GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 2589 
bury Aqueduct were being built ; for these have lobes at two levels 
which apparently fall into a water-plane with the Morseville pass and 
with a set of lobes on the Saxonville delta west of Lake Cochituate. 
The Saxonville plain deserves special mention, for it has lobes at sev- 
eral distinctly different levels. On the northwest side, where these lobes 
are best seen, the plain has clearly been built outward towards the 
northwest from an ice-front at the northern end of Lake Cochituate. 
The occurrence of lobes in increasingly lower and lower sets on this 
single plain indicates that the water-level fell while the delta was being 
built, possibly in successive stages. The way these lobes fall on the 
probable water-planes of Lake Sudbury is seen in Plate 4. Those of 
the highest set, at about 185 feet, are irregular, and have been measured 
only approximately. They seem to represent the stage of confluence. 
Below them, at the northwest corner of the plain, are two or three well- 
formed lobes, at 161-165 feet, which have a peculiar and very striking 
surface of cobblestones, as if all the finer sand had been washed down 
from them after the water-level fell, These lobes seem to fall on the 
Weston water-plane (the next one to be considered). Between two of 
them a lower lobe (at 149 feet) has been built ; and this falls on the 
Cherry Brook water-plane. Lowest of all is a terrace with several fine 
lobes, at 137 feet, which have a remarkably sandy composition. On the 
south side of the plain, southeast of Saxonville, lobes occur at 167-172 
feet, and a lower set at 152 feet. The 167-foot level, marked by at least 
two lobes, falls on the Morseville water-plane. The 172-foot lobe prob- 
ably represents the time when the waters were just falling to the level 
of the Morseville pass, or when the pass was first occupied by an outlet 
which cut down five feet before the 167-foot lobes were formed. In all 
cases, the higher lobes lead down to the lower lobes by smooth slopes. 
It seems probable, when these lobes are identified with the various 
water-planes, that the Saxonville sand plain was being built forward 
from a large ice-block in Lake Cochituate when the main part of the ice 
had retreated northward beyond Wayland, and perhaps even as far as 
Concord. 
Tae Weston Stace.— While Lake Sudbury stood at a level con- 
trolled by the Morseville pass, the ice-front receded from the Aqueduct 
plains towards the present position of the Massachusetts Central Rail- 
road. On reaching this position it must have uncovered the pass in the 
Sudbury-Charles divide that is occupied by the railroad. Although this 
pass now stands higher than the Morseville pass (over 160 feet, whereas 
the Morseville pass is 145 feet), its greater height seems to be due to 
VOL. XLII. — NO. 6 19 
