AGE 309 



TOPOGRAPHIC POSITION 



One of the strongest lines of evidence tending to show the great age of 

 the Windrow formation is its topographic position. Most of the known 

 occurrences are on the summits of the highest land in the vicinity. At 

 Devils Lake the gravels are over 500 feet above the surrounding country 

 and close to 900 feet higher than the rock bottom of the preglacial gorge 

 of the Wisconsin Eiver. The Waukon deposit is about 900 feet higher 

 than the rock bottom of the Mississippi Valley to the east, Avhile the 

 Windrow Bluff occurrence is over 800 feet above the rock bottoms of the 

 adjacent valleys. East of the last-named point the strata on which the 

 Windrow deposits rest have been entirely removed over thousands of 

 square miles, leaving the great central plain of Wisconsin. These topo- 

 graphic features took a long time to make and they seem to have been in 

 essentially their present form at the beginning of the Glacial period. The 

 harmony of the elevations above sealevel is an additional argument that 

 the different patches of gravel were deposited at essentially the same time. 



It has already been suggested, in the consideration of the origin of the 

 Windrow formation, that it was deposited by streams under conditions of 

 considerable relief. Such being the case, it follows that at the time of the 

 deposition of the gravels the present location of the deposits were the 

 lowest parts of the surface instead of the highest, as they are today. The 

 divides have migrated, so that what once was a valley bottom is now the 

 top of a ridge. Such has, doubtless, been brought about in part by the 

 resistance to erosion of the iron-oxide deposits, and an intervening period 

 of peneplanation is not a necessary consequence in the sequence of events. 

 The nature of the gravels suggests fairly wide valley bottoms with fairly 

 high divides. Hills doubtless rose to considerable heights along the 

 stream courses, and from their erosion the chert pebbles were derived. 

 The crevice at Seneca, Wisconsin, if formed at this time, indicates suffi- 

 cient relief to permit the formation of caves. If we imagine the country 

 as it appeared when the deposits were laid down and realize that not only 

 have the stream courses moved through the complete elimination of the 

 former divides and have entrenched themselves 800 to 900 feet below the 

 former levels of their valley bottoms, but that over thousands of square 

 miles they have totally removed everything to this thickness, so as to form 

 th^ great central plain of Wisconsin, then we obtain some conception of 

 the age of the Windrow formation. 



FOSSILS 



No fossils have been collected in any of the deposits which are contem- 

 poraneous therewith. The cherts have yielded fossils which range in age 



XXI — Bull. Geol. Soc. Am., Vol. 32, 1920 



