the west side of the map, the strike is east and west, but the dip varies from very 

 low in the north to steep in the central portion and back to low in the south. In 

 the central part of the area there is no consistency in the structural features in 

 that contours are pinched together in some places and widely spaced at others. 

 The east side shows a constantly changing dip and strike. Although it is quite 

 possible for such structural conditions to exist, it is not probable. 



In Figure 24-12, B shows the same control points contoured in a manner 

 that reveals two plunging anticlinal noses, two synclines, and a well-defined 

 terrace. This sheet was contoured, not to tie the widely separated control points 

 together in the simplest manner, but rather to develop the forms of any geologic 

 structures that might be suggested in the variations in the rate of dip or changes 

 in strike. In other words, this map bears the unmistakable marks of geologic 

 interpretation of the data. 



A knowledge of the general character and form of structures in the region 

 aids greatly in correctly interpreting the subsurface structure where the well 

 control is sparse. When the character of folding is known, an attempt should be 

 made to contour the widely scattered points so that the features shown bear out 

 the regional trends or tendencies. Often the subsurface geologist is called upon 

 to construct structure maps where little is known about the regional trends. 

 However, there are usually some clues in the datum elevations themselves. A 

 common but often erroneous assumption is that most of the higher wells are on 

 the highest parts of local structures, and most of the lower ones are on the lowest 

 points of the structures. When one is starting to contour the subsurface map, 

 it is better not to be too strictly constrained by the few scattered elevations 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 adhered to, the geologist 

 has considerable license, and he should endeavor to present a consistent and 

 feasible picture. 



Figure 24-12C illustrates the method by which the map B 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 1500 and 2100 feet on the west side and 1650 and 2040 feet 

 on the east. It is assumed that at these two localities the pairs of wells are aligned 

 somewhere 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. If one assumes a northwesterly strike 

 through points 2100 and 1500, the points 2415 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. 



466 



