MAIN MORAINIC SYSTEM IN THE GRAND RIVER LOBE. 443 



In the discussion of topography- just given the remarks have been of 

 a general nature in order that the salient features might be clearly set 

 forth. In that which follows the topography is described in more detail. 



In locating the ice margin in the valleys the writer has used the com- 

 monly accepted criteria of hunmiocky topography, accompanied often by 

 bowlders, placing the margin at the point where a change to level-surfaced 

 deposits set in. This has led to a different interpretation from that given 

 by Lewis and Wright in the report on the terminal moraine in Pennsyl- 

 vania,^ for their criteria were evidently somewhat different. They include 

 in the moraine only the till deposits and bowlder belts, and exclude the 

 large knolls and ridges containing stratified drift so commonly found at the 

 head of terraces, classing them as the product of water action in a recess 

 or embayment in the ice. That water played a prominent part in the form- 

 ation of these knolls of stratified drift at the head of glacial terraces can not 

 be doubted, but it is equally certain that these knolls constitute integral 

 parts of the moraine, since their form is such as would have been produced 

 only by the aid of the mechanical action of the ice sheet. The presence of 

 surface bowlders also shows that the ice occupied the valleys till the knolls 

 and ridges had acquired essentially their present form. The morainic 

 topography and the surface bowlders seem to the writer more essential 

 characteristics of a terminal moraine than the till. Furthermore, the strati- 

 fied material and the till in these valleys, as well as in the upland i;)ortions 

 of the moraine, grade into each other, or alternate in such a manner that 

 their border line is very indefinite, and in places the topography affords the 

 ouly^ reliable basis of interpretation. For example, in one valley, that of 

 Oil Creek, no till or bowlder belt was discovered, and as a consequence 

 Lewis and Wright were obliged to carry the ice margin across, where, 

 according to their critei'ia, there was no evidence of its presence. In this 

 ^'alley they placed the margin about 2 miles above the head of the terrace, 

 excluding, because of its gravelly constitution, the best developed morainic 

 topogi-aphy in the valley. 



The valleys which illustrate this difference of mapping of the morainic 

 border are those of the Conewango River, Jacksons Run, Big Brokenstraw 

 and Oil creeks, and Beaver River, in each of which the writer has extended 

 the morainic border southward from 1 to 3 miles beyond the limit set by 



1 Second Geol. Survey Pennsylvania, Kept. Z. 



