600 THE ICE AGE IN NORTH AMERICA. 



the ice, when for a short time water sorted and deposited 

 material over the lower stratum of till ; then there was a re- 

 advance, pushing along a vast mass of unsorted material over 

 the stratified stratum without disturbing it. In the deposit 

 already described near Germantown, evidence of as many as 

 four such marks of successive advances and retreats can be 

 seen. 



Again, near Oxford, in Butler county, a few miles up 

 the same stream (Four-Mile Creek) from Darrtown is jan ex- 

 posure of till where the unstratified character is perfectly 

 manifest in which I observed and photographed a piece of 

 wood, well preserved, projecting from the perpendicular face 

 of the bank about forty feet below the surface, and where no 

 land-slide could have occurred.* Equally good sections were 

 also seen on Aunt Ann's Run, near the city of Hamilton, in 

 the same county, and only about twenty miles north of Cin- 

 cinnati. 



Usually, as has been remarked, these buried deposits of 

 peat and wood have been assumed to imply the existence of 

 two distinct glacial periods. But, from what has been said 

 above, it would appear that the facts point rather to shorter 

 periods of advance and recession of the ice-front, analogous 

 to those which are now in progress in the Alpine glaciers, as 

 heretofore noted. That the interval between the two move- 

 ments noted at Darrtown was comparatively short is evident 

 from the fact that the fragments of wood found mingled 

 with the till, both above the stratum of stratified material 

 and below it, are identical in kind, and are in a similar state 

 of preservation. This locality is about twenty miles back 

 from the glacial margin. 



In the instances next mentioned of wood being found 

 imbedded in glacial deposits the locality is still nearer the 

 glacial margin, and, instead of being interglacial are pre- 

 glacial — that is, the vegetable remains have glacial deposits 

 over them but not under them. 



* See Fig. 14S, p. 577. 



