THE PLEISTOCENE OR GLACIAL PERIOD 875 



These stages were not equal. The early ones were longer and 

 the late ones shorter. 



I. The sub-Aftonian or Jerseyan glacial stage. In Iowa 

 there is a very old drift-sheet lying beneath the Kansan drift. 

 In the -area of the Keewatin ice-sheet, this sub-Aftonian drift- 

 sheet is not known at the surface, except as exposed by erosion. 

 In Pennsylvania and New Jersey, 1 the frayed edge of a very 

 old sheet of drift emerges from beneath the Wisconsin drift of 

 the region, and is, perhaps, the equivalent of the sub-Aftonian of 

 Iowa. 2 



II. The Aftonian interglacial stage. Overlying the oldest till 

 is a stratum of sand and gravel at some points, and beds of peat and 

 muck at others, with stumps and branches of trees. The surface 

 of the drift below shows evidences of an interval of erosion and 

 weathering. The organic remains in the interglacial beds indicate 

 a cool temperate climate; but as a cool temperate stage must be 

 passed through twice between successive glacial epochs, once as the 

 ice retreats, and a second time as it advances again, organisms 

 indicative of a cool climate do not necessarily show how warm the 

 interglacial epoch may have become. 



III. The Kansan glacial stage. The Kansan stage is repre- 

 sented by a sheet of till occupying a large surface area in Kansas, 

 Missouri, Iowa, and Nebraska (Fig. 582) . Theoretically it extends 

 under the later glacial formations to the northward, as far back as 

 the Keewatin center of radiation. Much of this sheet of drift, as 

 originally developed, has probably been rubbed away by later gla- 

 ciations. Presumably a similar sheet was formed by a contempo- 

 raneous ice-sheet spreading from the Labradorean center, but it has 

 not been certainly identified. The Kansan drift is clayey till, with 

 little stratified drift. Stream action seems to have been notably 

 inefficient at this time. This and some other consonant facts have 

 led to the abandonment of the former notion that vast floods inevit- 

 ably accompanied the melting of the ice. 



1 Salisbury, Ann. Kept, of State Geol. of N. J., 1893. 



2 The Albertan "drift" (province of Alberta, Can.) formerly thought to 

 be the probable equivalent of the sub-Aftonian, is probably not of glacial 

 origin (Calhoun). 



