346 FRA NK B URSLE V TA YL OR 



aggregation of spots, without any scheme of orderly arrangement, 

 as shown in Fig. 8. This appearance arises not only from the 

 fragmentary character of the evidences, but is greatly increased 

 by two other causes — by the shortness of the average interval 

 between the successive halting places, and by the extreme sinu- 

 osity of the ice-front at every halt. If the successive halting 

 places had been farther apart, say fifteen or twenty miles on the 

 average, instead of three and one-half, the continuity of the 

 successive ice-borders would have been more apparent ; for each 

 one would have stood out as a recognizable continuous indi- 

 vidual, inspite of the fact that it was represented only by a 

 sinuous line of fragments. 



In order to see the true relations in the Berkshires it is neces- 

 sary to revert briefly to some of the larger elements of the 

 situation. The Hudson valley was occupied by a great glacier 

 lobe, low and sharply pointed at the south, but rising to higher 

 and higher levels to the northeast along its eastern border until 

 it overtopped the highest summits of the Berkshires. It was 

 the ice of this lobe that overspread Berkshire county, and it was 

 its retreating eastern limb or margin which made the recessional 

 moraines. Each time the ice-front halted it fitted itself to the 

 rugged topography with which it happened to be in contact at 

 that time, projecting a series of tongues in the valleys and form- 

 ing high re-entrant angles on the intervening hills. Thus the 

 border of the Hudson valley lobe was made intensely serrate by 

 the ruggedness of the local topography, and it was this that gave 

 the course of the ice-border so many sinuosities and determined 

 the peculiar distribution of its associated deposits. When the 

 Hudson lobe began a movement of retreat, its border drew back 

 all along the line until it reached the place of the next halt; then 

 it halted all along the line, and whenever^it advanced it pro- 

 ceeded in the same manner. There is no reason to believe that 

 different parts of the margin of the lobe had dissynchronous 

 movements, such as would be the case if one part retreated while 

 another remained stationary or while still another advanced. 

 Everything we know tends to the conclusion that the movement 

 at all points was synchronous or in unison along the entire side 

 of the lobe. 



