132 Dr. BucKLAND on the Formation of Valleys hy Elevation, 



dinate beds of chert ; its greatest elevation is nearly the same as that of the 

 northern chalk-escarpment^ neither the one nor the other being high enough 

 to interrupt the view of the southern escarpment, which is seen from all parts 

 of the vale of Newbury ; as will be better understood by referring to the sec- 

 tion, than from verbal description. 



This unusual position of a valley, composed of green-sand, within the appa- 

 rent area of a chalk-basin, derives illustration from some other similar instances 

 which I shall mention, of valleys produced by the disturbance and removal of 

 the superior strata, and which occur as we follow the edges of the chalk, 

 south-westward from Kingsclere, through Wilts and Dorset towards Wey- 

 mouth. These all present the same features of a valley, circumscribed on all 

 sides by an escarpment, whose component strata dip outwards in all direc- 

 tions from an anticlinal line, running along the central axis of the valley. The 

 first of them occurs near the villages of Ham and Shalbourne, about 5 miles 

 to the west of the valley of Kingsclere, and differs from it only in the circum- 

 stance of the escarpment, which is continuous on every side but the souths 

 being less elevated, and the area within being less extensive. The western 

 end of this valley is separated from the upper extremity of the vale of Pewsey 

 by only a narrow and low bar of chalk, stretching like a bridge across the 

 green-sand. Without this bar, the valley of Shalbourne would have been 

 confluent with the upper end of the vale of Pewsey. 



Another valley of the same kind occurs at Bower Chalk, about 10 miles to 

 the east of Shaftsbury. In this case also the strata consist of chalk resting upon 

 green-sand : the inclosing bar or isthmus of chalk is known by the name of 

 Cleve Hill, and without it the valley of Bower Chalk would have been con- 

 fluent with that of Alvedeston, which expands into the plain of Shaftsbury. 



A third and still more striking example may be seen at Poxwell near Osming- 

 ton, about 7 miles on the north-east of Weymouth, being nearly elliptical in 

 shape, and in size not much exceeding two or three times that of the Coliseum 

 at Rome. In the circumstances of the strata dipping in every direction from 

 a central axis, and of the valley being inclosed by a circumscribing escarp- 

 ment, it perfectly coincides with the four valleys in chalk and green-sand 

 above enumerated, and differs only in the fact of the strata inclosing it con- 

 sisting of Portland stone and Purbeck marble. 



The drainage of these valleys is generally effected by an aperture in one of 

 their lateral escarpments, and not at either extremity of their longer axis, as 

 would have happened had they been simply excavated by the sweeping force of 

 rapid water; and as it is utterly impossible to explain the origin of any valleys of 

 this description by denudation or alone, indeed without referring the present 



