372 GEOLOGY. 



Valley trains often sustain significant relations to recer sional moraines 

 as suggested by Fig. 503. 



Where the water escaping from the ice spread over a plain instead 

 of being concentrated in valleys, the deposits took on a form more 

 like that of alluvial fans. By union, these fans often became exten- 

 sive, and are known as outwash plains, overwash plains, moraine plains, 

 frontal aprons, etc. They differ from valley trains much as alluvial 

 fans differ from flood-plain deposits. 



When the water which issued from the ice entered standing water 

 it tended to develop deltas. Where the edge of the ice was long sta- 

 tionary, the deltas often attained great size. They sometimes merged 

 laterally as alluvial fans do, giving rise to compound deltas, or 



Fig. 503. — Diagram to illustrate the relations of imperfect valley trains to recessional 

 moraines. The heads of the several trains are at 1, 2, 3, and 4. Examples of 

 this relation are common in northern New Jersey. 



subaqueous outwash plains. 1 Many such deltas are known about 

 extinct lakes in the glaciated area of the United States, and about 

 the borders of existing lakes, the levels of which have been lowered. 

 The iceward edges of the deltas, like the iceward edges of outwash 

 plains and valley trains, were sometimes in contact with the ice, and 

 took on a kame-like phase. Deltas were also built into the sea at 

 some points. 2 



Many of the valley trains, outwash plains, and deltas which developed 

 beyond the edge of the later, and especially the last ice-sheet during 

 the time of its maximum advance or during its retreat, are still well 

 enough preserved to be readily identified, but they have little repre- 

 sentation among the deposits left by the earlier sheets of ice. If they 

 were well developed in the earlier glacial epochs, as they doubtless 

 were in some cases, but apparently not in others, they have been largely 

 removed by subsequent erosion. Valley trains, outwash plains, deltas, 

 etc., developed during the advancing stage of an ice-sheet were over- 



1 The deltas about the extinct lake Passaic are an illustration. Ann. Rept State 

 Geol. of N. J., 1893, and Glacial Geol. of N. J. 



2 Stone, Mono. XXXIV, U. S. Geol. Surv., p. 371. 



