Imágenes de páginas
PDF
EPUB

extend out from the shore on the south side from 6 to 7 miles in a triangular form, the base being 14 miles long. The depth at low water over this area is only from 1 to 2% fathoms. In the middle of the entrance to the estuary are the two banks of D'Amford and Ratier, consisting of a base of clay covered with stones and large shingle, which are dry up to half-flood.

The cliffs on the north side of the estuary, which consist of chalk, stand back from the coast-line, and at their foot for 4 miles is a broad alluvial flat on which are situated Havre and Harfleur. The beach is covered with débris from the chalk cliffs, and at Hoc Point there is a large collection of shingle. The cliffs along this shore are continually wasting. At Cape la Hève the loss is estimated at a yard in a year, and during a tempest in 1862 between 15 and 16 yards were eroded. The lighthouses have twice had to be set back. In the twelfth century the Church of St. Addresse, on the top of the cliff, was nearly a mile from the coast-line on a spot now occupied by sand-banks.

The drift material along the coast north of the Seine sets in two opposite directions from Cape Antifer with the set of the tidal current—the main set of the flood tide and drift going northerly from the cape, and a counter-tidal current being deflected by the headland and drifting the shingle southward, which, working round the point at Cape la Hève, sets along the foreshore of the Seine to the Point of Hoc.

From Cape la Hève, which is 300 feet high, chalk cliffs containing flints extend along the coast in an almost unbroken line to Ault, 6 miles west of the river Somme, a distance of 142 miles, the average height being 200 feet. The annual loss of these cliffs has been estimated at a foot a year, or 5} million cubic yards. There are generally sixty seams of flints in the cliffs, varying in thickness from 3 to 12 inches, the average being taken at 3 inches, making a total thickness of the beds of 15 feet, or seven per cent. of the chalk. Taking the annual loss of the cliffs at 1 foot, the quantity of flints thrown on the beach at their foot would be half a million cubic yards a year. These flints soon get broken and ground into pebbles by the waves, leaving one-third of the original quantity on the beach in the form of shingle and twothirds as sand.?

It is estimated that the quantity of shingle drifted along the
1 “Portes Modernes,” par Cordemoy. Paris, 1900.
2 " Les Côtes de la Haute Normandie,” par De Lamblardie. Havre, 1789.

coast annually is 42,000 cubic yards at Treport, 39,000 at Dieppe, 23,400 at Valery-en-Caux, and 6500 at Fécamp. Large quantities are annually removed for ballast and for other purposes, 18,000 cubic yards being taken from the beach at Havre, and large quantities at Fécamp, Dieppe, and other places.

From Cape la Hève to Cape Antifer the coast is bordered for 11 miles by an unbroken line of chalk cliffs containing flints, which rise to a height of from 320 to 370 feet. There are continual falls of chalk and flints from these cliffs. Between Cape la Hève and the valley of St. Jouin, 3 miles south of Cape Antifer, large masses of shingle are banked against the foot of the cliffs, the drift of the shingle being southerly. On the north side of the valley the sea washes the foot of the cliffs.

At Etretat the chalk cliffs are broken by a wide valley, the low land on which the village stands being protected by a shingle-bank. Iport, another fishing village, is also sheltered by a natural high shingle-bank, the beach in front being encumbered by rocks which uncover at low water. The remains of ancient cliffs standing out seaward of the present coast-line are indicated by the Etretat Needle and the Guillemot and Vaudieu Rocks, which rise to a height of 53 feet.

The small port of Fécamp, 9 miles east of Cape Antifer, stands in a break in the high cliffs where the rivers Valmont and Ganzeville discharge into the sea. The west side of the harbour is protected by a high bank of shingle derived from the chalk cliffs, the easterly drift of which is stopped by the west harbour jetty extending out from the shore.

The coast between Fécamp and Valery-en-Caux consists of chalk cliffs with flints. The shingle, drifting from the west, accumulates against the west jetty of the harbour and forms a bar across the entrance. Occasionally the accumulation becomes so great that shingle which has worked into the channel has to be removed. Shingle accumulates in the greatest quantity in southwest winds.

The harbour of Dieppe is placed in a valley down which runs the rivers Bethune and Aure to the sea. The chalk cliffs rise to a height of 200 feet on both sides of the harbour, having seams of flint 6 inches thick and 2 feet apart. On the west side of the harbour the beach is about 300 yards wide, with a large accumulation of shingle, the pebbles being chalk flints derived from the

1“ Portes Modernes,” par Cordemoy. Paris, 1900.

cliffs; the drift to the east being stopped by several groynes, which are described at page 347. Formerly there were two sea approaches to Dieppe—one at the foot of the cliff to the east by a channel formed by the rivers Eaulue and Bethune, and the other at the foot of the rocks to the west by the river Arques. The west entrance has become choked by shingle, and a great part of the town stands on this shingle-bank.

Before the west harbour had become choked with shingle, vessels were able to go up the estuary as far as Boutrilles.

