THE ORIGIN OF THE VALE OF MARSHWOOD. 175 



Pen is one, are clear and certain proofs that a continuous sheet of 

 the same material once overspread the whole area. 



As it is intended that the Field Cluh should visit Pilsdon this 

 year, it seems a fit occasion to discuss the geology of the district in 

 relation to its present physical features, to explain the isolation and 

 great elevation of Pilsdon Pen and the other Greensand outliers, 

 and to account for the origin of the Vale of Marshwood. 



It is well known that Pilsdon and Lewesdon Hills are the 

 highest summits in Dorset, and they are also the highest Upper 

 Greensand hills in England, Pilsdon being 907 feet and Lewesdon 

 894 according to the Ordnance Survey of 1892. They are situate 

 on the watershed that divides the Valley of the Axe from the 

 Valley of the Char, which occupies the greater part of the Vale 

 of Marshwood. This "vale " is a broad plain, most of which lies 

 between 100 and 200 feet above the sea ; its floor consists of the 

 clays of the Lower and Middle Lias, and it is encircled by steep 

 slopes formed by the yellow micaceous sands of the Marlstone 

 Beds, the cincture of the hills being only broken on the south by 

 the gaps through which the rivers Char and Simenc escape to the sea. 



It may seem a paradox to say that the height of the Greensand 

 hills and great hollow of the Vale of Marshwood are due to one 

 and the same cause, yet it is true that they are so closely related 

 to one another that the history of the one involves the history of 

 the other. This history begins with the uplift of the strata which 

 took place in Miocene or Pliocene times and bent the beds into a 

 dome-shaped elevation, which is often called a pericline, i.e., an area 

 in which the strata are bent up so as to dip outwards in all 

 directions from a central spot or axis. 



I propose to ascertain the probable whereabouts of this centre 

 by a consideration of the levels through which the base of the 

 Upper Greensand passes in East Devon and West Dorset. It 

 might be thought that this spot could be found more easily by 

 examining the arrangement of the Jurassic rocks on the borders of 

 the Vale of Marshwood, but though these undoubtedly show the 

 existence of an anticlinal axis running in an east and west direction 



