Observations on the Recent Glacial Drift of the Alps. 259 



In the majority of cases, Alpine glaciers occupy narrow steep 

 valleys which afford them little opportunity to deploy as they un- 

 doubtedly would in more open ground, where they might present 

 phenomena analogous to those of continental or arctic glaciers ; 

 but in some cases, they terminate, or have recently done so, in 

 broader and less sloping portions of their channels, and thus 

 furnish some very valuable hints as to the probable action of 

 broad glaciers on less sloping floors. 



Alpine glaciers derive the material of their deposits from two 

 general sources, and their debris is correspondingly divided into 

 two general classes, 1st, that which falls upon them from above, 

 and 2d, that which they abrade from the rocks over which, or 

 against which they move. The first class is borne passively on the 

 ice stream, while the second is pushed or rolled along heneaih it. 

 The first is due to the accident of the glacier's position, the second 

 is the direct result of its own action. The first class is only pres- 

 ent when the glacier originates among towering peaks or flows 

 along precipitous slopes ; the latter presumably is always present. 

 At the edges of the glacier the two classes often mingle, and un- 

 doubtedly some of the surface debris finds its way to the bottom 

 through crevasses and moulins, so that the material carried along 

 beneath the glacier is greater than it would be but for the surface 

 burden; but, for the purposes of our study, this is unimportant. 

 It is imperative, however, that we distinguish between the super- 

 ficial and basal debris, as the former can have little or no repre- 

 sentative in so plane a region as that covered by our drift, and 

 can therefore throw no light upon its origin. This distinction is 

 very easily made, for the most part, in the case of the Alpine 

 glaciers mentioned; for the surface material is almost wholly un- 

 worn and angular, while the basal portion is usually abraded and 

 rounded in greater or less measure. 



The surface material forms in lines along the sides of the ice 

 stream, where it has fallen from above, constituting lateral mo- 

 raines; and where two streams unite, two of these lateral moraines 

 are brought together and form a line along the middle of the 

 joint stream, constituting a medial moraine. 



To the rock rubbish borne along beneath the glacier, the term 

 ground moraine, or moraine prof onde^ is applied. 



