i6o 



PHYSICAL GEOLOGY 



while it advances 500 feet, it is evident that comparatively little debris 

 will be left at any one spot, and no conspicuous hills or ridges will be 

 formed. (2) The velocity of the glacier, (3) the quantity of material 

 transported by it (p. 154), and (4) the amount carried away by the 

 stream which flowed from its end are also determining factors in the 

 size of terminal moraines. They sometimes reach a height of several 

 hundred feet, but heights of 100 or 200 feet are more common. If 

 the front of a waning glacier halts for considerable periods at different 

 points, a series of terminal moraines (also called recessional moraines) 

 will be left. 



The material of terminal moraines usually consists of a heterogene- 

 ous mixture of large and small pebbles and bowlders of different kinds, 



Fig. 145. — Moraine near Dansville, New York. (Photo. H. L. Fairchild.) 



embedded in clay and sand. Occasional patches of stratified sand and 

 gravel from the water of the melting ice also occur. All the glacial 

 debris is called drift, the unstratified is called till or bowlder clay, and 

 the stratified (sorted and laid down in water) is called stratified drift. 

 On glaciers which move between precipitous walls supplying great 

 quantities of talus, the lateral moraines will be large; and upon the 

 disappearance of the ice, especially if the retreat be slow, a high 

 ridge of unstratified drift will be left on each side of the valley. 

 Some of these are a thousand feet or more in height. The terminal 

 moraine of such a glacier may be comparatively insignificant. Ter- 

 minal moraines are breached by streams and are sometimes entirely 

 removed by them. Sometimes, however, the moraine constitutes an 

 effective dam for many years, behind which a picturesque lake lies. 



