.S04 THWAITES AND TWENHOFEL THE WINDROW FORMATION 



Howell lists a considerable number of fossils from the iron ore at 

 Waukon which were derived from the rock immediately below (11). In 

 chert pebbles collected by the senior author the following have been 

 identified : 



Favosites sp., Silurian or Devonian. 

 Atrypa reticularis Linn., Silurian. 



The chert pebbles in the Cretaceous conglomerate contain forms sim- 

 ilar to those of the recognized Windrow formation. Bain (4) lists the 

 following as identified by Calvin. The senior author has collected fossils 

 from the same horizon : 



Cijathophylluni sp. 



Bryozoa, Devonian? 



Bpirifer of. /S. eudora Hall, Niagaran. 



Favosites favosa Goldfuss, Niagaran. 



Favosites hisingeri Edwards and Haime. 



Favosites cf. F. hispidus Rominger, Niagaran. 



Streptelasma sp. 



Streptclasma spongaxis Rominger, Niagaran. 



Ptychophylluni expansuni Owen, Niagaran. 



Zaphrentis stokesi Edwards and Haime, Niagaran. 



Origin of the Windrow Formation 



previous views 



Strong considered the gravels near Seneca (25) to have been deposited 

 by ocean currents and cemented by iron oxide coming from 'ferruginous 

 springs or some other cause operating to precipitate iron from water. 

 Irving (14) considered that the gravels and pot4ioles at Devils Lake 

 record a higher level of the preglacial Wisconsin River. Chamberlin and 

 Salisbury ascribe the conglomerate at Seneca either to the marine Cre- 

 taceous or to the older drift (9). Salisbury first recognized the wide 

 extent of the upland gravels and correlated them with the high-level 

 gravels to the south, but did not make clear his views as to their origin 

 (20, 21). Howell ascribed the deposits at Waukon, Iowa, to accumula- 

 tion in a bog on a peneplain (12), a view earlier stated by Calvin for the 

 same deposit (6). Trowbridge, Shipton, and Hughes considered that 

 the gravels originated as stream deposits on a Tertiary peneplain (13, 22, 

 24, 26, 27, 28). 



CRITERIA OF ORIGIN 



Criteria relating to the origin of the Windrow formation may be di- 

 vided into four groups, as follows : composition and assortment of ma- 



