290 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
postglacial tilting, judging from the slant of the water-planes as marked 
by the lobes of deltas. As already mentioned, also, there is a pro- 
nounced drop in altitude of deltas just west of the pass in Wayland, 
from 190 feet to about 160 feet, the general altitude of the pass. 
Before the lowest part of the pass (north of the railroad, near the head 
of Cherry Brook) was opened, there was of course in all probability a 
partial evacuation of it, along the northern slope of the high ground 
south of Weston village. The well-formed sand plain cut by the railroad 
just east of East Sudbury station has lobes at 175 feet which may very 
likely have been built at this time. There are lobes also on both the 
north and south sides of the Saxonville delta, which fall into a tilted 
water-plane with this 175-foot delta. That there are so few deltas 
marking this stage is natural enough, because it is essentially a tempo- 
rary stage which was soon followed by renewed lowering of water-level 
as the ice withdrew so as to allow the water to pass eastward by lower 
paths, — first by the little forked brook half a mile northwest of Weston 
station, and later by Cherry Brook. These two probable lines of escape 
are so nearly at 2 common level that there has been no attempt made to 
differentiate two water-planes for them. 
THe Cuerry Brook Stace. —The deltas built while the Cherry 
Brook pass was a spill-way mark by far the best-determined water-plane. 
Lake Sudbury seems to have been maintained at this height from the 
time the Wayland deltas were formed until the ice-front had withdrawn 
eight miles to Concord ; for between Wayland and Concord the lobe 
altitudes of nearly all the deltas lie on a single slanting water-plane. 
The cross-section (Plate 4) shows how closely eight of these deltas fall on 
the water-plane. It is also remarkable that if the water-plane be ex- 
tended south from Wayland it just hits the 149-foot lobe of the Saxon- 
ville delta. This seems to show that the Saxonville delta was still being 
built out from the Lake Cochituate ice-block when the Cherry Brook 
pass was opened. 
The two large plains in North Sudbury seem to be the only deltas in 
the northern part of the basin whose lobes do not lie on the tilted 
Cherry Brook water-plane. Since there is no reason to doubt the accu- 
racy of measurement of their altitudes, they are thus far unexplained. 
The splendid big delta west of the railroad has lobes at 201 feet (14 to 
20 feet too high for the water-plane) ; the delta east of the railroad has 
lobes at 193 feet (6 feet too high). 
Tue Hopss Brook Stace. — It is possible that the pass at South 
Lincoln became an outlet to Lake Sudbury when the ice had retreated 
