DEPOSITS FORMED AT THE ICE MARGIN. l'.l< 



the finest part of then- load dropped. The channels leading strong streams 

 to the margin might receive esker-like deposits of coarse gravels and sand 

 irregularly deposited ; the open spaces near the ice margin, containing waters 

 of gentler movement, would become choked with kame-like mounds of finer 

 sand; and where streams of either class ran from the ice to the water in 

 front of it, sand deltas must have grown with greater or less rapidity. Their 

 growth would be in three directions. They would grow forward by con- 

 tinued addition of oblique layers to their sloping front; as the front ad- 

 vanced they would slowly grow upward by the addition of essentially hori- 

 zontal layers, after the fashion of ordinary deltas, in order to maintain a 

 gentle surface slope from head to front; and as the ice melted aw r ay, the 

 space that it evacuated at the head of the delta would be more or less com- 

 pletely filled by a backward growth at that part. If the feeding streams came 

 from beneath the ice they must needs rise to flow over the delta surface to 

 its front, and hence the backward growth at the head must have been at 

 such points in the form of up-hill deposition. These three classes of deposits 

 may be called fore-set, top-set, and back-set beds, shown in fig. 1 ; and it is 



F~0 UND /^\~T [C?l^i . 



Figure 1.— Ideal Longitudinal Section of a Sand Plain. 



manifest that the ratio of fore-sets to back-sets must be the same as the ratio 

 of the forward growth of the delta to the backward melting of the ice. 



When re-arrangement of glacial drainage leads the feeding streams away 

 to some other outlet, and when later meltiug or elevations of the land allows 

 the marginal waters to drain away, the deltas previously formed stand up 

 somewhat above the adjacent surfaces ; the steep, concave outlines of the 

 head of a plain, with its feeding eskers, kames, and kettles, mark the irreg- 

 ular margin of the melting ice; and the convex lobes of the front of a plain 

 mark the growing front of the delta. 



Verification of the Hypothesis. — The general correspondence of the fore- 

 going deductions with the facts is a sufficient assurance that our search for 

 explanation is in the right direction ; but certain facts of structure need re- 

 examination in the light given by our theoretical suggestions. Is there any 

 direct indication that the front of the plain grew forward by down-hill depo- 

 sition, while the head grew backward by up-hill deposition? The fine cross- 

 bedding frequently characteristic of both the fore-set and back-set beds leaves 

 no doubt on this point, when its significance is clearly perceived ; and for 

 this a brief digression is needed. 



