JRhythmic AccamuJatioii of Moraines. — Ujyharit. 415 
steepness of the ice ^lope, were efficient in the contigi^ous 
higher zone of the ice-sheet, including the upper part of the 
englacial drift and the clear ice of greater hights, to bear 
much of that drift forward to become superglacially amassed 
at the inward limit of the drift-enveloped ice. In this upper 
moraine, far travelled boulders would be plentiful, and some- 
times they would be almost unmixed with any boulders or fin- 
er drift of local or near derivation, as Chamberlin describes 
certain remarkable boulder belts in Ohio, Indiana, and Illi- 
nois.* Where the ice enclosed much drift, even these upper 
superglacial accumulations, ten to fifty miles back from the 
edge of the ice beneath the drift, may have attained a consid- 
erable depth, perhaps forming hills 100 feet or more in hight, 
before the deepening ablation exposed the englacial drift far- 
ther back, so that the next succeeding morainal line would be- 
gin to be established. 
Until such a later and higher moraine began to be formed, 
the basal glacial currents probably acted also very efficiently 
to carry forward much of the lower englacial drift and to heap 
it subglacially or englacially beneath the upper moraine. With 
the beginning of another and higher surface moraine, the belt 
of accumulation by the basal currents would be shifted in- 
ward to coincide nearly with the new superglacial accumula- 
tion. 
In the outermost zone of the waning ice-sheet, movement 
would be feeble or would whoU}^ cease, as Russell suggests, be- 
cause of the previous amassing of much englacial drift there. 
The basal currents farther back must accordingly pass up- 
ward, carrying some drift to the surface. During a long time 
the chief growth of a moraine belt took place, according to my 
view, at a distance of several or many miles back from the 
edge of the ice beneath the drift. When, in the gradual prog- 
ress of ablation, a new and inner moraine began to be amassed, 
the outer one soon ceased to receive additions, or they were 
comparatively insignificant, on account of the slackened cur- 
rents or stagnation of the outer ice zone. Finally, the com- 
bined superglacial and englacial or subglacial morainic ac- 
cumulations, bj'' the melting of the ice between them, became 
strictly a marginal or terminal moraine. 
*Jouinal of Geology, vol. i, pp. 52(50, Jan. -Feb., 1893. 
