The Natural History of Chautauqua 



where the stream crosses or flows into the old drift-filled valleys, and 

 then the shale wall is changed to one of till in which the bowlder clay 

 is sometimes one or two hundred feet deep, as is the case in parts of 

 the " Gulf " near Westfield. 



5. As the ice withdrew, with a south-facing front, it naturally inter- 

 fered with all north-flowing streams. It formed a dam and caused 

 many reversals of drainage. The St. Lawrence Valley was occupied 

 by the ice when the front had retreated north of the escarpment which 

 partly encloses the " grape belt." Therefore these north-flowing 

 streams could not drain by the present outlet but were empounded 

 and forced to take another place of outflow, and this was, of course, 

 the lowest point in the enclosing hills, a point which was naturally 

 higher than the present outlet. While Lake Erie was held at its upper 

 place of outflow it was building the upper beach, which has been called 

 the Sheridan beach. The outflow of this lake was then at Fort Wayne, 

 Indiana, into the Wabash, and the beach may be traced continuously 

 to this outlet. However, in the east this beach ends just southeast of 

 Silver Creek; and near its eastern end there is a tract of moraine. 



6. To the southward of the town of Hamburg, on the road to New 

 Boston, there is another morainic belt, and a second beach, which can 

 be quite continuously traced from west of Silver Creek nearly to Ham- 

 burg, begins to disappear as this town is neared. The last place at 

 which it could be distinctly determined is near Eden Church, south- 

 west of Hamburg; but a third beach from the top passes directly 

 through Hamburg, and has been found to disappear near Crittenden. 

 In each case, as the moraine is approached, the beach becomes less dis- 

 tinct and finally can be traced no further. 



7. This shows that while these beaches were being built at their 

 respective levels, the ice was standing at different places and was bring- 

 ing materials which were being laid down at its front in the form of 

 moraines. At first the ice front passed near Silver Creek and then 

 the upper beach was made, while the outflow of the lake was past Fort 

 Wayne. Then the edge of the ice withdrew for a distance until some 

 lower outlet was formed, and again to a still lower, more northern 

 point, when another and still lower outlet was established. One of 

 these last two outlets was past Chicago. 



8. At last the ice retreated far enough for the Erie Basin to take its 

 present outflow past Buffalo; but the valley of the St. Lawrence was 

 still ice-dammed, and Ontario was raised to the level of the overflow 

 of the Mohawk Valley. Thus temporarily the several Great Lakes 

 had their level raised by ice dams; and during this time distinct shore- 

 lines were formed. 



Adapted from R. S. Tarr, Cornell Bulletin 109. 

 108 



