MORAINES OF RECESSION 449 
deceived as to the amount of englacial drift by the highly 
deceptive appearance of pictures of débris-laden, melting ice- 
tongues. Chamberlin and Salisbury both draw attention 
repeatedly to the effect of the spreading of fine dirt so as to 
blacken the whole ice wall. When small streams of clear water 
wash this away, or the dark surface has been removed with a 
pick, comparatively clear ice is seen beneath, and yet some of 
these layers or thin lamine which they contain, may be the very 
ones that were supplying the blackening material. Large masses 
of englacial drift are rare and their forward motion is extremely 
slow, certainly much slower than that of the clean ice above, 
except where they occur, still more rarely, as lenses relatively 
high up in the glacier so as to have a considerable thickness of 
clear ice beneath them. In short, it seems to be shown that 
englacial drift is a far less voluminous constituent of ice-sheets 
than has been supposed by many. It keeps its importance, how- 
ever, as almost the only means of drift transportation by conti- 
nental glaciers like that which invaded the United States. But 
except under peculiar circumstances the amount in any given 
section of ice is almost insignificant. When all the circum- 
stances are taken into account it seems probable that the load 
of débris is as great or greater in the Greenland tongues than it 
was in the Laurentide lobes in Ohio and Michigan. The coast 
of Greenland is mountainous. The ice flows out among many 
nunataks and along the base of high cliffs, and no doubt over- 
rides many knobs and peaks and precipices from all of which 
it gathers material in such a way as favors its becoming super- 
glacial or englacial. Then, too, the exceedingly rough country 
is favorable to the production of high overthrusts by which the 
bottom layers with their débris may assume relatively elevated 
positions in the ice. 
When the ice-front was in Ohio or Michigan there was no 
chance for the formation of lateral moraines from cliff-fallings, 
nor of medial moraines from the detritus of nunataks. At that 
stage of glaciation the field of ice stretched away to the north- 
northeast without a break, and no land was exposed back of the 
