616 THE JOURNAL OF GEOLOGY. 



over roughnesses of bed in such relations that englacial material 

 there acquired may subsequently reach the surface of the ice in 

 such a position as to be added to the lateral moraine material 

 accumulated from above. This however is probably not an abun- 

 dant source of lateral moraine material. 



From the foregoing it will be seen that lateral moraine 

 material belongs to two distinct classes. The first class, that 

 descending from the mountain slopes above the surface of the 

 ice, is strictly superglacial. This at least is true of all that por- 

 tion of it which reaches the surface of the ice below the zone of 

 accumulation. That which reaches it within the zone of accu- 

 mulation may be temporarily buried by the snow of successive 

 winters, until the ice which carries it passes from the zone of 

 accumulation to the zone of wastage. While such material may 

 have a brief englacial history, it still belongs, to all intents and 

 purposes, with the first class of superglacial material. The second 

 class, that which was taken from the summits of prominences 

 which reached well up into the body of the ice, was at first sub- 

 glacial, but quickly became englacial as the ice closed together 

 beyond the prominence which gave rise to it. After a longer or 

 shorter englacial journey, it became superglacial, as the result of 

 surface ablation. Since the subglacial journey of such material 

 was exceedingly brief in most cases, and the englacial and super- 

 glacial journeys doubtless much longer, it may be called englacial- 

 superglacial drift. Englacial-superglacial bowlders should differ 

 from bowlders which have been superglacial throughout their 

 history, in that the former should show more evidence of wear. 

 This might be inflicted both during their brief subglacial journey, 

 and during their more protracted englacial history. 



Medial moraines .—Wherever two mountain glaciers bearing- 

 lateral moraines unite, the lateral moraines belonging to the 

 two margins which coalesce give rise to a medial moraine. Such 

 a medial moraine is no more than two lateral moraines joined 

 together. The derivation of the medial moraine material is 

 therefore essentially the same as the derivation of the lateral 

 moraine material. 



