152 THE GEOLOGY OF THE PURBECK HILLS. 



north of the fault, as at C, no sign of the fracture would be 

 apparent except at that juncture of the bedding planes 

 a'ong which the thrust took place, where pulverised debris 

 and slickenside polishing of the contiguous surfaces would 

 doubtless be evident. 



This description gives the simplest, and doubtless the 

 most probable, explanation of the phenomenon ; but it may 

 have been produced by less simple and obvious processes. 

 The researches of recent geologists in the Alps that great 

 example of the mightiest and most complex of strata dis- 

 tortions have thrown much light upon faults and overthrusts. 

 Lujeon's classical work on " The overfolding of the Alps," 

 and Dr. Suess in his " The Face of the Earth," Vol. I., have 

 shown that many apparently simple results were obtained by 

 far less simple processes than were formerly supposed. It is 

 quite possible that a more complicated folding of the upper 

 beds, perhaps resulting in reversed folds, may have taken 

 place at Ballard Cliff, before and during the thrusting south- 

 ward of the upper beds ; but if so the remains of these folds 

 would have been pushed up and over the others and denuded 

 away, so that no trace can now be found of them. This 

 would, however, depend upon the distance through which the 

 thrust took place. Presumably it was at least 400 ft., but 

 it may have been much more. It will be observed that while- 

 ordinary faults are examples of radial earth movements, and are 

 due, not to compression, but to the opposite an expansion of 

 earth surface, or rather a diminution of pressure below the 

 strata causing a wedge-shaped mass to drop down, or a radial 

 fracture to allow one side to sink thrust faults are due to 

 actual compression, and are tangential to the earth's surface. 

 In the Purbeck district we have many examples of the former, 

 as at Ulwell, and in the Purbeck beds in Durleston Bay, 

 but only one of the latter movement, which is comparatively 

 rare in this country, and due to far more violent forces than 

 the simple radial fault. 



Another point in the formation of this fault must be noted 

 which is the amount of denudation of the anticline which 



