162 A. J. Juhes-Broicne — The Vale of MarsJmood. 



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 Simene escape to the 

 sea and by a dry gap or pass above Chideock. 



The hills on the north side of the Vale are higher than those ou 

 the south side, and they form the watershed dividing the valley 

 of the Axe from the valley of the Char, which occupies the greater 

 part of Marshwood Yale. Pilsdon Pen and Lewesdon Hill are 

 the highest hills in Dorset, Pilsdon being 907 feet and Lewesdon 

 894 feet, according to the latest Ordnance Survey Map. It is only 

 on the Blackdown Hills in Devon that the Upper Greensand reaches 

 a greater height than this, and these hills, although so much farther 

 west, do not attain to more than 930 feet. 



It is obvious, therefore, that there must be some local reason for 

 the great height to which the Grreensand reaches in Dorset, and 

 yet I am not aware that any geologist has accounted for the fact. 



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

 hills and great hollow of the Vale of Marshwood ai-e 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, or 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 pi'obable 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 

 from which the strata slope to north and south, the curve to east 

 and west is not so apparent in them because they had received 

 a decided easterly tilt before the Greensand was deposited on them. 

 Moreover, the Jurassic rocks are broken by many faults, and only 

 a few of these aifect the Cretaceous strata, for most of them seem 

 to date from the Purbeck and Wealden periods, when the above- 

 mentioned tilting was produced. 



It is therefore by the position and relative heights attained by 

 the base-line of the Gault and Greensand that the periclinal uplift 

 of this district can best be determined, and by transferring the 

 boundary-lines from the published Geological Survey map to the 

 six-inch county maps, we can easily trace the rise and fall of this 

 base-line. The boundary-lines on the old Geological Survey map 

 are not everywhere correct, but I have good reason to believe that 

 this particular boundary is sufficiently accurate for our purpose. 

 The map (Fig. 1) is based on the new one-inch Ordnance map, and 

 the geological lines have been partially revised, so that it is more 

 accurate than the old Geological Survey map. 



