788 STUDIES FOR STUDENTS 



walls. A symbol like the cut will be found useful in recording 

 them upon the map. The figure at the end of the trough line of 

 the valley indicates the bearing in degrees to the east or west of 

 north, as the case may be. The slope lines on either side 

 pointed toward the trough are like Dana's dip symbols adjusted 

 in length to indicate the steepness of the 

 slope, a large slope line indicating a gentler 

 slope. The figures used in connection with 

 these lines indicate the height in feet of the 

 walls on either side, an estimate being 

 expressed by the ± sign. Very large and 

 important valleys are often omitted from 

 topographic map^s the topography of which is sketched, because 

 hidden from points at which the sketches were taken. 



Nature of contact surface of formations. — Contacts of forma- 

 tions wherever exposed should be carefully scrutinized, not only 

 to discover evidence of conformity or unconformity, but for indi- 

 cation of slipping, thrusting or faulting. The contact plane of 

 formations for two reasons is likely to be a locus of displace- 

 ment. In the first place contacts are likely to be planes of 

 weakness where a maximum of movement has occurred ; and in 

 the second place, displacement, whether by normal faulting or 

 thrusting, produces new contacts. It is, therefore, to be noted 

 at the contact of sedimentary formations : first, whether the 

 plane of contact is the natural result of sedimentation, and 

 whether conformable or not; second, whether the deformation 

 of the contact plane, if present, is such as may be explained by 

 accommodation of layers in flexuring, or must be accounted for 

 by the more violent processes of normal faulting or thrust. 

 With good exposures the direction of the plane of displacement 

 will probably allow of a decision between thrusting and normal 

 faulting. If insufficiently exposed the nature of the folding and 

 the general character of the deformation within the area may 

 afford a clue. Both normal faulting and thrusting may, how- 

 ever, have occurred at the same contact. 



