Geological and topographical maps, their interpretation and use, a handbook for the geologist and civil engineer . eed to draw a section along the line AB, and forthe sake of simplicity the surface of the ground will beassumed to be horizontal (Fig. 446). Now at the pointwhere the line AB crosses the outcrop of the fault thedotted bed is thrown against that marked with crosses,which is older. Applying the principle that the newer rock DISLOCATIONS 69 is on the downthrow side, Ave find that the downthrow istowards A. It is assumed here, for reasons which will presentlyappear, that the fault pos
Geological and topographical maps, their interpretation and use, a handbook for the geologist and civil engineer . eed to draw a section along the line AB, and forthe sake of simplicity the surface of the ground will beassumed to be horizontal (Fig. 446). Now at the pointwhere the line AB crosses the outcrop of the fault thedotted bed is thrown against that marked with crosses,which is older. Applying the principle that the newer rock DISLOCATIONS 69 is on the downthrow side, Ave find that the downthrow istowards A. It is assumed here, for reasons which will presentlyappear, that the fault possesses no hade, the fault-plane is vertical. It will also be necessary to find theamount of apparent dip along AB, as this will obviously beless than 40°, which is the true dip in the direction of thearrows. This may be obtained from the table on pp. 128,129, the use of which will be explained later. The positions of the various outcrops having beenmarked off to scale along AB (Fig. 446), the strata cannow be inserted, dipping at the appropriate angle, and the + + + +^ + + + + H-+ + + +^ <° o C ° n. («) (6) Fig. 44.—Plan and Section of a Normal Fault. section will be complete. On adding the parts removedby denudation, as indicated by the dotted lines, theamount of throw may be measured, being obviously equalto CD in the section. The above method is the one which will be found mostuseful in dealing with the fault-problems which occur inthe course of map-reading, but it is desirable to discuss thematter more fully in view of mining and engineeringproblems which may arise. It will be seen from Fig. 44a that one effect of thefault has been a lateral shifting of the outcrops, and theamount of this lateral shift is dependent upon three factors,namely: (1) the throw of the fault; (2) the dip of thestrata ; (3) the hade of the fault. 70 GEOLOGICAL AND TOPOGRAPHICAL MAPS In practice the problem which usually presents itselfis the determination of the amount of throw, and a
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