908 Subsurface Geologic Methods 



tions on the sheet. In some places the actual structural elevations probably 

 exceed those of the highest wells and at others are less than those of the 

 lowest wells. As long as the technical requirements of contouring are ad- 

 hered to, the geologist has considerable license, and he should endeavor 

 to present a consistent and feasible picture. 



Figure 478, C illustrates the method by which the map, B, in that 

 figure was constructed. A cursory inspection of the datum values of 

 the wells shows that the regional strike is roughly east and west over 

 most of the area. A high rate of dip is shown between elevation 1,500 and 

 2,100 feet on the west side and 1,650 and 2,040 feet on the east. It is 

 assumed that at these two localities the pairs of wells are aligned some- 

 where near the direction of full dip, and that from these wells to other 

 nearby ones, where a much lower rate of dip is suggested, the directions 

 are along components of the true dip. Therefore, the contours are drawn 

 in such a way that a consistent rate of dip is maintained. By assuming 

 a northwesterly strike through points 2,100 and 1,500, the points 2,415 

 and 980 are contoured with negligible variation in either dip or strike. 

 A similar procedure is followed for each locality where the distribution 

 and relative datum elevations of the wells provide the best control on the 

 rate of the dip and the local direction of the strike. These areas are then 

 joined by extending certain contours with values nearest those of scattered 

 datum points located between the "detailed" areas, as shown by the dashed 

 lines in the figure. These lines form the skeleton of the map, and it is a 

 simple matter to fill in the remaining contours. 



It is sometimes possible correctly to infer the presence and magnitude 

 of a fault by working according to the principles just outlined. In A of 

 figure 479 an anomalous dip is indicated on the east flank of the anticlinal 

 nose. Both dips and strikes are erratic in the eastern one-third of the 

 map. Now, in B of figure 479 the same control points have been carefully 

 contoured, particular care having been taken to maintain constant rates of 

 dip and gradual changes in strike. Instead of contouring through the 

 anomalous values 1,220, 750, 950, and 515, without regard to the struc- 

 tural 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 pro- 

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

 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 con- 

 tra! 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 



