GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 287 
south of the railroad (161 feet), and one north (163 feet). The two 
deltas at South Lincoln, halfway between the Wayland plain and the | 
Walden plain, had been found by aneroid readings to be of intermediate 
altitude, somewhere between 170 and 190 feet. Plotting the three 
accurately measured plains in cross-section, I found that they fell into a 
single straight line. Then, on the assumption that all the plains between 
Wayland and Concord kad originally stood in the same horizontal plane, 
and that this plane had since been tilted up on the north, I plotted the 
position of the two South Lincoln plains, and found that in order to he 
on this tilted water-plane the southern one should have lobes at 177 feet 
and the northern one at 180 feet. Hand-level surveys, made a few days 
later, fulfilled the prediction; for four lobes measured on the south 
plain averaged 179 feet, and three on the north plain stood at about 
182 feet. Inasmuch as the circuit for this levelling, from Lincoln station 
to the plains and back, was a very long one — three miles — involving 
a probable error of three feet, the results in this case were fully as satis- 
factory as could have been expected. The water-plane thus determined 
slants southward at the rate of about seven feet per mile. 
Having thus gained faith in the tilting hypothesis from the element 
of fulfilled prediction fn levelling, I continued to collect data, and found 
that in nearly every case the measurements of lobes fell on slanting 
water-planes which passed through one or the other of the passes in the 
rim of the basin. The final result of this grouping of deltas is shown in 
Plate 4, where all of the measured lobes are plotted to scale along a 
north-south line, and the slanting water-planes drawn through them. It 
will be seen that the Cherry Brook water-plane is determined by lobes 
measured on nine separate deltas. Between Wayland and Concord 
Junction all the deltas measured, except two, lie close to this single 
water-plane, as does also the Cherry Brook pass itself. The other water- 
planes are not so well determined ; but they explain all the features 
save a few which will be discussed later. 
On the basis of the Cherry Brook plane, then, there seem to be five 
parallel water-planes marked by the lobes of deltas in Lake Sudbury : — 
(a) The water-plane of confluence between Lakes Charles and Sudbury, 
ranging from 150 to 191 feet (sand plain at Waban Lake to two plains cut 
by the Sudbury Aqueduct south of Wayland). (6) The Morseville water- 
plane, marking the stage when Lake Sudbury drained down into Lake 
Shawmut through the Morseville pass. This stage is shown by lobes of 
the plains about Saxonville and Wayland. (c) The Weston water-plane, 
marking a stage of short duration, brought about by the uncovering of 
