TEAl^fSPORTATION BY FLOATING ICE. 21 



and a mass of it is called a moraine. Moraines are lateral, medial, basal, 

 or terminal, according to their situation with respect to the glacier. Moraine 

 stuff falling into crevasses is carried forward by the ice, and in this trans- 

 portation the stones often scratch one another or the solid rock. Moraine 

 stuff beneath the ice is known as a moraine profonde, or ground moraine. 

 In ordinary valley glaciers, such as those of the Alps, the ground moraine 

 forms but a small proportion of the moraine stuff. But where the whole 

 country is covered by ice, and no cliffs project above it, the whole of the 

 moraine stuff is beneath the ice or distributed through it. Most of 

 the mielting of the ice takes place at the surface. The melting waters then 

 run along on the surface until they reach a deep crevasse, down which they 

 pour, and make their escape by tunnels beneath the glacier. In this way 

 each glacier is drained by one or more subglacial streams. The waters of 

 these streams are usually muddy and heavily loaded with the finer detritus 

 resulting from the grinding of moraine fragments against one another and 

 against the underlying rock. In its impetuous course the subglacial stream 

 erodes its bed, sand-carves the rock, and forms potholes, like other swift 

 streams. During the winter, when the supply of water is diminishing, the 

 lower portions of the tunnels of the subglacial streams become clogged 

 with rounded sand and gravel. When the ice is thick, it is able to push 

 this gravel onward and finally deposit it as a part of the terminal moraine, 

 but a thin glacier will flow over its subglacial sediments without disturbing 

 even the lines of stratification. 



The general nature of the work done by glaciers, as stated in this brief 

 outline, has been established by the observations of so many persons that 

 it is here assumed without attempt at proof. Some controverted points will 

 be discussed hereafter. 



TRANSPORTATION BY FLOATING ICE. 



Icebergs. — Thcsc are masses broken off from the fi out of a glacier. They 

 caiTy more or less moraine stuff', which sinks to the bottom of the sea or 

 lake when the ice melts. 



Icefloes. — These ai'e composed of the ice formed along the shores of the 

 sea or of a lake They often contain numbers of the stones and bowlders 

 of the beach, frozen fast in them. Other things being equal, ice floes are 

 thickest where the tide rises and falls. In the spring they first. melt nearest 



