GLACIAL DEPOSITS 34:3 



places than others. Indeed, it thickens and thins out very 

 irregularly and in many places fails entirely either through 

 having never been deposited, as over many a rocky hillside in 

 New England, or through having been removed by running 

 water. Moreover, there are found in certain parts of the drift- 

 covered areas rounded hills of very symmetrical form, composed 

 of material identical with the till, but which must have been 

 deposited under slightly different conditions. These range in 

 height up to 200 or 300 feet, though rarely more than half that 

 amount. Such forms are known as drumlins. 



The terminal moraines represent those portions of the drift 

 which gathered near the edge of the ice sheet in the form of 

 submarginal accumulations, to be left as broad belts or ridges 

 of sand and gravel on its retreat. Such with reference to 

 their position to the margin of the ice are known also as 

 terminal, marginal, or frontal moraines. The materials of which 

 they are composed represent (1) that which accumulated be- 

 neath the edge of the ice while it was practically stationary for 

 a considerable length of time; (2) that dumped from the 

 surface at its margin; and (3) that pushed up by the ice sheet, 

 in front of itself during its forward movement. Such ridges 

 are not sharp as a rule, but broad and low, it may be from a 

 fraction of one to several miles in width. Unlike the subgla- 

 cial drift, — the till, — the materials are but loosely consolidated, 

 and but a small part, if any, of the boulders show the scarred 

 and abraded surfaces so characteristic of those of the till proper. 



The frontal moraine, occupying the southern and western 

 margin of the glaciated area, forms one of the most striking 

 and unique geological bodies. Composed of materials of a 

 most heterogeneous nature, ever varying, and limited in range 

 of variation only by the lithological character of the rocks to 

 the northward and eastward; in all degrees of coarseness and 

 fineness, from boulders of many tons' weight to particles too 

 small to be visible to the unaided eye, only obscurely and some- 

 times scarcely at all stratified excepting where subsequently 

 modified by running water; in the form of broad low hillocks, 

 domes, and ridges, — the moraine sweeps in an interrupted, sin- 

 uous belt from eastern Massachusetts to North Dakota and over 

 400 miles into British America, having a length, in all its wind- 

 ings and turnings, of not less than 3000 miles. 



The water arising from the melting ice sheet flowed off, in 



