Subsurface Maps and Illustrations 957 



The contours tell a different story about the uplift in the northeast 

 quadrant. The extremely high rate of thinning along the flanks and the 

 abrupt termination of the axis show that the sediments were truncated and 

 probably extended across the nose at an earlier date; therefore, this uplift 

 is younger than the sediments. The restored thickness contours are shown 

 as dashed lines, which represent the thicknesses of the limestone series 

 before truncation. 



The relative ages of the two uplifts shown in figure 516 cannot be 

 determined from the behavior of isopach contours representing the flanking 

 sedimentary rocks. Therefore, a lithofacies map is compiled, as shown 

 in figure 517. It can be seen that the facies trends conform to the general 

 outline of the granite area in the southwest corner, but are unaffected by 

 the one in the northeast. From the lithofacies it can be assumed that the 

 uplift in the southwest predates the sedimentary series and contributed 

 clastic materials. The northeastern uplift obviously is younger than the 

 sediments. 



Figure 518 shows a restoration of thicknesses based on the isopach 

 and lithofacies maps cited above. In constructing this map, it is assumed 

 that the changes in thickness will correspond in a general manner to the 

 change in facies. The cross section at the bottom shows the restored 

 formations after uplift but prior to complete truncation. 



Coloring of Geologic Maps 



Many kinds of maps require coloring, and since the maps that are 

 made by commercial organizations are not reproduced in color by print- 

 ing processes, they must be colored by hand methods. When several copies 

 of a map are needed, the coloring of the prints may develop into a bur- 

 densome task. For this reason, it is desirable to be familiar with a 

 number of different methods in order to select the one which will best 

 satisfy the needs of the project. 



Maps are usually colored for practical reasons, not merely for em- 

 bellishment. The main purpose in using colors is to increase the legibility 

 so that significant features can be seen at a glance. Because of this fact, 

 color contrasts are more desirable than color harmony with low contrasts 

 of tone or hue. On the other hand, "clashing" colors should be avoided 

 if pleasing effects are desired. 



The U. S. Geological Survey and various state surveys use only cer- 

 tain colors for rocks of specific geologic ages : one color range for Paleo- 

 zoic rocks, another for Mesozoic, and so forth. Subdivisions within a main 

 group are designated by patterns, such as ruling, stippling, and others. 

 Since it is not practicable in most cases to use patterns on hand-colored 

 maps, the color standards set up by the Geological Survey cannot be 

 maintained. However, some consistency can be practiced, and if the 

 colors are well chosen, the result in the finished maps will justify the dis- 

 crimination exercised in the selections. 



