structed to determine the deformation of rocks under different kinds of stresses. 

 The models discussed earlier are similar to maps in that they are built as an aid 

 to visualizing geologic conditions as they exist today. In contrast, the working 

 models attempt to determine the series of events that bring about these con- 

 ditions. 



ILLUSTRATIONS While subsurface maps of all kinds show geo- 



logic conditions in essentially horizontal 

 planes, sections show the details of strati- 



and Projections 



grapny or structure in vertical planes. IN either 



the map nor the section, alone tells the whole geologic story; and, for this reason, 

 any exhaustive subsurface investigation of an area must use both sections and 

 maps. Sections fall into two general groups, structural and stratigraphic, al- 

 though there are types that incorporate both stratigraphic and structural features. 



Figure 24-49^4 is a structure section plotted on a natural scale. This type 

 of section does not attempt to show any structural features between the wells, 

 such as might be indicated on structural maps of the area, but does show the 

 difference in elevation of the formations at the wells. 



Figure 24-495 is a stratigraphic section along the same line, also plotted 

 on a natural scale. In a stratigraphic section one continuous stratigraphic horizon 

 is selected as a reference line or datum; this line is drawn straight across the 

 sheet. All other formational boundaries are referred to this line according to 

 the thicknesses of the formation. Thicknesses are usually indicated by numbers 

 starting with zero at the datum horizon. In the drawing cited above, the datum 

 is the top of the Permian, and thickness numbers start at this point. Figure 

 24-49C shows the same stratigraphic section, but with the vertical scale, two 

 times as large as the horizontal. 



Exaggeration of the vertical scale, in either a stratigraphic or structural 

 section, introduces certain distortions which may suggest nonexistent geologic 

 conditions. This is particularly true in straight-line sections like those shown 

 in Figure 24-49. Sometimes the vertical scale is exaggerated as much as 50 

 times. Structural sections with greatly exaggerated vertical scales tend to exhibit 

 thinning of the formations where dips are steep, giving the effect of attenuation 

 on the flanks of structures and thickening on the tops of anticlines, bottoms of 

 synclines, and at any other places where the dips are flat. Rates of dips are like- 

 wise increased according to the amount of exaggeration of the vertical scale. 



Despite the distortions resulting from exaggerating the vertical scale, there 

 are also some advantages. Local structures in areas of very low dips may not be 

 discernible in sections drawn to natural scale. Where dips are extremely low, 

 there is little distortion except in the rates of the dips; therefore, some increase 

 in the vertical scale is obviously desirable. 



509 



