64 



IOWA STUDIES IN NATURAL HISTORY 



from Iowa into Winona and Houston counties in Minnesota ; 

 from the Minnesota line in Iowa to Dubuque ; from the Ga- 

 lena quadrangle in Illinois to the area south of Dubuque in 

 Iowa. (7) If the several summit areas of a district be pro- 

 jected until they meet, a surface is constructed which has 

 a relief of something less than 200 feet. (8) If the con- 

 structed summit plains of the several districts of the Drift- 

 less Area, as explained in (7) be projected across inter- 

 vening areas where summit flats are wanting until they 

 meet, an almost reliefless, gently south-sloping plain results 

 which covers practically the whole Driftless Area. Because 

 the surface so I'econstructed is well represented at Dodge- 

 ville, because there is at Dodgeville a large area of upland 

 flat, and because, from Dodgeville the flat may be traced 

 with certainty in all directions, this uppermost plain, re- 



Fig. 13. All idealised north-south cecticn in tVe Driftless Area, showing the gen- 

 eral relation of the Dodgeville p'a'n ti the rock formations. The upland surfaces 

 are found in large areas on the resistant Prairie du Chien, Platteville. Ga'ena, and 

 Niagara formations, but are wanting on the re'atively nonresistant Cambrian 

 sandstones, St. Peter sandstone and Maqucketa shale. 



constructed by projecting the upland flats until they meet, 

 is hereafter called the Dodgeville plain. 



Topography and rock structures are so intimately and 

 fundamentally related that it is always unsafe to draw im- 

 portant conclusions from analysis of topography before 

 these relations are understood. It is therefore necessary 

 that a careful study be made of the rock formations on 

 which the Dodgeville plain lies, the structure of these for- 

 mations, and the relative attitudes of plain and formations, 

 before interpretation of the Dodgeville plain is attempted. 

 Failure to give due weight to these relationships seems to 

 be responsible for certain errors of the past. 



The Dodgeville plain is underlain by different rock for- 

 mations at different places. In the Baraboo district of Wis- 



