GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 295 
Two fairly well developed deltas at North Sudbury which should 
fall on the Cherry Brook water-plane if the other deltas are correctly 
grouped, stand 6 and 14 feet too high, respectively. These were 
carefully levelled, and there is no reason to doubt their altitudes, 193 
feet and 201 feet. Ice-damming might account for this, or it is possible 
that the deltas were built in a little high-level lakelet held in by an 
irregular ice-front against the western side of the valley, but there is 
nothing to support these explanations save the necessity for them. Tilt- 
ing could explain the extraordinary height just as well, — but not tilting 
at the rate of 7 feet per mile. 
The extraordinarily low lobes on the northern side of the big Saxon- 
ville delta, at 137 feet, have already been spoken of; but their altitude 
has not yet been explained. These five or six lobes are unusually well 
formed, and compel attention. On seeing them, my first idea was that 
they belonged to the Cherry Brook water-plane ; but that plane, if ex- 
tended southward from Wayland with the 7-feet-per-mile slant, hits the 
Saxonville delta at 149 feet, where there is one lobe, to be sure, but 
only one, and to all appearances a lobe that registers simply the transi- 
tion from one long-lived water-plane, here 163 feet, to another permanent 
water-plane at 137 feet. 
Curiously enough, the ‘“‘too low” lobes of Saxonville and the ‘too 
high” 201-foot delta of North Sudbury can be brought by plotting into 
a single slanting water-plane which passes through the two Wayland 
deltas and just above the Cherry Brook pass, as shown in Plate 3. The 
Cherry Brook water-plane thus drawn has a slant of over 11 feet per mile. 
The deltas in Lincoln and Concord falling below this steeply inclined 
water-plane might perhaps owe their level to an outlet through the 
South Lincoln pass. 
Drawing other 11-foot-per-mile planes for the higher Saxonville lobes, 
one gets some curious surprises in the way in which lobes fall on single 
water-planes ; but the difficulties in explaining the occurrence of these 
water-planes are so great, and the slant of them is so extraordinarily 
steep, that little faith can be placed in them. 
Plotting the delta levels with respect to an axis in any other direction 
seems to bring no better results. Further detailed work is needed, if 
they are all to be brought into perfect harmony with a single slanting 
step-system. 
While the 137-foot lobes are so troublesome, in being too low for their 
water-plane, they have a peculiar value. Their altitude is a little below 
that of the very lowest col in the divide, — the Morseville pass, — which, 
