TNTRODUCTOEY : THE FORMATION OF MORAINES. 7 



the size of the glacier itself, and will also be greatly dependent on the cir- 

 cnmstances of the advance and retreat of the glacial mass. If the glacier be 

 very wide, and the lateral moraines small in proportion to the glacier itself, 

 the terminal moraine will be almost exclusively limited to the two sides 

 of the valley at the extremity of the glacier ; if the latter be narrow and 

 carrying a large quantity of detrital material, the lateral moraines will 

 coalesce and form a single terminal one, bai'ring the valley, except where 

 the stream of water which must issue from the extremity of every glacier 

 finds its way out. If the meteorological conditions are such that the glacier 

 during a long period neither diminishes nor lengthens, the terminal moraine 

 will become very large, and will gradually accumulate so as to cover a 

 considerable portion of the ice itself, as is the case with some of the large 

 glaciers of the Himalayan ranges. If, on the otlier hand, the glacier is, on 

 the whole, decidedly shrinking, the terminal moraine will bo less distinctly 

 marked, because the detrital material is always being dumped a little farther 

 up the valley. 



When the glacier is advancing the phenomena become somewhat com- 

 plicated. The ice, as it moves forward, shoves a portion of the detritus 

 of previous morainic accumulations before it, and rises over another portion, 

 which is then in a condition to be rapiflly acted on by the glacier stream, 

 and is soon converted into rounded Ijoulders, with the synchronous formation 

 of much gravel and sand of greater or less fineness. 



The phenomena of the lateral and terminal moraines are quite simple as 

 compared with those of tlie so-called "ground moraine," — that is, the detrital 

 material which lies under the glacier, and over which it moves. In regard 

 to the importance of this material there is a great diversity of opinion. 

 Before discussing the nature of the ground moraine, however, it will be 

 necessary to say something about the glacier as an originator of detrital 

 material, a subject in regard to which opinions differ most widely, some 

 geologists looking upon glaciers as mighty erosive agents, and ascribing to 

 them nearly all that has been done to give the earth's surface its present 

 form, while others take quite an opposite view. 



In regard to ordinary morainic material, there can be no doubt that the 

 glacier is simply the carrier, and not the originator. As to what goes on 

 above the sm-face of the ice there is no difficulty in getting at the facts; that 

 which is done under its concealing cover is mucli less satisfactorily made out. 

 And it must, in the first place, be called to mind that what takes place under 



