GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 267 
more little valleys it will shut in, and the more little lakelets there will 
be. Since also it is probable that the cols at the heads of these little . 
valleys have different altitudes, the lakelets formed at this stage of ice- 
retreat will not only be many in number, but will have different levels. 
Thus in the diagram (Fig. 2) the ice standing with its front at A.. A, 
north of the divide, shuts in four lakelets at different levels, indicated 
by the figures 2, 3, 4,5. Lakelets 6 and c overflow across the main divide 
through cols which have altitudes 3 and 2 respectively. It may happen, 
however, that a lakelet finds its lowest outlet through a col in a second- 
ary divide into an adjacent lakelet or across a secondary divide directly 
at the ice-front. The latter is notably the case in the temporary ice- 
front lakes of western New York, described by Fairchild (a, b, ¢, d, e, f). 
In the diagram, lakelet d drains westward into c, through a col 4 in the 
secondary divide which separates these two lakelets ; also, lakelet a flows 
eastward into 0, across the dividing spur directly at the ice-front, at the 
level 5. Many small lakelets, then, at different levels, characterize the 
first stages of withdrawal of the ice from a divide. 
As the ice-front melts back, it will retreat beyond the junction of 
certain of the smaller headwater valleys, so that the separate lakelets 
originally shut in, in those valleys, will merge into a single lake. Thus, 
in the figure, when the ice has withdrawn to B. . B, lakelets } and ¢ will 
coalesce. As the two lakelets unite, the higher one will drain down to 
the level of the lower, leaving its old outlet high and dry. At the mo- 
ment of coalescence, the submerged area will of course be equal to the 
sum of the two original lakelets ; but as the level of the higher one falls, 
the submerged area is partly reduced in extent, in a measure depending 
on the amount by which the higher lakelet is lowered, and its shallow- 
ness. Thus in the figure, when at stage B.. B lakelets b and ¢ unite, 
the level of 6 will fall from 3 to 2, the old outlet at 3 being abandoned, 
and its shore-line will contract to fit the lower contour. Still, under 
ordinary conditions, the lake of coalescence will be larger than either of 
the two original lakelets, for it contains all of one and part of the other. 
If, however, the outlet of the lake of coalescence is below the level of the 
bottom of the higher lakelet, it will drain the lakelet dry, and the lake of 
coalescence will then be no larger than the original lakelet was at the 
time the two united. In the figure, the total disappearance of a lakelet, 
due to coalescence with a much lower one, is shown by lakelet a, which 
between stages a.. A and B..B falls from level 5 to assume the level 
of the lake of coalescence, which is 3, and later 2, draining dry, however, 
because the new level 2 is below its lakelet bottom. Early in the with- 
