98 MAN AND THE GLACIAL PERIOD. 



with accuracy, on account of an enveloping deposit of fine 

 loam, technically called " loess." Loess is very abundant 

 in the whole valley of the Missouri River below Yankton, 

 South Dakota, being for a loug distance in the vicinity of 

 the river a hundred feet or more in depth. Over north- 

 ern Missouri and southern Illinois the deposit is nearly 

 continuous, but less in depth, and everywhere in that re- 

 gion tends to hide from view the unstratified glacial de- 

 posit continuously underlying it. 



A single instance of personal experience will illustrate 

 the condition of things. While going south from Chicago, 

 in search of the southern limit of glacial action, I stopped 

 off from the train at Du Quoin, about forty miles north 

 of where I subsequently found the boundary. Here the 

 whole surface was covered with loess, two or three feet in 

 depth. Below this was a gravelly soil, three or four feet 

 in thickness, which contained many scratched pebbles of 

 granite. A well which had recently been dug, reached 

 the rock at a depth of twenty feet, and revealed a beauti- 

 fully polished and scratched surface, betraying, beyond 

 question, the action of glacial ice. As we shall show a 

 little later, it is probable that, about the time the ice of 

 the Glacial period had reached its maximum development, 

 this area, which is covered with loess, was depressed in 

 level, and remained under water during a considerable 

 portion of the period when the ice-front was retreating. 



To such an extent is this portion of the area included 

 in southern Iowa, northern Missouri, southern Illinois, 

 and the extreme southern portions of Indiana and Ohio 

 covered with loess, that it has been difficult to determine 

 the relation of its underlying glacial deposits to the more 

 irregular deposits found farther north. At an early period 

 of recent investigations, while making a geological survey 

 of the State of Wisconsin, President T. C. Chamberlin 

 fixed upon the line of moraine hills, which can be seen 

 upon our map, running southward between Green Bay 



