558 MR. A. STRAHAN ON OVERTHRT/STS OF [JS T OV. 1 895,. 



Beds, but has been thrust southwards relatively to them ; similarly 

 tbe Forest Marble, Cornbrash, and Oxford Clay of Bincombe have 

 been thrust southwards over Kimmeridge Clay, Portlandian, and 

 Purbeck. The theory involves much speculation, but its difficulties 

 have to be weighed against the still greater objections to the former 

 explanation. 



We have to suppose that the Upper Cretaceous rocks rest upon 

 Oxford Clay, Cornbrash, and Forest Marble at no great distance- 

 north of the Bidgeway fault. That this is not improbable can be 

 seen by referring to the map of the country between Bridport and 

 Dorchester, where in three localities the Upper Greensand rests on 

 Oxford Clay or some older rock. This results partly from the 

 general dip of the Oolites, but especially is due to the great intra- 

 Cretaceous fault at Abbotsbury. Evidence is not wanting that a 

 similar succession may occur under the Chalk of Bidgeway, for 

 north of the intra-Cretaceous syncline of Upway and Portisham, 

 which introduces a Middle Purbeck outlier, the strata begin to rise 

 sharply northwards, as if to an anticline, before they are cut 

 through by the Bidgeway fault. If the Abbotsbury fault, or any 

 analogous disturbance, continues, it will assist in bringing the 

 Oxford Clay to the required position. 



The Bidgeway disturbance was not only contemporaneous with 

 that of the Isle of Purbeck, but so closely resembles it in its effects 

 that we may infer that it also is a thrust, and not a normal fault. 

 The Chalk must therefore have been pushed southwards relatively 

 to the Oolitic rocks, and not faulted down against them, the 

 movement resembling that of the thrust at Ballard Point, where, 

 however, Chalk was thrust over Chalk. 



In following the disturbance we have seen that the fracture often 

 follows the base of the Upper Cretaceous rocks, but more fre- 

 quently runs a little above it. If it anywhere ran below that base,, 

 then part of the Oolitic platform on which those rocks rest would 

 be caught up and moved with them. This seems to have taken 

 place at Bidgeway and Bincombe. 



For some reason, not improbably connected with the softness and 

 ' greasiness ' of the Oxford Clay, the thrust seems to have locally 

 split, the principal plane running in and partly below that formation, 

 while a minor movement took place along or a little above the 

 visual horizon, namely the Upper Cretaceous base. After a separa- 

 tion extending for about 1| mile, the major thrust-plane rejoined 

 the minor, a wedge of Oolitic rocks thus being enclosed between the 

 two. The effect of the minor thrust was to place Greensand, Lower,. 

 Middle, and Upper Chalk in succession on Oolites ; and of the major 

 thrust to place Forest Marble, Cornbrash, and Oxford Clay on 

 Wealden, Purbeck, Portland, and Kimmeridge Clay. According to 

 this view, the Portland Stone escarpment west of Bincombe does 

 not end at the village, but should pass under the Cornbrash to join 

 the corresponding outcrop east of the village, a question that might 

 be settled by boring. The structure, viewed on this theory, closely 

 resembles that of known overthrusts in older rocks. Large over- 



