246 GEOLOGY OF THE ISLE OF WIGHT. 



300 feet above the sea in the top of the cliff above Stetphill. 

 Similarly it is at a height of nearly 600 feet on the north side of 

 St. Martin's Down but falls to the south-south-east till it is 300 feet 

 above Ventnor. It is clear that the cliff from Blackgang to 

 Bonchurch does not give the line of strike, in as much as the base 

 of the Chalk falls from 500 feet at the former to 300 at the latter. 

 The true strike may be traced by drawing a line through the points 

 in each contour, at which it is intersected by the base of the 

 Chalk. Takingr the 600-foot contour first we find such a line 

 touches the northernmost point of the Chnlk on St. Catherme s, 

 Appuldurcorabe, and St. Martin's Downs. A similar line drawn 

 through the 500-foot contour is almost exactly parallel to this, 

 and at a distance of a little more than 1,000 yards from it, from 

 which it may be calculnted that the average dip amounts to 1 in 

 31 or a little less than 2°. Lastly a line drawn in the same way 

 with reference to the 400-foot contour runs approximately parallel 

 to the other two, but with a less decided bend, and therefore 

 more nearly parallel to the coast between Niton and Bonchurch. 



It will be noticed that the strike from Appuldurcombe west- 

 wards is south-west, curving round to the south-south-west in 

 St. Catherine's Hill, while from Appuldurcombe eastwards it is 

 only a little south of west. The difference is clearly due to 

 the influence of the Brixton and Sandown anticlines ; in St. 

 Catherine's we have the remains of an escarpment of Chalk which 

 must once have closed in this end of the Brixton area, while St. 

 Martin's Down forms part of a long escarpment which formerly 

 bounded the Sandown anticline on the south, eventually joining 

 itself on to the continuation of the Central Downs, as already 

 suggested. 



It has been remarked that there is evidence of the dip becoming 

 rather steeper in the Undercliff than it is in the Downs imme- 

 diately to the north. If to the three lines of contour above 

 enumerated we add a fourth, viz., the 300-foot contour, which the 

 base of the Chalk touches at Ventnor and Bonchurch, we shall 

 find that there is less distance between this and the 400-foot 

 contour, than there is between the 4(^0- and 600-foot contours, or 

 that, in other words, the gradient of the Chalk increases towards 

 the coast. 



The Brixton anticline first makes its influence perceptible in 

 the strike of St. Catherine's Down, as already mentioned. 

 Further west it becomes more marked, and the position of its 

 axis is indicated by the southward sweep of the Atherfield Clay 

 and Wealden Beds, but the axis itself lies just outside the coast 

 line. It seems to run about west-north-west, but curves round 

 to due west at Freshwater, and to W. 14° S. at the Needles. 

 Here it passes out to sea, but re-appears on the coast of Dorset, 

 everywhere throwing the beds into a nearly vertical position along 

 ita north side, and eventually bringing Oolitic rocks up to the 

 surface near Weymouth. 



It will be seen that the Brixton and Sandown anticlines form 

 two members of the great system of folds which have been 



