Subsurface Maps and Illustrations 



899 



gional structural maps may not truly represent the regional structure. The 

 departure of the contouring from that of a true structural picture is con- 

 trolled by the thickness of section eroded away, the relief of the uncon- 

 formity, and the average rate of regional dips. This discrepancy is greatest 

 where the dips are low and the relief on the unconformity is relatively 



Figure 472. Typical eroded anticline involving a major 

 unconformity in subsurface. 



high. It is purely a matter of chance that certain wells strike the horizon 

 of the unconformity at high points and others at low. Figure 475 shows a 

 case where four wells fail to reveal the presence of a structure because the 

 wells on the high part of the structure happened to strike locally low 

 points on the unconformity, while those structurally lower encountered 

 locally high points on the erosion surface. 



Sometimes a subsurface disconformity can be recognized by the types 

 or condition of the rocks in the drill cuttings; but very often the sample 

 work is not done with sufi&cient care to differentiate these materials, or the 

 diagnostic materials may be absent at the particular point where the well 



