G. F. Wright — Unity 'of the Glacial Epoch. 357 



public ten years ago during the early part of our investiga- 

 tions. In the map of the glacial boundary prepared for my 

 volume on The Ice Age in North America (p. 175), the legend 

 for the line marking the border of the glaciated area west of 

 Pennsylvania, is not " terminal moraine," but simply " southern 

 limit of the ice sheet." East of that point, indeed, a moraine 

 is marked as the boundary, but emphatic attention is called, 

 both in my own language and in that of Professor Lewis, to 

 the probability that here also an unexplored border of thinner 

 glaciated material is to be found south of the moraine. (Ice 

 Age, p. 135.) Recent observations enable me to determine 

 the extent of this ''fringe" as we proposed to call it, in the 

 Delaware and Susquehanna valleys, and to give an entirely dif- 

 ferent interpretation to certain facts in that region from that 

 proposed by Professor Salisbury in support of his theory of a 

 prolonged interglacial period. Of this 1 will speak under No. 7. 

 Suffice it here to say that the word " fringe " seems a fair term 

 to apply to such a bordering formation whose character is still 

 in dispute. Professor Chamberlin's phrase, "attenuated bor- 

 der," would, however, be equally good. But to insist on call- 

 ing it the drift of the first glacial epoch is to beg the whole 

 question in the terms chosen, and tends to serious confusion of 

 the public mind as to what the facts really are. The facts are 

 well stated in the sentence quoted from Professor Chamberlin. 

 But it would seem that occasionally in the discussion inferences 

 are mistaken for facts of observation. For example, it is pretty 

 generally assumed in the discussion that originally there was a 

 much greater amount of drift over the attenuated border than 

 there is now and that this has largely disappeared by erosion. 

 But there does not seem to be adequate ground for this sup- 

 position. A pronounced terminal moraine is by no means a 

 necessary result of an ice occupation. The extent of the de- 

 posits of till and other morainic material, depends in large 

 measure upon the amount of ice movement which has actually 

 reached a given point, which, in turn, depends on the time 

 during which the front of the ice remained stationary at a 

 given point. If there was a gradual approach to a given boun- 

 dary and a gradual retreat after a pause of moderate length we 

 could not expect a large accumulation of drift. It is too often 

 forgotten that the ice movement diminishes to zero at the very 

 border and that for some distance back of the border all the 

 effects of the movement are specially feeble. A fringe of 

 direct glacial deposit would, therefore, seem to be the normal 

 result near the border. By '" fringe " is meant an " atten- 

 uated border" of direct deposits from land ice such as might 

 naturally occur near the limit of an ice invasion. This, which 

 was at first supposed to be characteristic only of the Mississippi 



