48 EARTH FEATURES AND THEIR MEANING 



along the steepest line in the bedding plane. The dip angle is 

 always referred to a horizontal plane, and hence vertical beds have 

 a dip of 90. The device for measuring this angle of dip, the 

 clinometer, is merely a simple pendulum which serves as an indi- 

 cator and is centered at the corner of a graduated quadrant. A 

 home-made variety is easily constructed from a square piece of 

 board and an attached paper quadrant (Fig. 28 c), but the geolo- 

 gist's compass is always provided with a clinometer attachment 

 to the dial. 



Since the strike is the intersection of the bedding plane with a 

 horizontal surface, and the dip is the intersection with that partic- 

 ular vertical plane which gives the steepest inclination, the strike 

 and dip are perpendicular to each other. To represent them 

 upon maps, it is more or less customary to use the so-called T 

 symbols, the top of the T giving the direction of the strike and the 

 shank that of the dip. If meridians are drawn upon the map, the 

 direction or attitude of the T can be found by the use of a simple 

 protractor; and when entered upon the map, the exact angle of 

 the strike may be supplied by a figure near the top of the T, and 

 the dip angle by a figure at the end of the shank. It is the custom, 

 also, to make the length of the shank inversely proportional to 

 the steepness of the dip, so that in a broad way the attitudes of 

 the strata may be taken in at a glance (Fig. 29). It is further of 

 M advantage to make the top of the 



T a double line, so that some 



so symbol or color may show the 



60 correlations of the different expo- 



sures. To illustrate, in Fig. 29, 

 the symbol marked a represents 

 an outcrop of limestone, the strike 

 of which is 50 east of north (N. 

 50 E.), and the dip of which is 

 FIG. 29. Diagram to show the use 45 southeast. In the same figure 

 of T symbols to indicate the dip and & represents a shale outcrop in hori- 



stnke of outcroppings. , . , . . 



zontal beds, which nave in conse- 

 quence a universal strike and a dip of 0. An exposure of limestone 

 in vertical beds which strike N. 60 E. is shown at c, etc. 



Measurement of the thickness of formations. When forma- 

 tions still lie in horizontal beds, we may sometimes learn their 



