LABORATORY MODELS FOR STUDY IN THE INTERPRETATION OF GEOLOGICAL MAPS

The laboratory models which have been described on page 63, and are used to represent outcrops in the study of geological maps, are shown in Fig. 489. The drum-shaped blocks serve to represent massive rocks which occur in irregularly shaped masses such as batholites and flows. The long, narrow strips are for intrusive rocks in the form of dikes, while the larger blocks provided with a swivel joint are used for outcrops of sedimentary rocks, and after adjustment they give the dip and strike of the exposure. The wing bolts used in their construction should be of bronze, because of the effect of iron upon the compass. For the same reason tables should not be placed near iron beams or columns. All these blocks can be made by an ordinary carpenter, and should be available in sufficient numbers to arrange problems like those of Figs. 47, 48, and 490. With a view to supplying suggestions for other problems of the same general nature, the three additional field maps of Fig. 491 have been introduced.

The list of questions given below is intended to indicate the nature of some of the problems which the student should be asked to solve in the preparation of each map. The numbers in parentheses refer to pages in this book where further information is given:—

Stratigraphical

1. Of the formations represented what ones are sedimentary and what igneous (Chap. IV, App. B)?

2. Which formations, if any, are separated by unconformities (51-53)?

3. What is the order of age of the sedimentary formations (65)?

4. What are the exposed thicknesses of each of these formations (48-49)?

5. Do any of these values represent full thickness of the formation, and if so, which ones?

6. What is the age in terms of the sedimentary formations of each of the igneous rock masses (65)?

7. Which igneous rocks, if any, occur in batholites (143, 441)? Which, if any, in dikes (140)?

Structural

8. What formations, if any, have monoclinal dip (42)?

9. Indicate upon the map by dashed lines the crests of all anticlines and the trough lines of synclines.

10. Indicate by arrows the direction of pitch of all plunging anticlines and synclines wherever disclosed by changes of dip and strike (43).

11. Indicate the approximate position of all faults whose position is disclosed (58-61), and, if possible, state which limb is the one downthrown.

12. Prepare suitable geological sections.

Reading Reference

William H. Hobbs. Apparatus for Instruction in Geography and Structural Geology. III. The Interpretation of Geologic Maps. School Science and Mathematics, vol. 9, 1909, pp. 644-653.