Vol. 51.] TERTIAKY DATE IN DORSET. 555 



The Isle of Purbeck fault passes out to sea a mile farther west. 

 but cuts the cliff so obliquely that the section can be scarcely 

 interpreted. It can be seen, however, that the strata north of the 

 fault become horizontal within 30 yards, and then assume a south- 

 easterly dip, corresponding with that shown in the section. Here 

 we see the last of the great Isle of "Wight and Isle of Purbeck dis- 

 turbance, for no trace of it exists in the Weymouth promontory. 

 It must therefore die away in Weymouth Bay somewhat abruptly, 

 and not far from the point of its greatest intensity. 



(b) The Ringstead post-Cretaceous Movement. 



Disturbances belonging to both the earlier and later groups occur 

 near Kingstead. The one now referred to throws the Chalk and 

 Greensand up at angles of 40° to 75°, for a distance of less than a 

 mile at South Down Farm. It possesses the usual form of the folds 

 of this age, namely that of an arch with a steep plunge down to the 

 north, and, moreover, makes its appearance a little north of the 

 neighbourhood where the Isle of Purbeck fold dies away, a relation 

 that commonly holds between these disturbances. The Ringstead 

 post- Cretaceous disturbance lies very nearly in the line of the 

 Osmington Mill anticline, which, however, belongs to the earlier 

 group (see map, PI. XVII.). 



(c) The Chaldon and Ridgeway Fold and Overthrust. 

 (PI. XVII.) 



This great disturbance is second in importance only to that of 

 the Isle of Purbeck. It commences as an arch in the Upper Cre- 

 taceous rocks, which after rising gently from the south plunge 

 down north at 40° to 50°. The crest of the arch having been 

 denuded away, an irregularly oval inlier of the floor of Wealden 

 and Purbeck Rocks, on which the Upper Cretaceous strata were un- 

 conformably laid down, has been exposed. In these rocks also an 

 anticlinal arrangement prevails, which, however, has resulted from 

 a combination of the later with an earlier disturbance. 



The Gault and Greensand come into contact with different parts 

 of the Wealden and Purbeck group in the inlier in consequence of 

 the discordance that resulted from the earlier disturbance, and not 

 necessarily implying a fault. The fold keeps its course due west 

 for about 2 miles and then dies away in the Upper Chalk north of 

 Holworth. But almost immediately the great Ridgeway fold and 

 fault starts into existence about 400 yards farther south. Here 

 the evidence of displacement is clear, for different horizons of the 

 Chalk come into contact with the Purbeck rocks, the whole of 

 the Gault and Greensand being cut out. The Upper Cretaceous sub- 

 divisions all re-appear in normal sequence farther west, though for a 

 short distance only, and even here have probably moved more or 

 less over the underlying Oolitic strata. No such doubt exists west 

 of Sutton Poyntz, for the regular run of the beds is suddenly 



