310 THWAITES AND TWENHOFEL THE WINDROW FORMATION 



from the Ordovician to the Silurian and possibly the Devonian. This 

 gives positive assurance that the gravels are of post-Silurian age. 



AGE OF THE UNDERLYING FORMATIONS 



The youngest rocks known to certainly underlie the Windrow forma- 

 tion are of Devonian age. If the Guthrie County, Iowa, conglomerates 

 are part of this formation, then the Windrow formation is post-Carbon- 

 iferous. 



RELATION TO OVERLYING DEPOSITS 



The materials known to overlie the Windrow formation are the glacial 

 drift and the loess. This relation proves that the formation is older than 

 these deposits. 



HISTORY SINCE THE ADVENT OF GLACIATION 



The earlier glaciers found the topography of the Driftless Area not 

 greatly different from what it is today. They apparently entered the 

 central plains area of Wisconsin in Wood and Jackson counties, thus 

 showing that this plain is of pre-early-drift age, and that the Windrow 

 formation long antedates the time of these glaciers. The erosion which 

 has taken place since the retreat of the earliest ice in Wisconsin is a mere 

 nothing compared to that which has occurred since the formation of the 

 upland gravels and bog ores. 



Unless the time necessary for erosion of such great valleys and lowland 

 plains as have been formed since the deposition of the Windrow forma- 

 tion is much less than is generally thought necessary, the entire Tertiary 

 period does not seem too long, in the opinion of the writers, to have 

 brought about the existing stage of topography. It is time that in the 

 western mountains huge canyons have been carved since Middle Tertiary 

 time, but the conditions of climate and slope are there vastly different 

 from those which could ever have existed in Wisconsin ; so that the com- 

 parison is not fair. 



Summary 



The statements which follow summarize the general facts relating to 

 the Windrow formation and writers' conclusions based thereon : 



(1) The Windrow formation consists of gravels and conglomerates 

 with quartz and chert pebbles and of associated bog iron ores which in 

 part make up the cement of the conglomerates. Some sandstone and clay 

 are also present. 



(2) The formation rests unconformably on formations ranging from 

 the Huronian to the Devonian and possibly the Carboniferous. It is 

 overlain by loess and glacial drift. 



