354 ARTHUR C. TROWBRIDGE 



divides between the North Fork of Messenger's Creek and Skillett's 

 Creek, between the South Fork of Messenger's Creek and an 

 unnamed south-flowing stream, between Pine Creek and Otter 

 Creek, etc. Because the remnants of this plain within the district 

 cut across the quartzite beds and lie on St. Peter sandstone, and 

 because most of the streams of the district find their sources on 

 the plain, the remnants are believed to represent parts of a plain 

 of erosion developed at this level, but now mostly dissected. 

 This interpretation is greatly strengthened by the finding of a 

 similar plain, bearing the same relation to the older plain, and 

 cutting across the edges of eroded formations, at many places out- 

 side the district under consideration, as, for instance, in the Rich- 

 land Center quadrangle, in the Sparta quadrangle, in the Lancaster 

 and Mineral Point quadrangles, in the eastern portions of the 

 Waukon and Elkader quadrangles in Wisconsin, in Jo Daviess 

 County, Illinois, in Allamakee, Clayton, and Dubuque counties in 

 Iowa, and at various places in southeastern Minnesota. 1 



If this plain is correctly interpreted, it records the following 

 steps in the history of the district in general and of Devils Lake 

 gap in particular. After the 1,400-1,480 foot plain was developed, 

 the district was uplifted relative to the sea-level of that time to an 

 amount of approximately 200 feet, the streams were rejuvenated, 

 and reached grade again at levels 200 feet lower than the first 

 plain. 



It was during this cycle of erosion that Devils Lake gap was 

 re-excavated and the Upper and Lower Narrows were formed, or 

 re-formed if they are of pre-Cambrian age. In the formation of 

 these post-Paleozoic gaps problems of stream adjustment are 

 involved. Martin 2 expresses disbelief in the two peneplains in this 

 part of the country, considers that all post-Niagaran and pre- Wis- 

 consin erosion took place in a single cycle, and believes that the 

 Devils Lake gap, and the two Baraboo gaps in the North Range, 



1 O. H. Hershey, Am. Geol., XX, 246-59; U. B. Hughes, Proc. Iowa Acad. Sci., 

 XXIII, 125-32; W. D. Shipton, Master's thesis, University of Iowa Library; 

 U. S. Grant and E. F. Burchard, Lancaster Mineral Point Folio, U.S. Geol. Surv., 

 p. 2; A. C. Trowbridge and E. W. Shaw, Bull. No. 26, III. Geol. Surv., pp. 136-44; 

 A. C. Trowbridge, Bull. Geo!. Soc. Am., XXVI, 76. 



2 Lawrence Martin, Bull. No. 36, Wis. Geol. and Nat. Hist. Surv., pp. 63-70 and 177. 



