Vol. 51.] TERTIARY BATE IN DORSET. 561 



Still farther north a number of faults and folds in the Oolitic rocks 

 pass one after the other under the Chalk and Greensand without 

 displacing them in the slightest degree. They run about east and 

 west, and are nearly parallel to the later disturbances, but are 

 occasionally crossed obliquely by them, as at Litton Cheney. 



The age of these earlier disturbances can be fixed within narrow 

 limits. That they took place after the deposition of the Wealden 

 and before that of the Gault is proved by the great unconformity 

 between these rocks at Osmington and Chaldon. But their relation 

 to the Lower Greensand can only be inferred. This formation thins 

 gradually westwards in the Isle of Purbeck, and disappears altogether 

 between Mupe Bay and Lulworth Cove. Where last well exposed, 

 in Warbarrow Bay, it seems to pass down into the Wealden Beds, 

 the signs of erosion between the two noticed in the Isle of Wight 

 and elsewhere being unrecognizable. On the other hand, in the 

 same neighbourhood, the base of the Gault becomes increasingly 

 conglomeratic, with marked signs of erosion beneath it, suggesting 

 that the earth-movements, which were so active farther west, were 

 making themselves felt here also. The unconformity at the base of 

 the Gault, moreover, is not a local feature, but has been shown to 

 exist over a large part of England, including, as I am informed by 

 my colleague, Mr. Clement Reid, the Wealden area near Lewes. 



At the same time it must be borne in mind that the commence- 

 ment of the Lower Greensand marked a sweeping change in the 

 physical geography of this part of England, and that this formation 

 also rests unconformably upon Oolitic strata in Wiltshire and the 

 Midlands. It seems possible, therefore, that the movements com- 

 menced at the close of the Wealden, but that they were chiefly 

 accomplished at that of the Lower Greensand period. 



III. Effects of the Disturbances. 



The effects on the physical geography of the region of the two 

 systems of movements described above differ widely. The features due 

 to the earlier disturbances were planed off and smothered under Upper 

 Cretaceous rocks, and have had but little share in the existing con- 

 figuration. On the other hand, the later disturbances have directly 

 determined the drainage-system, the trend of the scarps, and, 

 indirectly, the form of the coast. It has been shown 1 that in the 

 London and Hampshire Basins the main lines of drainage follow the 

 synclines, while the tributaries flow at right angles to and off the 

 anticlines. Thus the Isle of Wight streams rose in the crest of an 

 anticline and flowed north to join the Erome, which follows a 

 syncline. In the Isle of Purbeck the water-parting held precisely 

 the same relation to the anticline, and the lines of drainage once 

 initiated have been maintained, although denudation has since 

 converted the anticlinal region into a valley, and left standing the 



1 'Geology of the Isle of Wight,' Mein. Geol. Sury. 2nd ed. 1889, chap. sv. 



