bo 
72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
pronounced that ohe may fairly assume that the back slopes of these 
deltas represent portions of the same ice-wall; and when these back slope 
lines, extended across the basin, connect with morainic patches on its 
rim, crossing the rim at points that allow an overflow-outlet at the level 
of the deltas under consideration, the inference of a continuous ice-dam 
along the line thus traced seems quite safe. If, on the other hand, 
in going northward one crosses rapidly a succession of deltas whose 
back slopes extend east and west, the inference is strong that these 
deltas were not formed at the same time, but successively, as the ice 
withdrew. 
Secondly, the levels of the lobe brows of the sand plains give an 
accurate measure of the levels of the temporary lakes in which they were 
built. The lobe brow measures the water-plane for the stage during 
which the lobe was built. A group of deltas of just the same altitude 
in a single basin probably belong to a single lake, whose level re- 
mained at that height at least so long as these deltas were being formed. 
The possibility of there having been several neighboring lakelets at 
identical levels should, however, be recognized and tested by an examina- 
tion of the topographic conditions and ice-border evidence. If two deltas 
are at different levels, they probably represent different lakes, or different 
stages in the same lake, — higher and lower water-planes, — unless there 
have been post-movements of the land, such as tilting. The question of 
the arrangement of sand deltas of different levels, and the light that it 
may shed on the history of a lake, will be considered later (p. 273). 
Certain features peculiar to individual sand plains are significant in 
showing how the ice retreated, its rate of melting, changes in lake-level, 
etc. That sand plains were built very rapidly — much faster than the ice 
melted back —is shown, for instance, by the extremely small ratio of 
backset beds to topset and foreset beds, as first pointed out by Professor 
Davis (a, 199). Kettle-holes, so commonly marking the places where 
ice-blocks were enclosed, or even buried in deposits of gravel and sand, 
have likewise been recognized as evidence that deposition was much 
faster than ice-melting. 
In certain cases, sand plains show a twofold level, having two sets of 
lobes, a higher and a lower set. These may fairly be taken to indicate 
a change of level of the waters while the delta was being built. The 
often observed fact that topset beds truncate the upper parts of foreset 
beds is believed by Professor Woodworth and others to indicate a change 
of level, giving a true erosion unconformity. 
