GOLDTHWAIT: SAND PLAINS OF GLACIAL LAKE SUDBURY. 291 
that far; but there ‘is no evidence of it in slightly lower lobes or in 
signs of scouring along the courses of the brooks that head in the pass. 
The height of the pass, moreover, is very nearly as great as the com-— 
puted height of the Cherry Brook water-plane. Consequently it seems 
reasonable to suppose that not until the ice had melted back beyond 
Lake Walden, so as to uncover a pass at the head of Hobbs Brook, three 
miles to the east of Concord, did Lake Sudbury suffer a marked lower- 
ing of level. With the opening of the Hobbs Brook pass, the water 
seems to have fallen about 50 feet, judging by the height of the lower 
deltas at Concord and Concord Junction. Since this new water-plane, 
50 feet below the other, begins where the Concord River begins, it 
would perhaps be appropriate to consider the opening of the Hobbs 
Brook pass as marking the death of Lake Sudbury and the birth of a 
new lake, Lake Concord. That the transition was not immediate is 
shown by lobes of intermediate altitudes on the sand plains near Con- 
cord Junction. 
Outlets of Lake Sudbury. 
In several papers on the temporary glacial lakes of central New York, 
Fairchild describes a large number of well-developed outlet channels. 
Next to delta deposits, these channels are the clearest records of the 
extinct lakes of this region ; and they are certainly the most striking of 
all. They are described as often heading in low marshy ground on a 
water-parting, called a ‘swamp col” by Fairchild (e, 36, 37, 38, 58, 61). 
As channels with definite walls, they run often nearly straight for a mile 
or so. One which drained Naples Lake, given as a good example of an 
abandoned river-bed, is described as “ over a mile long, 20 to 25 rods wide, 
with banks 15 to 25 feet high, and a flood plain of varying width,” and a 
‘“navement of cobbles and boulders in the bottom of the channel,” which 
“is still well shown through the vegetal accumulation” (a, 362). 
When, as in this case, the channel is cut in drift, the banks and flood 
plain seem to be well developed. Not infrequently, however, a channel 
has rock walls and a swampy bottom. Some of the old outlets of Gene- 
see Lake are rock gorges with swamps partly occupying the channel 
floors (c, 434, 435, 436, 437, 438). In certain cases, deltas built into 
these channels from side ravines have levels of from 70 to 80 feet above 
the channel floor, seeming to indicate a down-cutting of the floor 60 to 
70 feet since the channel first opened (c, 435). Other outlets of Genesee 
Lake are broad, with flood plains. At the lower ends of these channels 
are commonly plains or deltas of detritus, built out by the stream which 
