TEE DATE OF THE GLACIAL PERIOD. 579 



terior basin except in osars and kames, whose conditions of 

 formation were exceptional, but, on the contrary, abundant 

 proof of slow-moving waters and imperfect drainage, indicat- 

 i ng low slope of the surface. 



The later Glacial epoch, on the contrary, was character- 

 ized by strong glacial action, planing the rock-surface vigor- 

 ously, even up to the very limit of its advance. The glaciers 

 plowed up immense moraines about their edges, except on 

 smooth plains whose slope was away from the ice-movement. 

 The drainage was usually vigorous, and immense trains of glacial 

 gravel stretch away from the margin of the ice-sheet, reaching 

 great distances down the valleys and frequently filling them 

 to great depths with well-assorted material. The vigorous 

 action of the glaciers of the second epoch and the rapid drain- 

 age, in general, stand in marked contrast with the gentle 

 action and imperfect drainage of the earlier epoch. One of 

 the conditions that determined the distinction was probably 

 the difference in elevation that characterized the two epochs. 



The interval between these two leading epochs we regard as 

 the chief interglacial epoch, representing a greater lapse of 

 time and a greater change in the dynamic agencies of the age 

 than the several other interglacial intervals, or episodes of 

 deglaciation, which mark the complicated history of the Ice age. 



As belonging to the earlier Glacial epoch, we recognize two 

 drift-sheets that have been described by the geologists of the re- 

 spective States as occurring in southwestern Ohio, southern In- 

 diana, central and southern Illinois, eastern and southern Iowa, 

 northern Missouri, eastern Nebraska, and southeastern Min- 

 nesota. 



Between these occur, at numerous points, vegetal and fer- 

 ruginous accumulations and other evidences of a non-glacial 

 interval. To this horizon belong the larger number of de- 

 posits described under the term "old forest-bed," but very 

 many vegetal deposits so referred do not, in our judgment, 

 belong there, but are referable to several distinct horizons.* 



Others adduce as evidence of the distinct Glacial epochs 

 in North America the greater oxidization and general de- 



* " Driftless Area," pp. 214, 215. 



