STUDIES FOR STUDENTS. 617 



It sometimes happens that there are considerable hills in a 

 glacier's bed which the ice is unable to surmount, and it flows 

 around them. Temporarily, an ice stream may be said to be 

 divided into two by each prominence of this sort. These two 

 streams unite below the rock prominence. If the rock promi- 

 nence be high and steep, it may yield earthy and stony material 

 to the surface of the ice on either hand, just as the slopes above 

 an ordinary alpine glacier give rise to lateral moraine material. 

 Such a boss of rock protruding through the ice may give rise to 

 two lateral moraines, one on either side. These preserve their 

 distinctness around the projecting hill. But where the ice 

 streams on either hand coalesce below the prominence, the two 

 lateral moraines unite and become a medial moraine. In such a 

 case the ice passes over the lower slopes of the prominence which 

 it surrounds. When the ice has closed round such a prominence, 

 healing the wound which it made, some of the material plucked 

 from the slopes of the hill below the surface of the ice, will be 

 found in the ice above its base, that is in englacial position. 

 Some of it may be very near the upper surface of the ice, and 

 some of it will be at lower levels. As the ice passes on down 

 the valley, its surface is subjected to rapid melting. When the 

 uppermost twenty feet of the ice have been melted, all the 

 material which was englacial in this part of the ice will have 

 arrived at the surface, not because it was carried up to the sur- 

 face, but because the surface was carried down to it. It will be 

 seen that the greater the surface melting - , the greater the amount 

 of englacial material which will become superglacial, as the 

 result of ablation. This superglacial material with an englacial 

 history will first make its appearance along the line of the medial 

 moraine which has been formed by the union of the two lateral 

 moraines, since it is the ice along this line which, at the outset, 

 carries englacial material nearest the surface. With surface 

 melting, therefore, the medial moraine composed of superglacial 

 drift and formed by the union of the two lateral moraines, will be 

 augmented by the addition of englacial-superglacial drift. The 

 effect will be to widen the medial moraine, as well as to increase 



