402 



NATURE 



[August 23. 1900 



the numerous bays and coves have beaches on which are 

 accumulated shinsjle derived from the limestone, slate, green- 

 stone, and other rocks which surmount them, while in others 

 there is an absence of shingle and only sand is found. In the bay 

 lying between the headland of the Exe and that at Otterton 

 Point, the beach at Budleigh Salterton is strewn with quartzite 

 boulders and pebbles derived from a large bed contained in the 

 cliffs' bordering this part of the bay. These pebbles are of 

 a pink colour, some having marks on them like blood spots. 

 No stones of a similar character are found in the next bay, the 

 drift being stopped by some rock ledges which project out from 

 Otterton Point and form a natural groyne. The shingle in front 

 of Seaton consists almost entirely of the chert and flint derived 

 from the rocks at Beer Head. Beyond this, for several miles 

 there is no continuous bank of shingle, but accumulations are 

 to be found in the bight of the bays, the pebbles being derived 

 almost entirely from the gravel beds in the cliffs. The shingle 

 on the east side of Bridport Harbour is of a different character 

 from that on the west shore, and resembles in size and shape 

 that at Slapton, but the colour of the pebbles is different, 

 these consisting of flint instead of quartz. This small shingle 

 continues all along the coast, and up to the commencement of 

 the Chesil Bank. 



The drift of shingle along the shore only takes place above 

 the line of low water, and within the zone covered by the 

 horizontal range of the tide, and it does not accumulate below 

 the line of mean tide level, except where its progress is stopped 

 by encountering an obstruction, and when the quantity has 

 become so great as to extend out into deep water. When the 

 shingle encounters a river of any magnitude, it extends out in a 

 spit across the entrance to the estuary, causing the tides to be 

 diverted from their direct course, and to flow round the end ; 

 or else the channel becomes diverted from its course to the 

 leeward, and made to flow in a course parallel with the coast 

 for some considerable distance. Examples of this are found 

 along the coast here dealt with, in the Spits across the estuaries 

 of the Teign and the Exe, and that across Christc uirch Harbour, 

 and in the diversion of the streams at Seaton and (^harmouth. 



The Chesil Bank, which commences near Abbotsbury and 

 extends in a south-easterly direction to the island of Portland, 

 a distance of lof miles, has in its course diverted several small 

 streams, which now flow in a channel running parallel with the 

 bank. The width of this great mound of shingle is about 500 

 feet, and its height varies from 32 to 53 feet, its top being from 

 23 to 43 feet above high tides. 



Hurst Castle shingle bank extends out from the mainland at 

 the entrance to the Solent for i^ miles, terminating in a hook- 

 like formation on which stands Hurst Castle, erected in the 

 reign of Henry VHI. The bank slopes down across the Solent 

 for a distance of three miles, leaving only a deep narrow passage 

 between its foot and the Needles, in which is from 4 to 9 fathoms 

 of water. This shingle bank forms on its southern side a steep 

 submarine clifT from 20 to 70 feet in height, the face being very 

 steep and dropping almost suddenly from a dry bank to several 

 fathoms of water. So far as any record exists, this bank has not 

 increased or diminished in size or undergone any material altera- 

 tion since the castle was built. Eastward of the bank there is 

 no drift of shingle, the foreshore for several miles consisting of 

 a wide belt of alluvial deposit. 



Another lesson this stretch of coast appears to teach is, that 

 the theory which has generally received acceptance, that the 

 prevailing direction in which the shingle is drifted along any 

 given coast is always in tVie same direction as that of the pre- 

 vailing wind, is not founded on fact. This theory may be said 

 to have been settled on the facts brought forward in a paper 

 read at the Institution of Civil Engineers in 1853 on the Chesil 

 Bink, and the discussion which followed. 



A careful examination of the facts mentioned in that article 

 do not appear to warrant any such conclusion, but, on the 

 contrary, tends to disprove it. The local movement of shingle 

 along the south-west coast, and also along the other parts of the 

 seashore, are certainly not uniformly in accord with the direction 

 of the prevailing winds of this country. Approximately, the 

 wind in England blows for two-thirds of the year from the 

 south-west. On the east coast the general direction of the 

 drift is from north to south. On the south-west coast the 

 general direction is from west to east. From Beachy Head to 

 Dover it is north-east ; from Dover to the North Foreland, 

 northerly ; from the North Foreland to the mouth of the 

 Thames, westerly ; and north of the Thames south-west«rly. 



