338 HISTORICAL GEOLOGY 



The ice sheet could advance only when the rate of motion was 

 greater than the rate of melting of the ice front, and vice versa 

 in case of retreat. Thus it is true, though seemingly paradoxical, 

 to assert that the ice was constantly flowing southward even 

 while the ice front was retreating northward. Whenever, during 

 the great general retreat, the ice front remained stationary because 

 the forward motion was just counterbalanced by the melting, all 

 the ice reaching the margin of the glacier dropped its load to build 

 up a terminal moraine. Such a moraine is a more or less distinct 

 ridge of low hills and depressions consisting of very heterogeneous 

 and generally unstratified debris, though at times waters emerging 

 from the ice caused stratification. The depressions are usually 

 called kettle-holes. The so-called great terminal moraine marks 

 the southernmost limit of the ice sheet, and is wonderfully well 

 shown by the ridge of low, irregular hills extending the whole 

 length of Long Island. It is also more or less clearly traceable 

 across the United States, where it marks the southernmost limit of 

 glaciation. Terminal moraines farther northward are generally 

 not so long or sharply defined, though many have been located 

 and described. These are either terminal moraines found at the 

 southernmost limits of ice sheets which did not extend as far 

 south as earlier sheets, or recessional moraines formed during each 

 considerable pause of a waning or northward retreating ice sheet. 



When the ice front paused for a considerable time upon a rather 

 flat surface, the debris-laden streams emerging from the ice formed 

 what is called an overwash plain by depositing layers of sediment 

 over the flat surface. An excellent illustration of such an overwash 

 plain is all of that part of Long Island lying just south of the great 

 terminal moraine, and known as the Jamaica plain toward the west. 



When the ice front extended across a more rugged country, 

 with valleys sloping away from the ice, the large glacial streams, 

 heavy laden with debris, caused more or less deposition of materials 

 on the valley bottoms often for many miles beyond the ice front. 

 Such deposits, known as valley trains, are especially well developed 

 along many of the larger south-flowing streams of the glaciated 

 area. 



Glacial boulders (erratics) have alread}^ been referred to. They 

 are simply blocks of rock or boulders from the top of the ice or 

 within it which have been left strewn over the country as a result 

 of the melting of the ice. They vary in size from small pebbles to 



