The bank rests on Kimmeridge clay, which crops up on the bed of the bay off Chesilton one-third of a mile from low-water line in 8 fathoms of water. The shingle-bank extends out into the bay at the Chesilton end 200 yards, where the water is 7 fathoms deep. The bed of the bay between Abbotsbury and Chesilton generally is covered with sand, with a few scattered patches of gravel and shells. There are two ledges of rock running nearly parallel with the bank, one at about 5 miles from the shore in 10 fathoms, and the other at about 10 miles in 15 fathoms (see Plan of Lyme Bay, p. 130).

The mean inclination of the seaward face is described as being at the Portland end 1 in 5 from the top to a depth of 4 fathoms, 1 in 8 at 63 fathoms, and 1 in 30 at 8 fathoms, where the shingle terminates, and beyond which the bed of the sea consists of fine sand, clear of any shingle. At a short distance from the shore the pebbles are described as being encrusted with barnacles, the number of these and their size becoming larger as the depth of the water increases, showing that there is not much movement of the shingle under ordinary conditions far out from the shore. A subsequent examination after an unusually heavy gale showed the pebbles on the surface to be free from incrustation, which may have been due either to their movement by the waves, or by the older layer of pebbles being covered by those torn down from the bank during the gale.

At the Abbotsbury end the mean slope is described as being 1 in 7 to 3 fathoms, 1 in 11 to 5 fathoms, and 1 in 30 to 6 fathoms, where the shingle terminates.

During heavy on-shore gales the shingle is torn down from the bank, which then becomes much flatter than in calm weather, assuming a slope of about 1 in 9. After a continuance of calm weather or off-shore winds, the shingle gradually works back and the bank is restored to its normal condition; the slope then becomes much steeper, or from 1 in 3 to 1 in 2. At the level of spring tides the usual shelf is left. On the section, which is adapted from Sir J. Coode's paper, the dotted line shows the alteration in the face of the bank during a storm in 1852, the quantity of shingle then removed between Abbotsbury and Portland being calculated at 4,553,000 tons.

The pebbles consist principally of flints, these being of various colours, brown, black, and grey predominating, with a few red and other tints. Eight distinct varieties may be easily traced.

L

A considerable number are of greensand, chert, and also of the black flint underlying the oolite from the rubble cliff at Portland; whence also, and from the tips from the quarries, are derived some hard crystalline limestone and oolite pebbles.

There are also to be found a limited number of pebbles of sandstone and lias lime with pieces of carbonate of lime and Fuller's earth; also white and red quartz, light red quartzite, and occasionally jasper and a dark red stone which resembles a flint altered by heat. These latter have probably come from the raised beach at Portland Bill, where similar stones are found.

Some of the pebbles derived from the Portland cliffs may be traced as far as Abbotsbury, though the number is limited. At the eastern end of the bank, in the bight formed by its

junction with the Island of Portland, the beach consists entirely of black flints, chert, and hard oolite stone derived from the breaking up of the rock débris from the island. This part of the beach has increased since a landslip of the clay base, which occurred recently, when a large quantity of the overlying stone was thrown down on the beach.

The north end of the Island of Portland consists of a rubble cliff composed principally of angular black flints, chert, and hard oolite stone. From the wasting of these cliffs when the island was formed a considerable portion of the material of which the bank is composed has been derived, while at the present time it supplies what little addition is taking place.

The stones of which the bank is composed vary considerably in size, the largest being found at the Portland end. The graduation in size from the east to the west end of the bank is stated by Sir J. Coode to be so regular that a Portland fisherman,

if landed on the bank in the dark, would be able to tell his position by the size of the pebbles.

Dr. Vaughan Cornish grades the pebbles as follows.

The

third column, which is only an approximation, has been added.

With the exception of that part of the beach which lies at the foot of the bank in the bight near Chesilton, where the majority of the pebbles are not more than to inch in diameter, the stones at the Portland end vary from 9 to 2 inches in their longest diameter. There are a few large stones on the top, but the largest are collected together at about the level of high spring tides.

About half the pebbles on the top may be taken at about 2 inches diameter by 1 inch thick; about a quarter are about 4 inches diameter by 11⁄2 inches thick. Those at the lower part of the bank may be taken at about 1 inches diameter. The majority of the pebbles above high-water mark are flattened ovoids, showing that the principal agent in their movement has been a pushing rather than a rolling movement. Those below the water are more spherical in shape. With a heavy ground swell, some large pebbles, up to about 9 inches in diameter and containing 100 cubic inches, are thrown up from below low water, and a few of these are to be found on the top.