NO. 1608, VOL. 62] 



On the west coast, the drift is from south to north, up to the 

 middle of the Irish Channel ; and north of this, from south to 

 north ; and up the Bristol Channel from west to east. In all 

 these instances the movement is in the same direction as the 

 set of the flood-tide. 



Although this is the general direction of the drift, there are 

 numerous instances where, owing to the varying set of the tides, 

 the drift moves in three or four different directions within very 

 short ranges. Taking the example of Lyme Bay and the Chesil 

 Bank, the locality where the prevailing wind theory was estab- 

 lished, the facts as given by the author of the paper were : That 

 the prevailing direction of the wind on this part of the coast varied 

 between S.S.W. and S.W.^W., which is practically at right 

 angles to the Chesil Bank. If then the drift is in the direction 

 of the prevailing wind, this should lead to a north-easterly 

 movement. The bulk of the materials of which the bank is 

 composed are stated in the paper to have come from the cliffs 

 to the west of the bank, and therefore must have travelled in 

 a south-easterly direction. At the east end of the bank 

 the shingle derived from the debris of Portland moves 

 in a northerly direction. On the other side of the island of 

 Portland, in Weymouth Bay, the shingle is moved in a south- 

 westerly direction ; therefore, within the space of five or six 

 miles, the drift is in three opposite directions, not one of which 

 is in the direction of the prevailing winds, but all of which are 

 in the direction at which the flood-tide strikes the shore. 

 Further, it is correctly stated that the effect of winds from the 

 south-west tends to pull down the bank, which is restored 

 again to its normal condition during calms and north-east winds. 

 Along all tidal coasts it will be found that the general direc- 

 tion of the drift is the same as that of the flood-tide, and that 

 the beach material in bays is moved in the same direction as 

 that in which the wavelets due to the flowing tide break on the 

 beach. It is not contended that shingle is not drifted by waves 

 due to wind. On the contrary, it is a well-known fact that 

 shingle is frequently drifted, first in one direction and then in the 

 opposite, during the occurrence of gales blowing from different 

 quarters obliquely on the shore, and that the beaches are alter- 

 nately heaped up with material at one place and denuded at 

 another. This process, however, is only occasional and inter- 

 mittent, and beyond it there is a regular and continuous drift in 

 one given direction, the main course along the coast being in the 

 same direction as the flood-tide, the building up of shingle 

 banks being most active during calms and off-shore winds. 



After a long and careful investigation the writer has satisfied 

 himself that the building up of shingle banks and the regular 

 and continuous drift that takes place along the coast are due to 

 wave action caused by the flow and ebb of the tides. As the 

 great tidal wave moves along the deep water of the channels 

 surrounding the coast, its crest is in advance of the sides, which 

 encounter the friction of the shallower water. The swelling tide 

 therefore meets the shore at an angle oblique to its central 

 course. As the lateral flow of the swelling water comes in con- 

 tact with the shore it is checked and reflected back, causing a 

 series of small undulations or wavelets, which break at the margin 

 of the water. 



Although these waves are small, varying in height, according 

 to the condition of the tide and the slope of the beach, from 6 to 

 24 inches, they are constant, and never cease during the time 

 that the beach is covered by the tide, the number of them vary- 

 ing from ten to twenty a minute. Allowing a mean of fifteen, 

 this gives a total of 3600 impulses during the period that each 

 tide is acting on the shore. These wavelets are never absent 

 from the shore, except when absorbed by larger waves due to 

 wind. As the wavelets break, the water attains a horizontal 

 movement, and aided by the flood current lifts up and carries 

 forward coarse sand and pebbles in a movement oblique to the 

 coast Ime, and so gives them a slow but continuous forward 

 movement. 



The constant murmur that is heard on a shingle beach on days 

 when there is a total absence of wind, and when the sea is per- 

 fectly smooth and calm in the offing, attests the fact that the 

 pebbles on the face of a shingle beach are in constant movement. 

 These tidal wavelets are capable of moving and pushing up 

 the face of a shingle bank pebbles weighing from i or 2 ounces 

 up to 5 or 6 lbs. A calculation as to the mechanical power of 

 the water contained in an average sized wavelet shows that the 

 kinetic energy developed amounts to 165 foot lbs., which is 

 capable of lifting 9900 pebbles, each weighing 4 ounces, to a 

 height of I foot. W. H. Wheeler. 



