354 G. F. Wright — Unity of the Glacial Epoch. 



nomena of glacial erosion connected with it are generally 

 feeble. Glacial stride are indeed present, even in the periphe- 

 ral portion, but the surface of the rock is not usually exten- 

 sively planed. [5.] The whole aspect of the deposit indicates 

 an agency which spread the drift over the surface smoothly, 

 and relatively gently, with little forceful action. The drainage 

 phenomena are also of the gentle order. We have yet failed 

 to find evidence of very vigorous drainage connected with the 

 older drift of the interior basin except in osars and kames, 

 whose conditions of formation were exceptional, but, on the 

 contrary, abundant proof of slow-moving waters and imper- 

 fect drainage, indicating low slope of the surface. 



" The later glacial epoch, on the contrary, was characterized 

 by strong glacial action, planing the rock surface vigorously, 

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

 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 drainage, 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 ele- 

 vation 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 de- 

 glaciation, which mark the complicated history of the ice age. 



6. " As belonging to the earlier glacial epoch, we recognize 

 two drift sheets that have been described by the geologists of 

 the respective states as occurring in Southwestern Ohio, South- 

 ern Indiana, Central and Southern Illinois, Eastern and South- 

 ern Iowa, Northern Missouri, Eastern Nebraska, and South- 

 eastern Minnesota. 



" 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 deposits 

 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." 



7. Another supposed proof of a glacial period long preced- 

 ing that during which the moraine of the so-called " second 

 glacial epoch " was deposited is the greater oxidation of the 

 glacial drift south of this moraine. This is prominently men- 



