THE PLEISTOCENE OR GLACIAL PERIOD. 389 



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 glaciations. Pre- 

 sumably a similar sheet was formed by a contemporaneous ice move- 

 ment from the Labraclorean center, but this has not been certainly 

 identified in the region east of the Mississippi. It probably fell short 

 of the later advances there, and lies concealed beneath their debris, 

 so far as it has escaped destruction. The Kansan formation is a pro- 

 nouncedly clayey till, with exceptionally little assorted drift. Glacial 

 water action seems to have been notably inefficient. Observation 

 on this and the succeeding glacial formations has forced the aban- 

 donment of the earlier conception of vast floods as the inevitable 

 accompaniment of the ice-melting, the meagerness of marginal drain- 

 age in some cases being one of the strangest of all the strange 

 phenomena of the glacial period. No great deposits of sand and gravel 

 have been found in, or on, or leading away from the edge of this 

 formation. 



Originally the surface of the Kansan till sheet seems to have been 

 rather plane, but it has since been markedly eroded, and bears clear 

 evidence of great age as compared with the latest drift. As the next 

 younger sheet (Illinoian sheet) of drift overlaps its east border near 

 the Mississippi (Fig. 514), comparison along the junction shows that a 

 large part of the erosion of the Kansan drift took place before the 

 superposition of the Illinoian drift. A long intervening epoch is there- 

 fore inferred, an inference strengthened by the deep weathering of the 

 Kansan drift, and the pronounced decay of its bowlders. 



IV. The Yarmouth interglacial stage. 1 — The erosion just mentioned 

 is perhaps the best evidence of a prolonged interval between the Kansan 

 and Illinoian ice invasions; but in the tract where the Illinoian till 

 sheet overlaps the Kansan, in eastern Iowa, an old soil with deep sub- 

 soil weathering is found to have developed on the surface of the latter 

 before its burial. Some vegetable accumulations have also been pre- 

 served, a good instance being found near Yarmouth, Iowa, whence the 

 name was taken. Bones of the rabbit and skunk have been identified 

 from this horizon. A climate not essentially different from the present 

 is inferred. 



1 Leverett, Mono. XXXVIII, IT. S. Geol. Survey. 



