, occupied by lakes, will be seen to in- 

 dicate only the excess of erosion in one part of the glacial channel 

 ■over the erosion in the part next down-stream, (m) Similarly, 

 the ledges and knobs of rock that may rise here and there 

 in the less matured parts of the glacial channels will not be 

 regarded as surviving elements of pre-Glacial form and hence as 

 witnesses to the inefficiency of glacial erosion, but remnants of 

 much larger masses not completely worn down to a smooth channel 

 form. 



One difficulty in the theory here considered is our ignorance 

 regarding its fundamental postulate, namely, that glaciers are 

 destructive agents. It has been already noted that glacial erosion 

 ■can be observed only at the border and ends of a glacier ; whatever 

 goes on under the neve of the upper reservoir and under the heavy 

 ice-stream in the great channel cannot be seen. It is, however, 

 generally agreed that ordinary weathering can take little or no 

 part in sub-glacial erosion, and that even freezing and melting, 

 however important these changes are around the head of a neve- 

 reservoir, may be of less efficiency in detaching rock-masses at the 

 bottom of glaciers, because of the inferred constancy of temperature 

 beneath the ice ; but as to this, it has been suggested that changes 

 of pressure, just sufficient to cause melting and re-freezing, might 

 be as effective in opening joints and crevices as changes of tem- 

 perature would be ; and it may be added that scouring and plucking, 

 such as are observable under glacier-sides and ends, may be still 

 more effective under the heavy body of neve or of ice, in view of 

 the great pressure there prevailing. 



Moreover, it must be borne in mind that the rate of glacial 

 erosion, in whatever way it may be accomplished, need not be very 

 rapid, the only requirement in this respect being, as already noted, 

 that it shall be significantly more rapid than normal erosion ; and 

 furthermore, in any case the cwms and valleys of glaciated moun- 

 tains are the product of erosion of some kind, and in the absence 

 of direct evidence to the contrary, it is a priori no more unreason- 

 able to ascribe the erosion to large, heavy, sluggish ice-streams in 

 association with weatheiing on so much of the surface as rises above 



