176 THE ORIGIN OF THE VALE OP MARSHWOOD. 



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

 and west is not so apparent in them hecause 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 affect 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 determfhed, 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. 



Commencing with a traverse from west to east through Pilsdon 

 and Lewesdon, and starting the base-line of the Greensand at 

 Secktor, near Axminster, we find it there to be only about 320 feet 

 above sea-level, and thence it rises gradually eastward till it 

 reaches 580 feet at Birdsmoor Gate, 700 feet at the southern end 

 of Pilsdon, and about 770 feet on Lewesdon. Between Lewesdon 

 and Beaminster there are several faults breaking the Jurassic rocks, 

 but it is not certain that any of them displace the Cretaceous series, 

 and on Hackthorn Hill the base of the Greensand is close to the 

 500 feet contour. The distance from Lewesdon to this point is 

 four miles, and, assuming the fall to be gradual, it is a little, but 

 not much more, rapid than the rise from the west up to Lewesdon. 



Taking next a traverse through the southern outliers near the 

 coast, we find the Cretaceous base-line in Black Ven Cliff at about 

 320 feet above the sea. Thence it rises to about 350 feet in Stone 

 Barrow, and 400 and more on Golden Cap and Langdon Hill, and 

 finally to about 500 feet on Eype Down. Then comes a space of 

 four miles occupied by low ground near Bridport, and when 

 Greensand is next found on Shipton Hill its base has fallen 

 to 400 feet, sinking still lower eastward to 300 feet at Askerswell. 



