GEOLOGY OP SOUTHWESTERN IOWA 523 



In the past the writer has not favored the giving of taxonomic 

 value of formations to limestone caproeks and shale partings 

 that will not average a yard in thickness. However Condra and 

 Bengtson in their recent report on the "The Pennsylvanian For- 

 mations of Southeastern Nebraska" have for the different lime- 

 stones applied the term ledges, and have named and numbered 

 twenty-six ledges found in the Pennsylvanian of their state. Al- 

 though this method introduces a multiplicity of names it serves 

 a most useful purpose and avoids much circumlocution in de- 

 scription. As certain of these ledges can readily be recognized 

 in Iowa the Nebraska names and numbers will be adopted by the 

 writer. 



In the Page county report Volume 11, Iowa Geological Sur- 

 vey, Doctor Calvin described and named the Tarkio formation, 

 and gave as a type section one three miles northeast of Norwich. 

 In this section two ledges of limestone are showai divided by 

 twelve feet of shale. In the Nebraska report the Tarkio is only 

 given the taxonomic rank of a ledge, and the name is applied to 

 the upper ledge, while the lower ledge is named the Preston 

 ledge. This is an injustice to the late Doctor Calvin, as this for- 

 mation was clearly delimited above and below and a type section 

 given. However, to avoid complications in descriptions the Ne- 

 braska method is reluctantly accepted. In Volume 19, Iowa Ge- 

 ological Survey, the writer extended the City Bluifs beds of 

 Broadhead up to the Tarkio as at that time there seemed to be no 

 constant horizon to divide upon. The records of the different 

 coal mining shafts included only one hundred feet of shale un- 

 derneath the Pleistocene, directly above the cap rock of the Nod- 

 away coal, and above this thick shale no outcrops were known in 

 Iowa showing the rest of this interval. Also the records of core 

 drilling are not in close agreement with one another, as to the 

 stratigraphy found above this shale. It is now kno'wii that the 

 one hundred feet immediately underlying the Tarkio contains 

 three limestone ledges ; including the Tarkio, the Nebraska geol- 

 ogists have named them commencing with the highest Tarkio, 

 Preston, Fargo, and Burlingame. The Rulo has not been found 

 in outcrop east of Missouri river. Condra and Bengtson have 

 divided the strata in the interval between the Nodaway coal and 

 the Tarkio into two formations, the upper ono hundred feet be- 

 ing named the Nemaha formation, with its lower limit at the 

 Rulo and its upper at the Tarkio. They retain the name City 



