The Geological Story of the Isle of Wight

by J. Cecil Hughes

The lower course of the Medina is a submerged river valley, the tide flowing up to Newport. The river rises near Chale, and flows through a strip of alluvium, overgrown with marsh vegetation, known as "The Wilderness." This upper course of the Medina, from the absence of gravels or brick earth, has the appearance of a comparatively modern river. But the Medina has a further history. If you look at the map you will see branches of the Yar running south to north as transverse streams, but the main course is that of a lateral river. Look at the two chief sources of the Yar—the stream from near Whitwell and Niton, and that from the Wroxall valley. When they get down to the marshes near Rookley and Merston, they are not flowing at all in the direction of Sandown or Brading. They rather look as if they would flow along the marshy flat by Blackwater into the Medina. But the Yar cuts right across their course, and carries them off eastward to Sandown. When we look, we find a line of river valley with a strip of alluvium running up from the Medina at Blackwater in the direction of these two streams—a valley which the railway up the Yar valley from Sandown makes use of to get to Newport. There can be little doubt that these streams from Niton and Wroxall originally ran along this line into the Medina; but the Yar, cutting its course backward, has captured them, and diverted their course. They probably represent the main branches of the Medina in earlier times, the direction of flow from south-east to north-west instead of south to north being possibly due to the overlapping in the neighbourhood of Newport of the ends of the Brook and Sandown anticlines. The sheet of gravel on Blake Down belongs to this period of the river's history. The river must have diverted between the deposition of the Plateau Gravels and that of the Valley Gravels of the Yar. For the former follow the original valley, the latter the new course of the river.

We must now take a wider outlook, and see what became of our rivers after they had flowed across what is now the Isle of Wight from south to north. We have been speaking of times when the Island was of much greater extent than at present. Standing on the down above the Needles, and looking westward, we see on a clear day the Isle of Purbeck lying opposite, and we can see that the headland there is formed by white chalk cliffs like those beneath us. In front of them stand the Old Harry Rocks, answering to the Needles, both relics of a former extension of the land. In fact Purbeck is just like a continuation of the Isle of Wight. South of the Chalk lie Greensand and Wealden strata in Swanage Bay, and north towards Poole are Tertiaries. Clearly these strata were once continuous with those of the Isle of Wight. We must imagine the chalk downs of the Island continued as a long range across what is now sea, and on through Purbeck. A great Valley must have stretched from west to east, north of this line, along the course of the Frome, which runs through Dorset, and now enters the sea at Poole Harbour, on by Bournemouth, and along the present Solent Channel—a valley still much above sea level, not yet cut down by rivers and the sea—and down the centre of this valley a river must have flowed, which may be called the River Solent. It received as tributaries from the south the rivers of the Isle of Wight, and others from land since destroyed by the sea. There flowed into it from the north the waters of the Stour and Avon, and an old river which flowed down the line of what is now Southampton Water. Southampton Water looks like the valley of a large river, much larger than the present Test and Itchen. Its direction points to a river from the north west; and it has been shown by Mr. Clement Reid that the Salisbury rivers—Avon, Nadder, and Wily—at a former time, when they flowed far above their present level—continued their course into the valley of Southampton Water. For fragments of Purbeck rocks from the Vale of Wardour, west of Salisbury, have been found by him in gravels on high land near Bramshaw, carried right over the deep vale of the Avon in the direction of the Water. The lower Avon would originally be a tributary of the Solent River; and it enters the sea about mid-way between the Needles and the chalk cliffs of Purbeck, just opposite the point where we might suppose the sea would have first broken through the line of chalk downs. No doubt it broke through a gap made by the course of an old river from the south, as it is now breaking through the gap made by the old Yar at Freshwater. When the river Solent had been tapped at this point, the Avon just opposite would have acquired a much steeper flow, causing it to cut back at a faster rate, till it cut the course of the old river which ran by Salisbury to Southampton, and, having a steeper fall, diverted the upper waters of this river into its own channel.

Fig 8

Frost and rain and rivers cut down the valleys of the river system for hundreds of feet; the sea which had broken through the chalk range gradually cut away the south side of the main river valley from Purbeck to the Needles; and eventually the valley itself was submerged by a subsidence of the land, and the sea flowed between the Isle of Wight and the mainland.

A gravel of somewhat different character to the rest is the sheet of flint shingle at Bembridge Foreland. It forms a cliff of gravel about 25 feet high resting on Bembridge marls, and consists of large flints, with lines of smaller flints and sand showing current bedding, and also contains Greensand chert and sandstone, which must have been brought from some district beyond the Chalk. The shingle slopes to north-east. To the south-west it ends abruptly, the dividing line between shingle and marls running up steeply into the cliff. This evidently marks an old sea cliff in the marls, against which the gravel has been laid down.[16]

One or two comparatively recent deposits may be mentioned here. At the top of the cliff in Totland Bay, about 60 ft. above the sea, for a distance of 350 yards, is a lacustrine deposit, consisting in the main of a calcareous tufa deposited by springs flowing from the limestone of Headon Hill. The tufa contains black lines from vegetable matter, and numerous land and freshwater shells of present-day species—many species of Helix, especially H. nemoralis and H. rotundata, Cyclostoma elegans, Limnæa palustris, Pupa, Clausilia, Cyclas, and others.

On the top of Gore Cliff is a deposit of hard calcareous mud, reaching a thickness of about 9 feet, and forming a small vertical cliff above the slopes of chalk marl. It extends north a few yards beyond the chalk marl on to Lower Greensand. It has been formed by rainwash from a hill of chalk, which must once have existed to the south. The deposit contains numerous existing land-shells, especially Helix nemoralis and other species of Helix.

Between Atherfield and Chale at the top of the cliff is a large area of Blown Sand. The sand is blown up from the face of the cliff below. It reaches a thickness of 20 feet, and possibly more in places, and forms a line of sand dunes along the edge of the cliff. The upper part of Ladder Chine shows an interesting example of wind-erosion. The sand driven round it by the wind has worn it into a semi-circular hollow like a Roman theatre.