294 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
have been formed by the action of a powerful stream that was the outlet 
for Lake Sudbury. Whatever water spilled over at Weston, down Cherry 
Brook, at South Lincoln, or at the head of Hobbs Brook, would have run 
into Stony Brook; and marks of scouring, such as boulder pavements 
and pot-holes, should be expected wherever the course of the stream was 
down a steep slope. 
Heading on the southeast side of the Hobbs Brook pass, Hobbs Brook 
flows south into Stony Brook, a large part of its course having been 
dammed up to form the Cambridge Reservoir. On the northwest side 
of this reservoir, about a mile north of Prospectville, near where the 
parkway joins Concord Avenue, is a small exposure of bare rock, rounded 
and hummocky, as if worn by a powerful stream. About a quarter of a 
mile below the reservoir, along the course of the brook, is a pavement of 
boulders about 100 feet wide and 300 feet long, —again suggesting the 
former occupancy of the course of Hobbs Brook by a powerful stream at 
a time when the pass above it served as a spill-way. 
Similar evidences might be expected below the Morseville pass, in the 
brook that drains Little’s Pond, but I have looked in vain for them. 
The brook that heads in Weston village shows no sign of scouring. 
Just where its course begins to be steep, a millpond has been formed, 
and the aspect of the ground is otherwise changed by artificial grading. 
Cherry Brook exhibits no boulder pavement. 
Direction and Rate of Tilt is questionable. 
Judging from the fact that 8 of the 11 accurately levelled 
deltas between Wayland and Concord fall nearly in line for a single 
water-plane when plotted with respect to a north-south line, as in Plate 
4, and that they do not harmonize so closely when plotted with re- 
spect to any other direction, it seems safe to say that the direction of 
maximum tilt is somewhere about due north and south. The rate of 
tilt measured along this slanting water-plane of the Cherry Brook stage 
is a little over 7 feet per mile, steeper than what might be expected 
from the evidence of Gilbert, Fairchild, and DeGeer. The other water- 
planes all lie parallel to this well-defined Cherry Brook plane, — that is 
to say, the same tilt is recorded by all. 
Over against the striking conformity of delta levels to the 7-feet-per- 
mile water-planes, there are a few details which suggest that the tilt is 
even steeper and the grouping consequently somewhat different from 
that just given. 
