On the top of the cliffs between Ventnor and Bonchurch, at a point called Highport, is a stratum of flint gravel carried down from the top of the down. The shore here is strewn with large flints fallen from the gravel. The substance of many of the flints has undergone a remarkable change. Instead of black or dull grey flint it has become translucent agate, of splendid orange and purple colours, or has been changed into clear translucent chalcedony. In the agate the forms of fossil sponges can often be beautifully seen. The colours are due to iron-charged water percolating into the flint in the gravel bed, but further structural changes have altered the form of the silica; chalcedony having a structure of close crystalline fibres, revealed by polarized light: when variously stained and coloured, it is usually called agate. Many of these flints, when cut through and polished, are of great beauty. The main force of the tides along these shores is from west to east; and so there is a continual passage of pebbles on the shore in that direction. The flints in Sandown Bay have in the main travelled round from here; and towards the Culvers small handy specimens of agates and chalcedonies rounded by the waves may be collected.
Photo 2
Photo by J. Milman Brown, Shanklin. Scratchell's Bay—Highly Inclined Chalk Strata
The extensive downs in the centre of the Island are largely overspread with angular flint gravel similarly formed to that of St. Boniface. Of other beds of gravel, which have been washed down to a lower level by rivers or other agency we shall have more to say later.
The Chalk strata in the Isle of Wight are of great thickness. In the Culver Cliff there are some 400 feet of flintless Chalk (Lower and Middle Chalk), and then some 1,000 feet of chalk with flints. There is some variation in the thickness of the strata in different parts of the Island, and the amount of the Upper strata, which has been removed by denudation, varies considerably. The average thickness of the white chalk in the Island is about 1,350 feet.[9] Including the Lower Chalk, the maximum thickness of the Chalk strata is 1,630 ft.
The divisions of the chalk we have so far considered depend on the character of the rock: we must say a word about another way of dividing the strata. It is found that in the chalk, as in other strata, fossils change with every few feet of deposit. We may make a zoological division of the chalk by seeing how the fossils are distributed. The Chalk was first studied from this point of view by the great French geologist, M. Barrois, who divided it into zones, according to the nature of the animal life, the zones being called by the name of some fossil specially characteristic of a particular zone. More recently Dr. A. W. Rowe has made a very careful study of the zones of the White Chalk, and is now our chief authority on the subject. The strata have been grouped into zones as follows:—
Zones. Sub-Zones. | ||||||
| Upper Chalk. |  | Belemnitella mucronata. Actinocamax quadratus. | ||||
| ||||||
| Micraster cor-anguinum. Micraster cor-testudinarium. Holaster planus. | ||||||
