58 IOWA STUDIES IN NATURAL HISTORY 



thin in all places, and, lying as it does between two resist- 

 ant formations, does not affect topography greatly. So far 

 as their effect on topography is concerned the Platteville 

 and Galena formations are a unit. The Alexandrian for- 

 mation varies in thickness and resistance and affects topo- 

 graphy in such a way as to be inseparable from the Niaga- 

 ran formation in some places, and from the Maquoketa in 

 others. There is an unconformity between the St. Peter 

 and Prairie du Chien formations, which causes both to vary 

 in thickness, but the sum of their thicknesses is nowhere 

 far from 300 feet\ The Cambrian formations are all weak 

 but vary slightly in resistance. Devonian and Pennsylva- 

 nian formations were perhaps deposited over part or all of 

 the region, but, if so, they have been eroded away. The 

 erosional history of the present surface started with the 

 final withdrawal of the Paleozoic seas and continued 

 through the Mesozoic and Cenozoic eras. 



STRUCTURE 



Although in most of the Driftlcss Area the strata dip in 

 a general southwesterly direction, there is a northeast- 

 southwest axis crossing the Mississippi river between La 

 Crosse, Wisconsin, and Winona, Minnesota, and passing 

 north of Sparta, north of vv^hich the beds dip northwesterly. 

 In other words, the structure is that of a low anticline with 

 its axis near the north edge of the region, plunging to the 

 southwest, with a long limb to the south and a relatively 

 shorter limb, so far as the Driftless Area is concerned, to 

 the north. Most of the work on which this paper is based 

 has been done south of the crest of the arch and the relation- 

 ships between topographic forms and structure are con- 

 sequently best known there. However, Mr. Patton and the 

 writer worked in the axial area and to some extent north 

 of the axis in Minnesota and Wisconsin in 1917. 



The average direction of dip of the formations south of 

 Winona and Sparta, as determined by .twenty-eight compu- 

 tations is S. 26° W., and the average amount 14.6 feet per 



1. Trowbridge, A. C, la. Acad. Sci., Vol. XXIV, pp. 177-182. 



