THE ORIGIN OF THE VALE OF MARSHWOOD. 177 



Along this line of country then, as along the first, we seem to have 

 a gradual rise and fall in the height of the Cretaceous base-line. 



We will next trace the rise and fall of the same line from 

 north to south. This is hest shown on the western side of the 

 area. North of Thorncombe village the base of the Greensand lies 

 at about 450 feet, on the south side of that outlier in the same 

 latitude it is nearly 500, by Lambert's Castle it is about 600 feet ; 

 thence it falls to 550 feet below Coney's Castle and to 350 feet at 

 Stonebarrow, 2| miles further south. 



On the eastern side of the district the regularity of the rise is 

 broken by faults, but we find it rising to a maximum of 600 feet 

 on Drakenorth Hill, east of Poorton, falling thence rapidly both to 

 the north and to the south. Even where it is faulted up again on 

 Eggardon Hill it does not seem to get much above 400 feet, and 

 at Combe, near Litton Cheney, it is down to about 300 feet. 



We may fairly assume that the centre of the uplift, or pericline, 

 will be found by drawing lines between the points where the base- 

 line reaches its greatest height, namely, from Lambert's Castle to 

 Drakenorth Hill, and from Lewesdon to Eype Down. The 

 intersection of these lines occurs a little east of Monkswood above 

 the low ridge which forms the watershed between the Char and 

 the head branch of the Simene brook. We may take this spot 

 as the approximate centre of the pericline, which appears to have 

 an elliptical shape, its longest axis being from east to west and its 

 shortest from north to south. We can even form a good estimate 

 of the height to which the base of the Greensand reached over 

 this centre by prolonging the actual rise of the base-line in the 

 Pilsdon outlier, for at the north end of Blackdown, by Stony 

 Knap, it is at 500 feet, rising thence to 700 feet below the Pen, 

 and if this rise were continued south-eastward to the spot above- 

 mentioned it would bring the base to a height of 877 feet. 

 Assuming the thickness of the Greensand there to have been 

 180 feet, the Chalk wouh? have come in at about 1,150 feet. 



The relative levels of sea and land varied, of course, at different 

 epochs of Tertiary time, but we are quite warranted in believing 



