ESKERS IN THE VICINITY OF ROCHESTER, NEW YORK. 175 



the surfaces of eskers (99). Upham has described eskers in New 

 Hampshire covered with alkivium (115). Valley terraces and delta 

 materials may bury them also (21). 



Till, as well as large boulders, is very rare over the surfaces of 

 the broad places, the "plains", of esker courses. 



Relation to tJic rock surface beneath. The relation of the esker 

 to the surface upon which it rests is an unknown quantity in most 

 cases, the lack of knowledge beingdue to the paucity of sections that 

 reveal the bases of the eskers. Where examination is possible it has 

 been found that in the case of a large number of eskers the gravels 

 either penetrate to the bed rock beneath, or below the surface of the 

 till on either hand. However, a number of eskers rest on the till 

 directly with little or no evidence of erosion of the till beneath. 

 Geikie states that eskers rest often on boulder clay, but more often, 

 perhaps on solid rock (40). Leverett has mentioned a number of 

 examples of till erosion beneath eskers in the glaciated portion of 

 the interior (62, 63). 



TiiME OF Formation. 



It is universally conceded that eskers were formed during the 

 waning stage of the continental ice mass. It is probable that the 

 eskers were formed for the most part after the formation of the 

 drumlins. 



Condition of the ice at the time of esker formation. The ice at 

 the time of esker formation was thin, having a thickness of but 300 

 feet or even less (99). High hills, ridges and divides probably pro- 

 jected above the ice as nunataks. Alovement was slight or lacking 

 altogether, surface ablation was rapid with the consequent formation 

 of considerable volumes of water (45, 46), a "general recession of a 

 nearly stagnant sheet of ice" was taking place (61). 



Rate of Growth. 



It is generally admitted that the rate of esker growth was rapid, 

 very rapid. The streams in which the eskers were formed while 

 torrential in character deposited the gravels and sands rapidly. The 

 character of the stratification, the cross-bedding and irregular bed- 

 ding, the rapid changes in dip and steep dips, all point to conditions 



