BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 32, pp. 293-314 June 30, 1921 



WINDEOW FORMATION; AK UPLAND GEAVEL FORMATION 



OF THE DRIFTLESS AND ADJACEIfT AREAS OF 



THE UPPER MISSISSIPPI VALLEY ^ 



BY F. T. THWAITES AND W, H. TWENHOFEL 



{Presented orally before the Society December 30, 1919) 



CONTENTS 



Page 



Introduction 294 



Description of the Windrow formation 294 



Occurrences 296 



Wisconsin 296 



Iowa 299 



Minnesota 301 



Illinois 302 



Other States 302 



Fossils 302 



Origin of the Windrow formation 304 



Previous views 304 



Criteria of origin 304 



Composition and assortment of the materials 305 



Sedimentary structures 306 



Distribution of the deposits 306 



Nature of the underlying surface 306 



Conclusions as to origin 307 



Age of the Windrow formation 307 



Previous opinions 307 



Criteria relating to correlation 307 



Lithological similarity 308 



Topographic position 309 



Fossils 309 



Age of the underlying formations 310 



Relation to overlying deposits 310 



History since the advent of glaciation 310 



Summary 310 



Bibliography 312 



1 Manuscript received by tlie Secretary of tiie Society October 7, 1920. 



XX — Bull. Geol. Soc. Am,. Vol. 32, 1920 (293) 



