and strikes are erratic in trie eastern one-third of the map. Now, in B of Figure 

 24-13 the same control points have been carefully contoured, particular care 

 having been taken to maintain constant rates of dip locally and gradual changes 

 in strike. Instead of contouring through the anomalous values 1220, 750, 950, 

 and 515, without regard to the structural conditions thus developed, a more 

 methodical plan was used. In this case the contouring should be developed 

 from the east and west edges of the area toward the locality of erratic elevations, 

 each side being treated independently of the other. It is essential that the spacing 

 of the contours depict a consistent structural condition, which can usually be 

 attained only by trial and error in drawing the contours. When this procedure 

 is followed, the presence of a fault with a throw between 200 and 400 feet is 

 clearly indicated. 



The preceding examples are given to illustrate the importance of developing 

 the subsurface structure with a few sparsely distributed control wells. In order 

 to accomplish this result, it is necessary to think geologically — to visualize the 

 various structural forms as if they were solid models and to contour these forms 

 in a manner that will withstand critical geologic analysis. As mentioned earlier, 

 it is a very elementary task to contour a sheet technically correct. The geologist 

 must go further — his map must be technically correct and geologically feasible. 



Isopach Maps 



An isopach is a line of equal stratigraphic thickness, and an isopach (or 

 isopachous) map is one which shows by means of isopachs the variations in 

 stratigraphic thickness of a stratum, formation, or group of formations. As in 

 the case of structural maps, isopach maps may be either surface or subsurface, 

 depending upon the class of data used in their construction. The subsurface 

 isopach map is based primarily upon formation thicknesses determined from well 

 cuttings or electric logs. 



Although isopachs must be drawn to agree with thicknesses plotted on the 

 map, their spacing and the nature of thickening and thinning may be guided 

 largely by other known facts concerning the source of sediments, their relative 

 rates of deposition, truncation, and so forth. An isopach map drawn strictly to 

 the numerical values and without regard to the geologic reasons for thickening 

 and thinning of the formations is likely to present a picture difficult to reconcile 

 with other geologic facts. 



The isopach map A in Figure 24-14 is drawn according to thicknesses shown 

 at the wells. No consideration is given to the reasons for the body of thicker 

 sediments in the central part of the area or the changes in rate of thickening 

 in the two regions where the formations are absent. The same points are con- 

 toured in B of Figure 24-14 with much better effect. The close spacing of the 

 contours from zero to 200 feet on the west side of the map indicates the area 



468 



