348 W. UPHAM — GLACIAL ACCUMULATION AND INVASION. 



of 700 miles from the Scioto river in Ohio eastward to Marthas Vineyard 

 and Nantucket, where this boundary passes into the Atlantic ocean, 

 series of moraines, continuous with those of the upper Mississippi region 

 and of the same general age, form the outer margin of- the drift or are 

 nearly coincident with that margin but distant from one or two to 15 or 

 20 miles back from it. All these moraines, whether situated far back 

 from the glacial boundary or near to it or upon it, give good evidence, 

 by the volume and contour of their drift accumulations, that the ice- 

 border while forming them halted in its recession and usually made at 

 least some slight readvance or invasion of territory which it had relin- 

 quished or which even in some districts it never before had occupied. 



ATTENDANT DRIFT EROSION, TRANSPORTATION AND DEPOSITION. 



The country inclosed by the moraines has been powerfully eroded by 

 the ice, its strise being found upon practically the entire rock surface. 

 Large accumulations of drift, which I think to have been almost wholly 

 engiacial during the time of that erosion, being held and carried forward 

 in the lower part of the ice-sheet, perhaps to the height of a quarter of 

 its whole thickness, are spread over the strongly glaciated bed-rocks, and 

 much of this drift is amassed in steep, irregularly grouped hills of excep- 

 tionally bowldery drift, extending in belts one to five miles or more in 

 width. These moraines, here parallel, there interlocking or overlapping 

 one another, are traced in a complex series from Manitoba and the Da- 

 kotas east to Long island, New England and the Atlantic. 



On the smooth areas of drift beyond the moraines in the Mississippi 

 basin the proportion of the ice inflowing from the north and bringing 

 the bowlders and finer drift derived from the granitic, gneissic and other 

 crystalline rocks of northern Wisconsin and Michigan, of Minnesota, and 

 of Canada, may have been a tenth or less, or a fourth or a third part, of 

 all the ice covering those areas. That the northern ice was much less 

 than the amount supplied by the snowfall seems demonstrated by the 

 attenuation of both the ice and its drift. It was not, as I believe, an ice 

 invasion, but mainly snow and ice accumulation, with a proportionally 

 small inflow of northern ice, bringing the greater part of the drift. The 

 glacial currents were too feeble to accomplish much erosion, and most of 

 the engiacial drift borne in by the ice currents from the north was prob- 

 ably laid down as subglacial till during the time of culmination of the 

 ice-sheet and while the ice was finally retreating.* 



In the drift areas bordered by the moraines, upon which, indeed, the 

 ice-front doubtless bodily moved forward, as a general rule, for short dis- 

 tances while amassing the morainic hills, the steep frontal slopes and 



* Bull. Geol. Soe. Am., vol, 5, 1894, pp. 83, 84. 



