Subsurface Maps and Illustrations 931 



is extremely low. This profile is, in a sense, the structural datum of the 

 cross section. It is also the reference line from which all succeeding geolo- 

 gic boundaries are drawn. 



Now, from the Jurassic isopach map, thicknesses of the Jurassic along 

 the line of section are obtained where isopachous contours cross the line. 

 The thicknesses are plotted above the structural profile. The top of the 

 Jurassic is then drawn through these points. The same procedure is fol- 

 lowed with the Triassic and Permian isopach maps to draw the contacts 

 stratigraphically below the structural reference line. 



The surface profile can be taken from a topographic contour map. 



Figure 496, A, is a log map. It consists of well logs plotted to any 

 adaptable scale in their respective locations on the map. The example 

 shows the bases of the logs at the map location, but they may be plotted 

 with the tops of the logs or any selected continuous stratigraphic horizon 

 on the logs at the respective map points. When the logs are plotted, they 

 may be joined by formational correlation lines, as shown. 



Figure 496, B, is a panel map of the same area as that shown in 

 the log map. In the panel map it is possible to show changes in lithologic 

 facies, pinch-outs, and other stratigraphic conditions occurring between 

 the wells. 



As only the front panels are shown in their entirety, they should be 

 drawn first. In other words, the lowermost panels on the page are drawn, 

 then the next higher, and so on to the top of the drawing. Panels joining 

 wells along north and south lines are omitted, for they would appear only 

 as single lines on the map. 



The stratigraphic isometric projection is a special adaptation of the 

 panel map. Figure 497, ^, is a base map with a few principal streams 

 and well locations. Figure 497, B, is the isometric projection made from 

 this map. 



In order to construct an isometric projection it is necessary to have 

 the map contained in a rectangular grid, unless the land lines, as in the 

 case illustrated, provide such a grid. This grid, which may be drawn to 

 any scale, regardless of the scale of the map, serves only the temporary 

 purpose of placing map features correctly on the perspective drawing. 

 Instead of a grid, coordinate scales were used in the figure, and parallels 

 were drawn from the section lines. 



The isometric projection is referred to as a 20° or 30° projection, 

 depending on the perspective effect desired and the construction necessary 

 to produce this effect. An isometric projection is always less than 45°; 

 for a 45° projection is simply a map rotated 45° on the sheet, and there 

 is no foreshortening as in any perspective drawing. 



Referring again to figure 497, B, the upper corner of the projection, 

 is the northeast corner of the map. The east and west sides of the map are 

 drawn at an angle of 30° to the horizontal (in a 30° projection). Thus, 

 the northeast and southwest 90° angles of the map become 120°, and 



