Island

A Geological Voyage Around the Isle of Wight - 1847

by Gideon Algernon Mantell, Esq., LL.D, F.R.S

As during the summer months, excursions round the Island are almost daily made by steamers, an opportunity is afforded of obtaining a general view of the geology of the coast in the course of a few hours. To a person acquainted with the physical structure of the country, such a voyage is alike agreeable and instructive. The steamers leave Cowes or Ryde, and take an easterly or westerly course, according to the state of the tide.

Proceeding from Ryde to the eastward, we coast along the northern shore of the Island, which is almost everywhere covered with vegetation to the water's edge; a low bank, of cliff, of the freshwater eocene marls and limestones, being the only indication of its geological structure. St. Clare, the charming seat of Col. Vernon Harcourt, stands on an eminence of these deposits. Doubling the eastern point of St. Helen's, Bembridge Foreland appears, surmounted by the range of chalk hills that terminates at Culver Cliff. We cross the mouth of Brading Haven; the little town of Brading is seen nestling in a grove of elms at the foot of the chalk downs, which form the western boundary of the view. The upper horizontal freshwater strata on the northern extremity of Whitecliff Bay next appear, and a good view is soon obtained of the bay, bounded to the south by the magnificent chalk cliffs of the Culvers.

On reaching the extremity of Whitecliff Bay, the vertical section of the highly inclined chalk strata, which forms the eastern promontory of the Island, is finely displayed. The dip of the strata, varying from 50 to 70 deg, towards the north, is clearly marked by the layers of flint nodules, which are distinguishable a quarter of a mile from the shore. Culver Cliffs, which are the eastern termination of Bembridge Downs, are between 300 and 400 feet high, and about a quarter of a mile in breadth. When off the promontory, a very interesting geological view is obtained; at one glance we see the nearly vertical chalk, bounded on the right (north) by the eocene strata of Whitecliff, and on the left (south) by the greensand of Redcliff.

Upon doubling the headland, we enter Sandown Bay; the lower chalk, firestone, galt, and greensand, appearing in succession, like sloping bands of yellowish white, green, blue, and deep red; the latter denoting the ferruginous sands [iron-sand] of the lower group. Sandown Fort is now visible, and marks the low tract consisting of the Weald clay. The new church and little town of Sandown, are seen on the brow of the sand cliffs which form the western side of the bay, and extend towards Shanklin, retaining the same dark red ferruginous colour as those at Redcliff. The eastern termination of the southern range of chalk, forming the lofty downs above Shanklin, next appears, and seaward, the high but subordinate cliff of greensand at Dunnose Point. The dip of the strata in the cliffs on each side of Sandown Bay, and the anticlinal axis formed by the elevation of the Wealden, may be distinguished by means of a good telescope.

Off Dunnose, we lose sight of the inner range of chalk downs, and gain the region of the Undercliff, which from the sea presents, in the foreground, a line of fallen masses of rock and strata scattered along the sea-shore; and above, a verdant terrace covered with trees and shrubs, with here and there houses peering out from amid the foliage – the barrier of firestone capped with chalk, surmounting the terrace on the north, and bounding to view for a distance of six miles. The ledges of cherty firestone projecting like lines of masonry along the face of the inland cliff, may be distinguished, if the steamer keeps within the usual distance of the shore.

Nearing Blackgang, St. Catherine's appears towering above the western termination of the Undercliff. The Sandrock hotel is seen standing on a ledge high up the precipitous escarpment of the greensand, which here forms the sea-cliff, near the foot of the inland cliff of firestone; the latter rises to within 200 feet of the summit, which is nearly 800 feet above the level of the sea. The ruins of the tower of St. Catherine surmount the whole. The white chalk is seen forming the top of the downs, and appearing from the sea as an inconsiderable layer, immediately over Blackgang.

From Blackgang to Atherfield the cliffs are entirely composed of the greensand, and the general direction of the beds from east to west may be easily traced. The clays and sands along this part of the coast present a very curious appearance. The lower strata are generally dark, indeed, almost black, but the uppermost are of a light ferruginous colour; and where ever springs are thrown out by the clay partings, the cliffs are stained with broad streams of ochreous yellow. This appearance is correctly attributed by Sir Henry Englefield, to the action of the water that percolates from the surface of the soil through these strata, and issues out on the face of the cliff; the oxidation of the sands and clays is the result of the decomposition of the sulphuret into peroxide of iron. This change is often accompanied by the formation of crystals of sulphate of lime or selenite. The abundance of nodules of pyrites in these clay cliffs, as well as in those of the Wealden, is so great, that considerable quantities of this mineral are collected on the shore by the cottagers, for sale to the copperas manufacturers.

Approaching Atherfield, the undulated outline of the distant horizon on the north, denotes the re-appearance of the central range of chalk downs, which we lost sight of behind Dunnose Point, and is here seen extending in a south-westerly direction towards the southern shore. The station-house on Atherfield Point marks the locality where the junction takes place between the Wealden and the greensand.

The next interesting geological features of the coast, are the cliffs at Brook Point, but the steamers pass at so great a distance from the shore, that the general bearing of the strata only can be distinguished. In the long sweep exposed in these bays, the greatly elongated arch, formed by the anticilinal axis of the Wealden, may be distinctly made out in a clear state of atmosphere. Shalcombe Down, now forms the inland distance on the north; and as we near Compton Bay, the middle chalk range is seen extending to the shore, and the sea washes the base of the magnificent line of chalk hills from Afton Down to the Needles.

The position of the greensand, galt, and firestone, beneath the white chalk, in the cliffs west of Compton Chine, may be distinguished from a distance, owing to the contrast of colour presented by the upper and lower groups; as in Redcliff in Sandown Bay. Freshwater Bay is now gained, and the cliffs of the Mainbench, and of Scratchell's Bay, and the Needles, stand forth in all their sublimity. There is one point in Scratchell's Bay in which a magical effect is produced, by the sudden appearance of the richly coloured cliffs in Alum Bay, between the pure white pinnacles of chalk.

As we sail round the Needles, the distant Isle of Portland is seen on the western horizon, and on the north are the shores of Hampshire. The vertical strata of Alum Bay, and the freshwater series at Headon Hill, are now on our right; and we perceive that the structure of this north-western extremity of the Island, is the counterpart of the north-eastern promontory at Culver Cliff and Whitecliff Bay, which we sailed by in the earlier part of our voyage. Colwell and Thorness Bays are next passed, and we land at Yarmouth at the end of our excursion.