936 Subsurface Geologic Methods 



and two synclines, the axes of which are parallel and trend diagonally 

 across the block. Since the block is drawn in perspective, these parallel 

 lines must also be shown with the same degree of convergence; therefore, 

 it is necessary to select a new vanishing point on the horizon (2 in the 

 figure) . When the folds are constructed according to the convergence 

 of lines into this point, the perspective in the geologic features will be 

 the same as that in the block. 



The lower right diagram utilizes three vanishing points, as indicated 

 by the construction lines. Point 3 is the focus for the lines bounding the 

 fault plane, and 2 controls the lines cutting off the corner of the block. 



Figure 499 shows a structural map and a one-point perspective block 

 of the same area. Both have been shaded with a pencil to emphasize 

 the structural relief. A contour map shaded in this manner is called a 

 "shadow-graphic map." There are two methods for attaining the shadow 

 effect. The simplest is by hand shading, as mentioned. In both the map 

 and block it is assumed that the source of light is the upper-left-hand 

 corner. All structural surfaces facing this corner receive the greatest 

 amount of light, and those sloping toward the lower-right-hand corner 

 bear the heaviest shading. High points have high lighting; low areas, 

 dark shadows. 



A more cumbersome method consists in first soaking a contoured 

 map until it can be moulded into ridges and depressions conforming to 

 features shown by the contours. This may be done by working the sof- 

 tened map over a mass of wet papier-mache. When the modeling is com- 

 pleted, the shadow-graphic map is obtained by photographing from directly 

 above with one source of light, preferably from the upper-left-hand corner. 

 Obviously, this is a much more tedious method than shading the map with 

 a pencil. 



Facies Maps 



In stratigraphy it is axiomatic that the lithologies of any formation 

 change in some manner from one part of a basin to another. The degree 

 of variation and rate of change may be small or large, depending upon 

 the physical conditions of the basin and adjacent terrain, the chemistry 

 of the waters, the climate, and many other factors that determine the 

 type of sediment laid down. Therefore, a single formation may be a 

 coarse conglomerate at one locality, a sandstone or shale at another, a 

 limestone at a third, and all three lithologies at some places. Facies changes, 

 although perhaps not so drastic as the example given, are the usual and 

 normal condition; and it is often of the utmost importance to the strati- 

 grapher to determine the characters of the variations and where they occur 

 and then to have some means of showing the change on maps. 



A large number of methods have been devised by geologists to illus- 

 trate facies changes on maps. Some of these have been mentioned earlier, 



