944 Subsurface Geologic Methods 



map is made for each of the principal lithologic classes: coarse elastics, 

 fine elastics, precipitates, and evaporites. The control points used in con- 

 touring are wells whose logs have been broken down according to these 

 lithologic classes and tabulated on the form shoAvn in figure 502. 



Figure 505 is the isolith map of coarse clastic rocks : i.e., sandstones, 

 arkoses, and conglomerates. The thicknesses shown represent the aggregate 

 thickness of all rocks falling in this classification, regardless of the thick- 

 ness of the individual beds in which they occur. Thus, of* two control 

 points, each indicating a thickness of 100 feet, one might be made up of 



Figure 506. Shale isolith map. 



two 50-foot members, the other of five 20-foot beds. They have the same 

 values on the isolith map. 



The sandstone isolith map shows not only the aggregate thicknesses 

 of the coarse elastics, but also the areas where certain kinds of sandstones 

 predominate. The shale isolith shows the aggregate thicknesses of the 

 shales and a further differentiation on the basis of color. Likewise, the 

 limestone and evaporite maps indicate areas of different types. There are 

 many other ways of differentiating within each of the lithologic groups, 

 and, depending on local conditions, it may be advantageous to set up other 

 main classes. These examples are given to indicate how lithologic facies 

 may be drawn on maps in both a qualitative and quantitative manner. 



It has been necessary here to differentiate areas by contrasting pat- 



