286 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
A study of the map will bring out the following facts as to the general 
arrangement of deltas. 
In the southern part of the basin, about Lake Cochituate, the sand 
plains lie at about 170-180 feet, i.e., the highest lobes mark a water- 
plane at that altitude; but in more than one case there are lobes at 
different altitudes on a single sand plain, as if marking changes of level 
during the construction of the delta. This is remarkably true of the 
big sand plain just east of Saxonville, which has at least four different 
sets of lobes. 
The two sand plains north of the last, which are cut by the Sudbury 
Aqueduct, and the plains northwest of them in Sudbury, are higher, — 
about 190 feet—although these also have one or two lower sets of 
lobes. 
Going on towards Wayland, one finds there a sudden drop in altitude 
of the sand plains down to about 160 feet, represented by the three 
plains near Wayland station. The plain cut by the Massachusetts Cen- 
tral Railroad west of the Sudbury River stands at an intermediate 
height, however, 175 feet. 
From Wayland northward through Lincoln to Concord there is a 
steady increase in height of the sand plains, — 160 feet at Wayland, 175 
feet at South Lincoln, and 195 feet at Lake Walden. Across the river 
on the west side of the valley, the plains show the same sort of an in- 
crease in height going north, through 175 feet near Sudbury Centre to 
nearly 200 feet around North Sudbury, Maynard, and Concord Junction. 
Right at the north end of the Sudbury basin, where the Concord valley 
begins, there is a second pronounced drop in level of the sand plains 
from 200 feet to about 150 feet. The high plateau east of Concord vil- 
lage is the farthest north of the 200 feet group of deltas. Beyond that, 
the gravels occur only at lower levels. 
In general, then, the discordance is not haphazard, as it would he if it 
depended on the ice-blockade theory. It is quite systematic ; the plains 
rise from Cochituate north to Wayland, drop down there from about 190 
to 160 feet, then rise steadily from 160 to 200 feet at Concord, where 
there is a second drop. There is a strong suggestion, in short, of a 
slanting step-system. 
Water-Planes slanting southward, Seven Feet per Mile. 
The first three sand plains to be measured with a hand-level were the 
Lake Walden plain (194 feet) and the two deltas east of Wayland, —one . 
