112 TJie Ahiericdii (ieohKjiaf. Auirust, l.s9:). 
t'e2t in th? de?plj ice-covered ( entr;il ]);>rtioii of (ireenlaucl. 
Other features esp?'cially noted are th ■ very distinct stratifi- 
cation of th-^ ice and its dilferi>ntial forward motion, ])roduc- 
in^ not only this stratilication but also sigmoid folds and 
overthrust faults, wh'r're the upp.'r layers move faster than the 
lower and th^s? in turn faster than the friction-hindered base. 
In just the sam? Avay, as I have shown in the foregoing pages, 
the accelerated currents of the waning ice-sheet during ths 
temperate Champlain epoch overrode each other in succession 
from the highest to the lowest on the moraine forming border, 
bearing a great amount of superglacial drift to the margin. 
If a mild temperate climate could bring to Greenland the con- 
ditions of the Champlain epoch, its thick ice-sheet in the in- 
terior under rapid ablation would fully illustrate, as the Mal- 
asi)ina glacier even now does in a cDnsiderable degree, the 
formation of the great series of morainic drift hills which 
mark stages in the reti'eat of the continental ice-sheet§. 
Marginal Moraines chiefly a Characteristic of the Cham- 
plain Epoch. 
From this discussion of the origin of marginal moraines, it 
will be seen that their accumulation belonged chiefl}" to the 
Champlain epoch of land depression, restored warmth, and 
mainly rapid glacial retreat, interrupted by times when the 
ice-sheet for several years or decades of years held a nearly 
stationary position. According to the supposition that two 
inches of daily summer ablation was approximately equalled 
by the glacial onflow, whenever the ablation was at a faster 
average rate, as three or four inches daily, the ice receded, de- 
positing the smoother till sheets between the hilly marginal 
moraine belts. 
During the stages of ice accumulation, up to the maximum 
of the glaeiation and to the lowan stage, I think that the ice- 
sheet eroded much drift on its central area and bore it for- 
ward in the basal quarter or third of the whole thickness of 
the ice, depositing much of it, however, as subglacial till with- 
in fifty miles, more or less, back from its front. When the 
final recession of the ice carried its border gradually back- 
ward ov.u* all its area, I believe that tlie process of subglacial 
drift deposition continued, forming the ground moraine or 
lower part of the till progressively as the ice border withdrew. 
