Subsurface Maps and Illustrations 933 



on the projection is the same number of units along corresponding lines. 

 Any point on the map can be accurately located on the projection by 

 scaling along coordinates; and this procedure must be followed in locating 

 a suflicient number of control points, such as the confluences of streams, 

 road intersections, and well locations, to insure accurate sketching be- 

 tween these points. 



When the wells are located on the projection, logs are plotted at the 

 locations on a vertical scale adapted to the scale of the projection so 

 that the desired effect is attained. The isometric base is considered as a 

 level plane. Therefore, if it is assumed to be at sea level and the logs 

 are placed so that the sea-level point on the log is adjacent to the map 

 location, then the panels will represent the structure. Of course, if the 

 bottoms of the wells are above sea level, then the plane of the projection 

 should be at some datum plane high enough to cut all of the wells. 



The plane of the projection may also be considered a stratigraphic 

 datum, i.e., the top of a formation; and all the logs are then placed with 

 this horizon at the location point. Another common practice is to draw 

 subsurface geology entirely below the plane of the projection and surface 

 geology above. The principal objection to this method is that the drawing 

 may have neither a structural nor a stratigraphic datum. The panel pro- 

 jection in figure 497, B, is drawn with the plane at the top of a forma- 

 tion; this, therefore, is a stratigraphic projection. The panels have not been 

 completed in order to permit a clearer view of some of the map features. 



Block Diagrams and Other Illustrations 



It is of the utmost importance that the geologic concepts developed 

 as a result of studies in structure or stratigaphy be shown in some manner 

 that is most comprehensible to those who have only occasional contact 

 with the projects. Maps and cross sections sometimes fail in their purpose 

 of conveying to others certain complex geologic conditions, mainly for 

 the reason that each is two-dimensional, one in the horizontal plane, the 

 other in the vertical. Block diagrams effectively combine the features of 

 both maps and sections and are, therefore, an indispensable mode of il- 

 lustration. 



Geologic block diagrams are constructed according to certain prin- 

 ciples of projection and perspective. Space here does not permit going 

 into all the details of block diagrams: only the fundamental principles 

 necessary for constructing the simplest illustrations can be given. 



The two upper blocks in figure 498 are examples of the simplest pro- 

 jection. All opposing sides are parallel to each other, and, because of 

 this feature, they can readily be drawn with a drafting machine or a 

 triangle and straightedge. Distances along the front and back edges and 

 all lines parallel to these edges are drawn to the scale of the map. Dis- 

 tances along the sides and parallel to the sides may or may not be to the 



