68 



Prof. E. Hull — Submergence of the Isle of Wight. 



is almost perfectly level, and it breaks off in a well-marked scarp 

 overlooking the valley of the Medina at a level of (about) 350 feet 

 above the stream ; the upper surface of the plateau is about 4U3 feet 

 above O.D. 1 The terrace is formed of rudely stratified beds of gravel 

 and lenticular bands of yellow sand and loam ; current lamination 

 being slightly apparent in some places; the whole resting on the 

 Lower Greensand. The pebbles, which are generally well rounded 

 and waterworn, consist mainly of flint; but there are a few of grit, 

 ironstone, and detritus of Lower Greensand. The whole deposit 

 is about 50 feet thick, of which about half the thickness is laid 

 open to view in the large gravel-pit above Blackwater station. On 

 reaching the surface of the plateau one perceives, at a distance of 

 half a mile, the rounded ridge of the Chalk rising slightly ahove it, 

 and we easily recognize that we are walking on the floor of the old 

 sea-bed which terminated against the uprising ridge of the Chalk, 

 from which the materials were largely derived. 



Headon Hill. — This remarkable outlier is described by Mr. Bristow 2 

 and other authors; and its relations to the terrace of St. George's 

 Down can scarcely be mistaken. In composition, in altitude, and in 

 its relations to the Chalk ridge, the conditions are almost identical, 

 except that Headon lies to the north of the ridge, St. George's 

 Down to the south of it. The distance between the two is ahout 

 17 miles, and the gravel-beds of Headon rest on the Upper 

 Tertiary strata of the Isle of Wight series. Like the St. George's 

 beds, the Chalk ridge formed the limit in one direction to the range 

 of the gravel-bed ; towards the north and west it breaks off in 

 the well-known cliff of Alum Bay. The height of the summit is 

 (according to Bristow) 370 feet ; my aneroid measurement makes 

 the height 388 feet ; in either case the level approximates to that of 

 St. George's Down; and the important point remains to be noticed, 

 that once we reach the 400 feet level, and ascend the ridge of the 

 Chalk clowns, these beds of stratified gravel cease, and are replaced 

 by local flint-gravel derived from the underlying Chalk, and left 

 behind after the calcareous matter had been dissolved away by 

 suba^rial agencies. 3 



It is unnecessary to go into further details in order to show that 

 after the upheaval and denudation of the Tertiary beds of the Isle 

 of Wight the district was submerged, beds of sand and gravel, 

 derived mainly from the waste of the Cretaceous rocks, were laid 

 down, and the whole were subsequently elevated to an extent of 

 about 400 feet, as indicated by the levels of Headon Hill and St. 

 George's Down. The relations of the gravel-beds to the older rocks 

 enable us to fix definitely the amount of this elevation ; and this is 

 the point to which I wish particularly to call attention to in this 

 short communication. 



1 By aneroid measurement. The gravel of St. George's Down has been described 

 by Mr. Codrington. 



2 "Geol. of the Isle of Wight " : Mem. Geol. Survey (1862), p. 104. 



3 The two varieties of gravel are very well shown on the Survey map already 

 referred to. 



