GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 279 
The Horizontal Step Scheme applied to Lake Sudbury. 
The four points in the last paragraph may be considered in order : 
(a) First, regarding the maximum height of the sand plains. Accord- 
ing to the contours of the Framingham sheet, some of the sand plains of 
the Sudbury valley are above 160 feet. The splendid large delta at 
North Sudbury, for example, is bordered by a 180-foot contour ; the 
large plain east of Maynard, occupied by the American Powder Com- 
pany, is shown by a 200-foot contour ; and the plains in Concord near 
Lake Walden and east of the village rise apparently to at least 180 feet. 
Careful levelling not only confirms the idea that these plains are well 
above 160 feet, but shows that many other plains which from the map 
might be taken as 160 feet in elevation are really 10, 15, or 20 feet 
above that level. A glance at the map (Plate 5) will show this. 
(b) The sand plains of Lake Sudbury do not fall even approximately 
into horizontal water-planes at 160 or 145 feet. The contours, of the 
U. S. G. S. map, poor as they are, express rather clearly the strong dis- 
cordance of levels. Taking the contours as truthful only within 20 feet, 
one finds that the plains do not agree in altitude. One at Wayland, for 
instance, is bordered by a 140-foot contour, while that at Maynard, 
about eight miles further north, rises to 200 feet. The real test of ac- 
cordance, however, comes when careful levelling is done to determine 
accurately the levels of lobe brows of the sand plains. The figures on 
the map (Plate 5) show the great range of altitudes that accurate levelling 
brings to light. Out of sixteen different sets of lobes, measured in this 
way, only four measure within five feet of 160 feet, and they run all the 
way from 137 to 201 feet. 
(c) Several things point to the probability that the Cherry Brook 
pass once drained Lake Sudbury. Just west of it in the valley, near 
Wayland, the level of the sand plains suddenly drops from about 190 
feet to about 165 feet, the approximate level of the pass. Along the 
courses of brooks that head in the pass, as one follows them down-stream, 
are suggestions of former occupancy by more powerful streams, in the 
from of boulder-paved stretches, aggraded floors, and pot-holes. The de- 
tail of this evidence will be given later. While not at all strong, it is at 
least suggestive, when taken in connection with the better evidence of 
water-planes above the level of 160 feet. 
(d@) The Morseville pass near Steep Rock, and the course of the brook 
from that point southeast through Little’s Pond to the Charles River, 
show no sign of scouring. The evidences of outlets are in general so 
