STRUCTURE OF THE DRIFT BORDER. 41 



was -probably in part formed by the ice in passing over rock ledges and in 

 part collected from the calcareous portions of the underlying sheet. It 

 does not seem at all probable that the variations in the depth of the leached 

 material are due entirely to leaching which has occurred since the Illinoian 

 sheet was deposited. The portions which are leached to great depth seem 

 to be no more readily pervious to water than those in which the leaching 

 has extended to a depth of only 6 or 8 feet. 



The ridge marking the western limits of the Illinoian drift, in Lee 

 County, is in places thickly set with bowlders, but as a rule it appears to 

 carry no more bowlders than the portion of the same sheet in the plain 

 tracts immediately east. The bowlders and smaller rock constituents of 

 the Illinoian drift are found to differ somewhat from those of the sheet 

 that underlies it, there being certain rocks found in it that are not found 

 in the underlying sheet, while other rocks differ in abundance in the two 

 sheets. Several bowlders of red jaspery conglomerate, apparently from 

 the Huronian outcrops north from Georgian Bay, have been found in this 

 county on and east of the ridge that marks the Avestern limits of the 

 Illinoian drift, and these are thought to point decisively to the Labradorian 

 invasion. There are also quartzite rocks present in the Illinoian drift that 

 have not been seen in the sheet beneath it and which probably were derived 

 from sources not far distant from the Huronian that bears the jaspery 

 conglomerate. The cherty beds of the Burlington limestone that outcrop 

 along the Mississippi and its tributaries have been incorporated in the 

 Illinoian drift sheet and transported westward to the extreme limits of that 

 sheet. They point with certainty to the influence of the Labradorian 

 invasion. 



The Illinoian till sheet in Lee County, as also in counties to the north, 

 is separated from the underlying Kansan till sheet by a weathered zone 

 accompanied by beds of black muck and peaty material. This was first 

 brought to the writer's notice about ten years ago, in a well sunk near the 

 village of Yarmouth, in Des Moines County. For this reason, and because 

 it is not liable to be a source of confusion by duplication in other parts of 

 the glaciated region, the name Yarmouth has been proposed to cover the 

 interglacial interval between the Kansan and Illinoian. 1 The village of- 



1 The weathered zone (Yarmouth) between the Kansan and Illinoian till sheets, by Frank 

 Leverett: Proc. Iowa Acad. Sei., Vol. V, pp. 81-86, 1898; Jour. Geol., Vol. VI, 1898, pp. 238-243. 



