274 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
grouping of deltas into sets, each set being composed of deltas of a 
single level or water-plane, and being succeeded by a lower set on the 
iceward side. ; 
This step arrangement of deltas, in descending groups as one proceeds 
in the direction of ice retreat, has apparently been recognized by Grabau 
in his study of Lake Bouvé, and has been applied by Clapp and Crosby 
to the deltas of Lake Charles, Lake Nashua, and other temporary ice-front 
lakes of eastern Massachusetts. 
The area embracing what Professor Crosby has called glacial Lake 
Sudbury is the field in which I have attempted to apply one or the 
other of these two schemes of arrangement of sand plains, — that of 
confusion of levels or that of horizontal steps. 
Glacial Lake Sudbury. 
The area occupied by the temporary glacial Lake Sudbury is a belt of 
lowland on the Framingham sheet (U.S. G. 8.), reaching from Lake 
Cochituate northward to Concord. It constitutes the valley of the lower 
part of the Sudbury River, which at Concord joins the Assabet to form 
the Concord River, and as such flows north through Billerica to the 
Merrimac. The length of the old lake basin, from Framingham to 
Concord, is about eighteen miles ; its average width is about four. That 
this basin was occupied by a temporary lake as the ice withdrew is well 
shown by the abundant sand plains which are scattered about in it, and 
which rise generally to the level of the lowest points on the rim of the 
basin, near by. 
The approximate form and extent of the lake can best be seen on the 
accompanying map (Plate 5), in which the probable shore-lines are drawn 
along the contour which lies nearest the level of the adjacent sand deltas. 
Since these deltas vary nearly 70 feet in elevation, the shore-line follows 
different contours in different parts of the basin, as will be recognized 
when the map is compared with the U. 8. G. S. contour map. 
The country on the western side of the basin is much higher than that 
which forms the southern and eastern rim, being as a rule well above 200 
feet in altitude. The Nashua basin, which lies west of the Assabet and 
Sudbury, was occupied in late glacial times by a temporary lake whose 
level was much higher than Lake Sudbury, and which drained eastward 
into it, as Professor Crosby has shown (b). The waters of Lake Sudbury, 
then, could not have escaped to the west; they must have reached 
south and east into the lower basin of the Charles River. 
