296 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
judging from data secured from the Metropolitan Water Board, must be 
over 140 feet. As mentioned on page 281, this condition of things 
seems to be explainable only by tilting. — 
The Theory of Many Lakelets. 
In a recent paper on the sand plains of glacial Lake Charles, Clapp 
states his belief that the ice melted off from eastern Massachusetts in an 
extremely irregular fashion, so as to form a network of marginal lakelets, 
in which, here and there, the deltas were built. This theory seems to 
account for the prevalent discordance of altitude in neighboring deltas; 
and it is based on observations which Clapp sums up under four heads 
(c, 207). In reviewing these features in order, let us consider whether 
they are to be seen in Lake Sudbury or not. 
(1) The plains in the northern part of Lake Charles, according to 
Clapp, are distinctly marginal in their position, — grouped about the 
shores and islands of the extinct lake, as if they had been built not in 
an open water body but in single lakelets which formed along the sides 
of the basins while the ice still occupied the central portions. This 
marginal distribution of sand plains is not distinctly the rule in Lake 
Sudbury. Most of the large deltas are not far from the rim of the basin ; 
but since there was shallower water near the rim, it is natural that there 
the deltas were most quickly built and thus grew to largest size. 
(2) Clapp remarks that in several cases the ice-contact slope extends 
clear around the sand plain, except on that side where there is higher 
ground. In some cases there is an approach to this in Lake Sudbury — 
e. g. the large Wayland plain, the Walden plain, and the sand plain 
which lies a mile due west from South Lincoln station; but as a rule 
about half the border of a delta in ice-contact and the rest is either defi- 
nitely lobate or flattish, as where there was shallow water. 
(3) Typical deltas in Lake Charles are said to be associated with 
effluent eskers on the south, marking the course of subglacial or super- 
glacial outlets of the Jakelets. The supposed connection of a number of 
lakelets by these streams accounts for the fact that although no two 
deltas are at exactly the same level, the discordance of their altitudes is 
slight. In the Sudbury valley, feeding eskers are common. Effluent 
eskers are rare. One which runs southward from the southeastern 
corner of Walden Pond connects the main Walden plain with the plain 
at Baker’s Bridge. The plain one mile due west of South Lincoln 
station may have been enclosed by ice on all sides; for its southern 
