80 ICE RECESSION IN NEW ENGLAND 



greater thickness of the varves can hardly be due to decreasing 

 depth because of sedimentation at the locality, since this lies 

 about 125 feet below the level of the glacial lake, and since the 

 varves are exceptionally well developed. Shallow- water varves 

 have heterogeneous material and thin and indistinct winter 

 layers, as well as wavy surfaces. Nor is it likely that the reason 

 is drainage, because this ought to have been felt over a large 

 part of the lake, if not over the whole of it. The fact can hardly 

 have been caused by a shorter distance from the locality to the 

 mouth of the eventual Mill River of that time, since this would 

 have discharged into a bay three miles northwest of it, i.e. as 

 far away from here as from localities 19 and 21. So it seems 

 most probable that the zone indicates an approach of the ice 

 border which during year 4635 or somewhat later overrode the 

 clay, crumpling up its upper part. It would have been a local 

 tongue, extending from the northwest, since deposition of clay 

 proceeded at other points in the vicinity. If, however, the lake 

 during years 4586 to 4634 did not stand at its highest level 

 (cf. p. 52), it may be possible that the thick varves 

 were due to some of the other possible reasons mentioned as 

 alternatives. 



North of Northampton, close to my localities 21 and 22, 

 Emerson (1898, p. 686, and PI. 18, Fig. 3) in 1880 found in a 

 long railway cut, in the midst of alternating till and sand beds, 

 contorted varve clay. The overlying till was no doubt deposited 

 by the land ice during a marked oscillation of its border. The 

 unusually complete disturbances of the clay at localities 21 and 22 

 also make it probable that they once or several times were 

 overlapped by the ice. 



To sum up, there are in the Northampton-Amherst zone 

 different facts which seem to indicate oscillations of the ice edge. 

 If the conditions are correctly explained, the ice border returned 

 to locality 20 during year 4635 or somewhat later, after having 

 left it more than 258 years earlier; it returned to locality 24 about 

 year 4800, or some 350 years after it first left; and to localities 

 21 and 22 at some other date not known. Furthermore, the 



