GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 285 
groups, each group being composed of deltas whose lobe brows fall 
into a single tilted water-plane. In each group one should find, as he 
proceeds icewards, a steady increase of altitude of the brows, until he 
reaches a new water-plane in the vicinity of a lower outlet, where a 
sudden drop of altitude to the lower water-plane would be followed by 
a new group of ascending deltas. Such an arrangement may conven- 
iently be called a slanting step-system. 
Methods and Results of Levelling in Lake Sudbury. 
To determine whether the Lake Sudbury deltas lie in a slanting step- 
system or only in haphazard fashion, a good deal of detailed levelling 
was necessary. Through the kindness of the Division Engineers of the 
a 
> = b 
a 
Figure 5, 
Effect of tilting a step-system of three horizontal water-planes, (a) after the ice has with- 
drawn from the region, or (2) while the ice is retreating across the region. In the lat- 
ter case, the water-plane which is first abandoned is tilted more steeply than its 
successors. 
various railroads and of the Engineers of the Metropolitan Water Board, 
I secured bench-marks which in almost every case I could conveniently 
use as a base, —such as the top of the rail at a certain bridge, road- 
crossing, or turnout, or the capstone of a culvert along an aqueduct line. 
From such a starting-point, with a rodman, I ran levels over a circuit as 
short as possible, ranging from a few hundred feet to three miles, using 
in some cases a surveyor’s spirit-level, but more often a Locke hand- 
level, which proved to be a much more rapid means of levelling, and 
nearly as accurate. The probable error by this method was not over 
one or two feet per mile. On the map (Plate 5) all of these accurately 
determined levels and the lobe brows from which the average height has 
been computed are shown in black figures, while other altitudes deter- 
mined by aneroid or rough computation are shown in parenthesis. 
These last-mentioned levels involve an error of from 5 to 20 feet. All 
altitudes are referred to mean low water, Boston City base. 
