RECESSIONAL ICE BORDERS IN BERKSHIRE, MASS. 349 



Still others may be at a standstill. The cause of this lack of 

 unison in movements is found in the fact that Alpine glaciers are 

 fed from separate snow-fields. Their gathering grounds are on 

 the high flanks of mountain peaks or ranges, and each glacier 

 has its own basin, or cirque, in which its snow accumulates. 

 The varying conditions of snowfall in individual storms, and in 

 different months and years and periods of years, furnish abundant 

 reason for their individual peculiarities and dissynchronous 

 movements. 



But although their forms often bore some resemblance to 

 Alpine glaciers, the ice-tongues of the Berkshires were not of 

 the Alpine type. They were not fed by independent snow-fields, 

 but were all simple offshoots from one ice-mass — the Hudson 

 valley lobe. They were all fed from one source, and whatever 

 affected that source affected them all alike. There appears to 

 be ample reason, therefore, for believing that the recessional 

 halts in the Berkshires were separate individuals without over- 

 lappings. 



But while these conclusions may be safely applied to the Berk- 

 shires, and would probably be applicable in other regions where 

 the relations were equally simple, it is not intended to imply 

 that they would be a safe guide everywhere. There is much 

 reason to believe that the great lobes, like the Hudson valley and 

 Lake Ontario lobes, had movements somewhat dissynchronous, 

 so that as between two such lobes the principle of correlation 

 here suggested might not apply. 



In the Berkshires, however, this seems to be the one thing 

 needed. It seems to furnish the only possible basis of correla- 

 tion by which the fragments of the recessional moraines can be 

 connected together and the ice-borders reconstructed as they actu- 

 ally existed. With one or two ice-borders clearly made out in con- 

 tinuous form for a sufficient distance to show their general trend, 

 with branching and interlacing series of terminal deposits made 

 by ice-tongues at consecutive halts in the main valleys, and with 

 a moderate amount of interpolation between adjacent fragments 

 applied according to rule as given above, it becomes possible to 

 reconstruct in continuous form all of the recessional ice-borders 



