312 



PROF. W. MOKEIS DAVIS ON 



[Aug. 1909, 



they should join the main valley at accordant levels ; and the spurs 

 that slope into the main valley between the side-valleys should be 

 of normally subdued slope, if the main valley-floors are broad. 

 But the normal development of accordant junctions for a group of 

 side-valleys may be prevented by a highly specialized arrangement 

 of side-glaciers, which shall conspire to hold their ends at the side- 

 valley mouths, as already shown in fig. 12 (p. 310), and more clearly 

 in fig. 13 (below). Hence, in all such cases the floors of the group 



Fig. 13. — Diagram showing the origin of hanging lateral 

 valleys under protective glaciers. 



of hanging side-valleys must be the preserved parts of a previously 

 eroded mature valley-system, and as such must, irrespective of the 

 size of the side-valleys, hang at an altitude appropriate to a corre- 

 sponding part of the main valley-floor preserved above the next 

 upstream rock-step. Furthermore, (/) there can be no more 

 groups of hanging side-valleys than there are rock-steps in the 

 main valley-floor ; and all the members of a group of hanging side- 

 valleys must occur between two successive rock-steps in the main 

 valley-floor. On the other hand, (g) large or small lateral valleys, 

 not occupied to the mouth with side-glaciers, must enter the main 

 valley at accordant level with it, and yet they may be in close 

 neighbourhood with hanging lateral valleys. 



It should also be noted that (h) the long-maintained halt of a 

 group of side-glaciers at their valley-mouths, as required for the 

 production of hanging side-valleys, is a much more specialized 

 requirement of this theory than is an equally long-maintained halt 

 of the main glacier at some unspecified point in its larger valley 

 where a rock-step is to be made. The halt of the main glacier 

 requires only a long-maintained uniformity of glacial climate, and 

 while so long a halt is in itself certainly remarkable, it is not 

 complicated by having to stand in a definite relation to any non- 

 climatic element. The long halt of a group of side-glaciers at their 

 valley-mouths, however, requires not only the same long-continued 

 uniformity of climate, but the maintenance of the climate at just 

 such a condition as shall cause a number of independent side- 

 glaciers of unlike dimensions to end in a definite relation to a line 

 of an altogether different quality, namely the line of the main 

 valley to which the valleys of the side-glaciers are tributary. 

 When it is noted that the drainage-basins of neighbouring hanging 

 lateral valleys vary greatly in area and in altitude, it appears 

 extremely improbable that any glacial climate could cause the 