At Abbotsbury the pebbles vary from inch to 2 inches in diameter, a few being from 4 to 6 inches, the average being about

inch. They consist principally of flints of various colours, the majority being brown, with a fair proportion of yellow and black chert and pink quartzite.

West of Abbotsbury, and between there and Bridport, the bank, which abuts on the cliffs, consists principally of small spheroids of to 1 inch in diameter, and is of the same character as that on the east of Bridport Harbour, 8 miles distant. Below low water the shingle at Abbotsbury is generally large and spherical in form, while the pebbles below the water at Portland are smaller than those in the bank.

The bank faces nearly south-west, from which direction the heaviest gales come, frequently also accompanied by the highest tides. It is exposed to an unbroken fetch across the Atlantic. There is a depth of from 35 to 57 feet immediately in front of the bank at high water at the Portland end, and of 30 to 45 feet at the Abbotsbury end.

The rise of the water at spring tides is 101 feet, but the crest of the waves is stated to frequently run up the slope of the bank from 8 to 10 feet vertically, and during exceptionally heavy gales the waves, breaking on the bank, project the water and spray to a height of from 60 to 70 feet, or 20 to 30 feet higher than the top of the bank, which falling on the land on the inside has occasionally flooded the houses.

As an illustration of the force exerted by the waves in gales, the following instance is recorded in Sir John Coode's paper: During the height of a gale in November, 1824, a sloop of 100 tons burden, having on board a cargo of ordnance stores and heavy guns, was thrown on the Chesil Bank, and was carried on the top of a wave to its crest. This vessel was ultimately launched on the land side into Portland Roads. During the same storm the village of Chesilton was overwhelmed.

Considerable difference of opinion exists as to the source from which the pebbles composing this bank have been derived, and as to the direction in which the pebbles drift.

Sir John Coode, in his paper, expressed the opinion that the drift was from west to east, except at the east end, where the movement is in a northerly direction for one mile along the shore; and that "the only possible source from which the shingle of the Chesil Bank can have been derived is between Lyme Regis and Sidmouth, where there are chalk cliffs with numerous flints." He adduced, as a proof that the drift must have been from the west, the fact that a few of the pebbles of a dull red colour marked with red spots are of the same character as some he found near Budleigh Salterton, where also on the beach he found jasper pebbles. As already pointed out, however, these pebbles may have come from the raised beach at Portland.

The fact that the largest pebbles are to be found at the furthest distance from the source of supply was met by the statement that large pebbles are moved more readily than those which are smaller, and that the largest pebbles are always to be found on the leeward end of a beach.

Sir J. Prestwich, on the other hand, in his paper, stated that in his opinion the bank has been formed partly of pebbles derived from the south end of Portland and from the sea-bed westward thereof, but principally from the remains of an older beach; and that the movement of the material had been from south-east to north-west. In support of this theory, he cites the facts that the largest pebbles are found at the eastern end, and that they gradually diminish in size as the bank proceeds westward; that if the movement were eastward, there could not be that constant increase in the dimensions of the bank as it trends towards Portland without a discharge of the overloaded end into Portland Roads; and the common occurrence of pebbles of greensand, chert, and black flints from the Portland beds, with fragments of the harder limestone and oolites, all of which must have come from Portland, and some of which, although in very small specimens, are to be found as far north as Abbotsbury. Beyond these pebbles of local origin, Sir J. Prestwich traced the source of supply to a beach which formerly existed about 10 miles further seaward than the present shore-line, and remains of which are still to be found at Portland Bill, at Hopes Nose near Torquay, at Brixham, and in Start Bay. The remains of this beach at Portland Bill are situated 21 feet above the present beach, and extend over a length of 11 miles, ranging inland the sixth of a mile, and covering about 100 acres with a thickness varying from 4 to 10 feet. The pebbles of which this raised beach is composed vary in size from 2 to 4 inches longest diameter, with a few of larger dimensions, and consist chiefly of flints, greensand, chert, the harder oolites, and Portland black flint, with a not inconsiderable number of red and purple sandstone, red and grey quartzites, light and dark red porphyry, white and red quartz, and slate.

The pebbles from this old beach, Sir J. Prestwich considered, are scattered over the bed of the bay, and he points to certain places within the line of the original bank and the present shore from Portland to Tor Bay, where the soundings denote gravel to prove this, and states that these pebbles have been and are still thrown up by the waves on to the Chesil Bank.

The few isolated patches of gravel scattered over the bay, as indicated by the references on the Admiralty Chart, if indicative of a former beach, cannot be reckoned as a source of supply of pebbles to the beach. They are situated in deep water beyond