66 



IOWA STUDIES IN NATURAL HISTORY 



levels. Wide areas of the plain lie on the Prairie du Chien, 

 Galena, Platteville and Niagara formations respectively. 

 The St. Peter sandstone underlies patches of the plain in 

 very small areas only, and the Maquoketa formation is not 

 known to form summit flats at all. In traveling from north 

 to south three distinct belts are crossed, in each of which 

 there are large upland remnants of the Dodgeville plain and 

 between which there is none. (Figs. 13 and 14) 



Assuming that the Dodgeville plain is a geometrical 

 plane, it is possible to ascertain its dip and strike if the 

 relative positions and altitudes of three points on the plane 

 forming a triangle are known. If the elevations of a cer- 

 tain stratigraphic horizon under the three points on the 

 plain can be ascertained it is also a simple matter to com- 

 pute the dip and strike of the strata and get the relative 

 directions and amounts of dip of plain and strata (Fig, 15). 



, Plain 1300* 

 Stratum 1240' 



O 



Plain 1250* 



Stratum 1000' -« Pkjin JZOO' 



B Stratum i loo' 



Fig. 15. Diagram sliowing liow tlie dip and strike of a i>lain and of a stratum can 

 be determined if the positions and altitudes of three points are l^nown. A, B, and C 

 are three jxjints on the surface plain at 1300, 1200, and 1250 feet respectively. On 

 the line AB there is a point D at which the altitude is the same as at C, 1250'. The 

 line CD is the strike of the ]ilain. The direction of dii> is obtained by constructing 

 line EB at right angles to CD through B. Reading the direction of EB the dip is 

 found to be S 14° E. From E to B the surface falls 50'. Scaling EB the amount 

 of dip of the plane is found to be 4.4' per mile. 



Knowing the elevations of a stratigi'aphic horizon under points A, B, and C to 

 be 1240, 1100, and 1000 feet respectively, locating the 1100' point on AC at F, BF 

 becomes the strike of the strata, the dip CG is S 37° W, and the amount of dip is 

 7' per mile. 



The results of 13 such computations, based on points south 

 of the axis of the arch are given in the following table. 



