STRATIFIED DRIFT . 955 



likely to become continuous by lateral growth. Such border 

 plains of stratified drift differ from valley trains particularly 

 (l) in being much less elongate in the direction of the drain- 

 age ; (2) in being much more elongate parallel to the margin of 

 the ice; and (3) in not being confined to valleys. Such plains 

 stood an especially good chance of development where the edge 

 of the ice remained constant for a considerable period of time, 

 for it was under such conditions that the issuing waters had 

 opportunity to do much work. 



Thus arose the type of stratified drift variously known as 

 ovcnvash plai?is, morainic plahis^ and inorai?iic aprons. These over- 

 wash plains are sometimes found with a width of several miles. 

 Like the valley trains, they are topographically and genetically 

 distinct, and their relations are well known. They have been 

 abundantly described in the literature of glacial geology, and it 

 is, therefore, not needful that more be said concerning them at 

 this point. ^ 



Overwash plains may sometimes depart from planeness by 

 taking on some measure of undulation, of the sag and swell 

 (kame) type, especially near their iceward edges. The same is 

 often true of the heads of valley trains. The heads of valley 

 trains and the inner edges of overwash plains, it is to be noted, 

 occupy the general position in which kames are likely to be 

 formed, and the undulations which often affect these parts of the 

 trains and plains, respectively, are probably to be attributed to 

 the influence of the ice itself. Valley trains and overwash plains, 

 therefore, at their upper ends and edges respectively, may take 

 on some of the features of kames. Indeed, either may head in 

 a kame area.^ 



Occasionally a morainic apron, or stratified drift in the gen- 

 eral position of a moraine apron, is affected by numerous sags 

 without corresponding elevations. This topographic type has 



' This type is described in the following places, among many others : 

 Chamberlin, 3d Ann. Report U. S. Geol. Surv., p. 303 ; Journal of Geology, 



Vol. II, p. 533. Salisbury, Ann. Report of the State Geologist of New Jersey, 1892. 



p. 97. 



- Salisbury, Ann. Report of State Geologist of New Jersey, 1892, p. 94. 



