320 Geological Society . 



the other of intra-Cretaceous (between Wealden and Gault) age. 

 The former includes the Isle of Purbeck fold (which is the continua- 

 tion of the Isle of Wight disturbance), the Ringstead fold, the 

 Chaldon and Ridgeway disturbances, and the Litton Cheney fault. 

 In the latter are placed the anticline of Osmington Mill, the syncline 

 of Upton, and a part of the anticline of Chaldon ; farther west the 

 Broadway anticline and Upway syncline, a fault at Abbotsbury, 

 and many other folds come into the same group. These earlier 

 movements led to the well-known unconformity at the base of the 

 Upper Cretaceous rocks. 



The Isle of Purbeck fold is accompanied by a large thrust-fault, 

 by which the uppermost zones of the Chalk have been pushed in a 

 vertical position under gently inclined lower zones. On the same 

 line of disturbance at Lul worth Cove, the squee/ing-out of plastic 

 strata from a part of the fold where compression has been great, and 

 the folding and packing away of such strata in a part where there was 

 a tendency to gape, is described. Farther west the same disturb- 

 ance is accompanied by inversion of a great thickness of beds, great 

 compression, with vertical crush-planes and nearly horizontal slide- 

 planes. The latter slope southwards, and the roof has moved north- 

 wards and upwards over the floor ; these slide-planes have accom- 

 panied the phenomena of inversion. 



The Ridgeway fold and fault resemble those of the Isle of Pur- 

 beck, but for some distance the thrust-plane has split, a part of it 

 cutting into the Oolitic floor on which the Upper Cretaceous rocks 

 were laid down, and causing a wedge of Oxford Clay, Cornbrash, 

 and Forest Marble to be thrust over Wealden, Purbeck, Portlandian, 

 and Kimmeridge Clay. 



The Litton Cheney fault is connected with an anticline in the 

 Chalk and Greensand which has been superimposed upon a syncline 

 in Kimmeridge Clay and Corallian. 



The intra-Cretaceous disturbances have been distinguished by the 

 fact that Upper Cretaceous rocks rest undisturbed upon them, the 

 difference in inclination amounting sometimes to 40°. This move- 

 ment may have commenced before the Lower Greensand was laid 

 down, but took place principally between the deposition of that 

 formation and the Gault. 



The features produced by the earlier movements were planed 

 down before the Gault was deposited, and have had no share in 

 producing the existing physical geography. The later movements, 

 on the other hand, have determined the lines of drainage and the 

 great physical features of the region. 



