580 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



Indeed, its uniform size for so long a distance scarcely admits of any other 

 explanation. This is represented on PI. XXXV, C, and its end appears in 

 the section, fig. 32, p. 578. 



The next striking peculiarity is that, while the grand teiTaces are 

 heaped high on the east, north, and west of the basin, and across its 

 westeiii portal, the flat bottom is, and has always been, bare till, and 

 scarcely a trace of sand can be seen upon the south slopes — nowhere 

 enough to mark the water stand. Finally, the highest ten-ace on the east, 

 at 1,000 feet, has no counterpart on the other side. 



I have expressed upon the map (PI. XXXV, C) the explanation which 

 seems to me most plausible, so far as this was possible. In the first place, 

 the highest sands on the east of the basin, at 1,000 feet, seem to have been 

 deposited by waters coming down the eastern of the two northern valleys 

 when the basin was still nearly filled with ice, and, as these sands are on 

 the same level as the lowest portion of the ridge to the east, the waters 

 would seem to have escaped east into the West Branch Valley. This 

 lowest ground is just north of the section line. The ice ban-ier (b 8, PI. 

 XXXV, C) placed on these lake beds (1 p^ PL XXXV, C) may well have 

 been somewhat farther west, as the lower-level waters have worn into and 

 terraced these sands on the west. 



Some temporary posture of the ice tui-ned the waters of this eastern 

 stream out across its surface in a course directed toward the portal, and 

 the sand filling this channel sank to foiTQ the great esker as the ice melted. 

 With the retreat of the ice from the basin the asbestos mine valley on 

 its south rim was set free at a level of 830 feet above the sea, furnishing 

 a permanent waste weir for its waters south into the Belchertown Lake, 

 along the course described on page 576, and as the ice still filled the whole 

 Connecticut Valley opposite, it completed the barrier across the portal on 

 the west. The lake basin was then rapidly filled by sands pushed south 

 as great deltas from the two northern valleys, and the waters coming down 

 from the north between the ice and the west slope of Hygeia entered the 

 basin at the portal and sent a third delta into the basin, thus completing the 

 teiTace on the western side. The life of the lake at this stage was a very 

 brief one, and when the deltas had advanced halfway across the bottom the 

 ice barrier (b^) failed at the portal and the waters escaped, breaching the 

 portal terrace and moving soiith by a channel, still well marked, which runs 



