414 Geology of the South- East of Dorsetshire. 



Art. III. Illustrations of the Geology of the South-East of Dorset- 

 shire. By the Rev. W. B. Clarke, A.M. F.G.S. 



No. 1. The vertical and curved Chalk Strata of Ballard 

 Head, near Swanwich. 



It is well known to the geological student, that an elevated 

 ridge of chalk runs through the peninsula of Purbeck, from 

 the vicinity of Weymouth on the west, to the high land of 

 Ballard Down on the east; and that, passing through the 

 Isle of Wight, it finally makes its appearance on the coast of 

 France, between Dieppe and Boulogne. It is interrupted at 

 Ballard Head, the Needles, and Culver Cliff, by sections, 

 which disclose either vertical or highly inclined strata, con- 

 taining broken and fragmentary flints, embedded in hard com- 

 pact chalk, of the nature of marble. 



This ridge is evidently a portion of the Isle of Wight 

 chalk basin, from which it has been separated by a crack, 

 longitudinally produced, and thence upheaved by some vast 

 action from below. 



The junction of the vertical and other portions of the 

 chalk basin are seen admirably displayed at Ballard Head 

 under circumstances of extreme interest. Thence the chalk 

 passes under the sea and the plastic clay series of the trough 

 of Poole, milking its appearance again between Wimborne 

 and Hinton Martell, at the distance of from twelve to fifteen 

 miles ; whence, ranging past the British station of Badbury 

 Camp, which forms a lofty eminence on the north of the 

 Stour, the connexion is kept up with the western boundary 

 of the basin at Blandford, and thence with the vicinity of Dor- 

 chester ; occasional pits of chalk occurring at Lytchett, &c, 

 on the south of the Stour. From Hinton Martell to Purbeck, 

 the surface of the chalk appears to be excavated into such 

 hollows as generally form the surface of that rock ; but, from 

 circumstances to be considered hereafter, it also appears that 

 movements have taken place in this district as well as in the 

 vertical Purbeck range, which have dislocated the overlying 

 deposits of the plastic clay. It may suffice to mention, that 

 the river Stour has found a passage at Blandford through the 

 chalk, and that at Spetsbury and Sturminster it flows imme- 

 diately at the foot of a cliff formed by a section of the chalk ; 

 and as, at the former place, there appears to be a series of 

 gentle terraces, rising one above the other, in the chalk above 

 the river, on the side of the hill from which the cliff is 

 formed, as there is also at Arlett Mill, near Corfe Castle, it 

 would seem that the chalk has been raised by successive im- 

 pulses of elevation, and that in the strain consequent upon 



