CHAPTER 11 



SUBSURFACE MAPS AND ILLUSTRATIONS 



JULIAN W. LOW 

 The term "subsurface map" may be somewhat confusing in that 

 nearly all types of both surface and subsurface maps display features 

 that are actually concealed at the surface of the ground by soils, alluvium, 

 and other types of overburden. The geologic formation or horizon con- 

 toured on a surface-structure map may lie beneath other formations over 

 the greater part of the map area, as shown in the cross section in figure 

 467. In this figure, points 1, 2, and 3 are outcrops of the datum horizon 

 where direct instrumental observations can be made. Point 4 is an out- 



FiGURE 467. Geologic cross section compiled from surface data. 



crop of bed C, stratigraphically below the datum, and 5 6, 7, and 8 are 

 outcrops of beds B and A, stratigraphically above the datum. Control 

 points a, b, c, d, and e are computed from instrumental observations 

 obtained on the outcrops. The position of the datum surface from 1 to 2 

 and 3 to e is restored above the actual surface of the ground, and at all 

 other places the datum is covered by other formations. 



Figure 468 is a subsurface cross section. Neither the datum nor the 

 two key beds A and B crop out at the surface. The control points, a, c, 

 and e are determined from the logs of wells which penetrate the datum 

 bed. Control points 6 and d are computed from the drilled points on 

 key bed A. The similarities in the section shown in figures 467 and 468 

 are obvious. Indirect methods are used in the construction of both; yet 

 one is a surface section and the other a subsurface. The principal differ- 

 ence in these sections and in surface and subsurface maps is that the 

 surface map or section is constructed from surface data; that is, from 

 outcrops. The subsurface section or map is constructed from data sup- 

 plied by wells that have penetrated recognizable formations. 



