14 



INTRODUCTION. 



the weather, and particularly of frosts, while in others the appear- 

 ances may be caused by the roots of trees ; again, many cases seem 

 to require some more powerful disturbing cause, and are suggestive 

 of glacial action. It is of course necessary to distinguish these 

 from any original disturbances w^hich may have affected the beds. 

 (See Fig. i8.) 



Faults are disturbances in the strata whereby older rocks, or 

 older beds of the same formation, are brought into abrupt contact 

 with newer. They are fractures in the earth's crust attended by the 

 upheaval of one side of the ground and the downthrow of the 

 other ; the amount of the shifting is termed the throw, and the dip 

 or direction underground of the line of disturbance is its hade. 

 The hade is generally in the direction of the downthrow. Care 

 must be taken not to confound faults with unconformities, where a 

 newer set of strata abut on a cliff of older rocks against which they 



Fig. 2. — Cutting near Uphill (Bristol and Exeter Railway).^ 



b Carboniferous Limestone, faulted against, a. Lower Lias. 



were deposited. This may, however, be proved by the sedimentary 

 nature of the newer deposit, which would probably exhibit con- 

 glomeratic conditions. 



Faults generally approach a straight line in the direction 

 they take, although of course they are much modified in their 

 superficial outline by the shape of the ground and the amount of 

 the hade. The material filling a fault is sometimes termed the 

 'clog' or 'Fault rock'; this of course varies according to the nature 

 of the rocks affected, being either clay, rubble or breccia. In 

 many instances Faults have become the receptacles for mineral 

 matter. 



Reversed faults are so termed when the dislocated beds overlap 



^ This woodcut should represent the cutting on the eastern side of the railway, 

 but unfortunately tlie drawing was not reversed on the block. 



