T. C. Charnberlin — Diversity of the Glacial Period. 183 



the northward and remain long enough to permit the accumu- 

 lation of a soil equal to that of the present and then advance 

 again while the country still lay at a low altitude. It certainly 

 could not be supposed that the ice would retreat so that low- 

 gradient deposits should take place six or seven degrees of 

 latitude back from the edge of the ice without general degla- 

 ciation also taking place. Now, in the Mississippi Valley we 

 have silt deposits spreading over many thousands of square 

 miles and connecting themselves with the second main drift 

 sheet of the series given above. They overlie the oldest drift 

 throughout large areas but are separated from it by a soil hori- 

 zon. (The older drift seems to have its own silt-appendices 

 in some regions also.) These silts occur on the highlands ad- 

 joining the main rivers throughout the older glaciated regions 

 from Ohio to Nebraska, and they reach northward to the west- 

 ern part of Wisconsin, the southeastern part of Minnesota, 

 the southwestern part of Minnesota, the southeastern part of 

 Dakota and the northern part of Nebraska, and from these 

 tracts they extend without detectable interruption to the gulf 

 region. In other words, they reach seven degrees north of 

 the extreme drift limit in the Mississippi region and six de- 

 grees in the Missouri region. To account for the phenomena 

 in harmony with the hypothesis of glaciation by elevation, it 

 appears to be necessary to suppose that the northern region 

 was elevated to a height sufficient to cause an ice sheet to 

 creep down to a point south of the latitude of 38 degrees on 

 a plain of great extent; that this was followed by a depression 

 sufficient to cause the withdrawal of the ice for an unknown 

 distance and to remain withdrawn long enough for a soil and 

 for peat and forest beds to accumulate to thicknesses fully 

 comparable to those of the present; that there was then an 

 advance sufficient to form the second till which overlies this 

 forest bed, followed by a retreat under such conditions as to 

 permit silts to gather over the wide areas indicated for six or 

 seven degrees northward and at considerable heights above the 

 present streams, spreading well back on the uplands. Now a 

 depression that would permit a deposit of these silts through- 

 out this great extent and at these heights, carries with it the 

 presumption of the removal of the ice from the whole north- 

 ern region, because the depression is assumed by the hypothesis 

 to have been caused by the ice itself, and to have been pro- 

 gressively greater to the north., and so, unless an entirely in- 

 consistent supposition be interpolated, the northern region 

 must be presumed to have been lower than it is to-day. The 

 great extent of silts, therefore, in the Mississippi region car- 

 ries with it the presumption of complete deglaciation of the 

 northern region, if elevation be assumed to be the chief cause 



