Vol. 5 I .] TERTIARY DATE IN DORSET. 551 



the Broadway anticline and the Upway syncline to the north-east 

 of it (see map, PL XVII.). Farther west, and also of the earlier 

 age, are a large fault at Abbotsbury, a syncline at Litton Cheney, 

 and a large fault at Chilcombe, together with other minor folds and 

 dislocations. 



The latter or post-Cretaceous group includes the great Isle of 

 Purbeck fold (which is the continuation of the Isle of Wight disturb- 

 ance), the Kingstead fold, the Chaldon and Ridgeway disturbances, 

 and lastly the Litton Cheney fault. 1 



As a consequence of the earlier set of movements, the Upper 

 Cretaceous rocks rest with a conspicuous unconformity upon the 

 upturned Jurassic and Wealden strata, coming into contact with 

 each in quick succession. On the top of this unconformity and of 

 the disturbances which caused it were superimposed the later 

 movements, leading to the strange complications which it is my 

 object to attempt to unravel. For this purpose it will be best to 

 eliminate first of all those folds which we know to belong to the 

 later group by the fact that they affect Upper Cretaceous rocks. 



(1) Post-Cretaceous Disturbances. 

 (a) The Isle of Purbeck Fault and Fold. 



This disturbance commences east of the Isle of Wight and dies 

 away in Weymouth Bay. 2 At Ballard Point, near Swanage, it 

 strikes the coast at right angles, giving rise to the remarkable 

 section figured by Webster. The section in PL XVIII. fig. 1 gives 

 the cliff-sections, in part reduced from photographs, and at the same 

 time shows the supposed structure below the sea-level and above 

 the present land-surface. In this and the other sections prominence 

 is given to the base of the Upper Cretaceous rocks by a dark line. 



The cliff has been so accurately described by Webster, Clarke, and 

 others that it will be unnecessary to repeat more than the salient 

 features. It will be noticed that the dip increases regularly on the 

 south side until it reaches verticality near and under the fault. 

 This, with a knowledge of the thickness of the various subdivisions, 

 renders it possible to restore the curve of the monocline upwards. 

 The fault itself exactly follows a bedding-plane in the curving 

 Chalk above, but cuts across the edges of the vertical strata 

 beneath ; it therefore originally ran in part of its course along, but 

 in other parts across the bedding. The position of the Belem- 

 nitella (B. mucronata and quadrata)-7jOnes proves that the move- 

 ment along the fracture was the reverse of that of a normal fault. 

 For the curving strata above the fracture must have been thrust 



1 A disturbance of post-Cretaceous age at Winterborne Abbas falls nearly 

 into the line of the Litton Cheney fault, but lies outside the area which 

 I have examined. 



2 In the ' Geology of the Isle of Wight,' Mem. Geol. Surv. 2nd ed. 1889, 

 p. 239, the anticline of the Isle of Wight is said to commence off the coast of 

 Devon. I take this opportunity of correcting that statement, for which I alone 

 was responsible. 



2k2 



