122 A MANUAL OF TOPOGKAPHIC METHODS. 



Now, let this operation be extended farther. As a stream builds its 

 ridge higher it soon reaches a condition of instability and it then forsakes its 

 bed for an adjoining lower course. It builds this up and in turn abandons 

 it. So in time it builds up a dry delta, or, as it is called, a fan, made up of 

 a radiating group of abandoned ridges marking its former courses. 



Lake terraces are formed by the collection of material at the water's 

 edge. Whether brought down by gravity alone or transported by water, 

 its descent is checked on reaching the water and it accumulates at the 

 water's edge. 



GLACIAL DEPOSITION. 



The northern part of the United States was, in recent geologic times, 

 covered by a sheet of ice, a glacier of continental dimensions. Its bound- 

 aries, within the United States, included New England, New York, north- 

 ern Pennsylvania, Ohio, Indiana and Illinois, all of Michigan, Wisconsin, 

 Minnesota and the Dakotas, much of Iowa, and northeastern Montana, 

 The glacier had a southern movement, but this advance southward was, 

 on the whole, neutralized by the melting of the ice on the southern bor- 

 der. In cold seasons, the movement of the glacier gained on the power of 

 the sun's heat to melt it, and it advanced southward. In warm seasons, 

 it retreated northward. The action of this glacier in originating and modi- 

 fying topographic forms was twofold. It eroded and carried away material 

 and it deposited material. It is the latter result that is considered here. 



The material, consisting of bowlders, gravel, and sand borne by the 

 glacier was deposited as it melted, and consequently is most abundantly 

 distributed in the neighborhood of its southern boundary. Owing to the 

 recent character of the deposits,' they have been little eroded. Lakes, 

 swamps and waterfalls abound in the region in question. The terminal 

 moraines which mark the limits of the glacier consist of an irregular mass 

 of material, thrown down in the greatest confusion, with crooked winding 

 streams and sink holes. There is no symmetry or law in its disposition, 

 but it is made up of details, which bear no relation to its whole. On this 

 account it must be sketched piecemeal. The topographer must go all over 

 it, picking up each detail by itself, and necessarily the control must be 

 equally minute. 