To the east of Dieppe the shore is bordered by precipitous chalk cliffs, which rise to a height of over 300 feet, and are washed by the sea at their foot, where there is a rocky shelf covered at low water. Five miles east of Dieppe, in a recess between two projections, is a beach the surface of which is from 2 to 9 feet above low water, and a narrow beach of sand and gravel continues to Treport.

Treport lies in a valley below the cliff at Mont Huon, which is 322 feet high. The entrance to the harbour is much encumbered by shingle drifting along the coast from the west. To prevent this the jetties have had to be lengthened from time to time.

Between Treport and Ault the chalk cliffs are broken by nine valleys, the shore being bordered by a rocky ledge dry from 10 to 15 feet at low water for a quarter of a mile from the cliffs, and having a narrow sandy strand.

The chalk cliffs end at Ault, the coast becoming low and marshy, and being protected from the sea by a bank of shingle 350 yards wide, which slopes down to the land, and is backed by sandhills which continue to La Hourdel. The village of Cayeux is surrounded by the dunes. In westerly gales a great quantity of blown sand is heaped up in front of the town and also carried inland. In front of the dunes is a sand beach which dries at low water for half a mile. The shingle on the beach gradually diminishes in quantity, and finally disappears at the entrance of the river Somme, 6 miles from the termination of the chalk cliffs.

Formerly the shingle drifting along the coast accumulated at Hourdel Point at the entrance to the Somme, beyond which it never passed, causing it to advance 20 feet a year; but a wooden jetty and groynes which were placed on the beach have stopped further drift.

The estuary of the Somme lies between the town of Cayeux and

St. Quentin Point, the space between these headlands being filled with sand (which is uncovered at low water) to a height of from 2 to 16 feet. North-east of the Somme the coast is low and bordered by ranges of sand-hills to Routhianville Point, fronted by a strand which dries for half a mile at low water. It is bordered by low land and marshes, the greater part of the bay having been reclaimed. Since the river and the stream flowing into it above Abberville have been diverted by the Somme Canal, the sand has accumulated in the estuary and nearly filled up the bed.

North of Routhianville is the estuary of the river Authie, the entrance to which is 2} miles wide, and extends the same distance inland, the whole area being covered with a large accumulation of sand and broken shells, which dries at half-ebb. The tendency of the sand-banks of this estuary is to advance northward, Routhianville Point having moved in that direction 550 yards between 1835 and 1878.

Further northwards the shore continues low and sandy, and is bordered by a range of sand-hills of moderate height, with a sandy strand half a mile wide. At the back of the sand-hills is low land, beyond which is a second range of dunes 1} to 2 miles inland.

Further to the north-east is the bay of Etaples, a large sandy estuary 2 miles wide, forming the outlet for the river Canche. It is filled with sand and broken shells, which dry at half-ebb. The sand-banks of this estuary, like those of the Authie, have a tendency to move northward, the two points which border the estuary having moved 330 yards since 1835. To prevent the destruction of the navigation by the filling up of the estuary, the channel has been fixed to one course over a length of 2 miles by training-walls of chalk blocks.

Boulogne, further along the coast, is situated in a valley. The cliff, composed of Jurassic rocks and chalk, is 300 feet high, and continues to Vimereux, where it is bordered by sand-hills which extend to Audercelles. Beyond this the land rises from the shore in a succession of hills and valleys, and in places the shore is bordered by perpendicular cliffs with a sandy beach at the foot the third of a mile wide, and a rocky shelf 350 yards wide.

Cape Grisnez stands out as a bold dark grey chalk headland, which marks the change in the direction of the coast-line from north to east. The rock, which is precipitous, is 167 feet high. Large masses of broken rock lie at the foot of the headland partly covered with sand, which dries at low water for a quarter of a mile.

[graphic]

Extending out from the shore on the north of Cape Grisnez in an east-north-easterly direction for 44 miles is a bank of sand and broken shells, the shoalest part being from 4 to 6 feet above low water. This bank has been caused by an eddy tidal current due to the sudden change in direction of the coast-line, the tide running past the headland in a north-easterly direction, and then curving round suddenly to the east along the altered direction of the coast.

At the termination of the rocks to the east of Cape Grisnez the shore becomes low and sandy, bordered by sand-hills. Cape Blanc Nez is a high cliff of white chalk, the coast eastward being steep to Sangatte, where commences the low sandy shore which continues along the remainder of the coast to the Texel. The sand-bills are low to within 1} miles of Sangatte, whence they rise in height towards Calais, and spread out into several parallel ranges.

The sandy beach at Sangatte is a quarter of a mile wide, increasing at the west pier of Calais to 1000 yards, with an inclination of 1 in 60.

Between Calais and Gravelines the coast is bordered by sandhills, which in some places have become so low that they have had to be protected by an embankment. In front of the dunes a sandy strand extends out nearly a mile from the shore, with an inclination of 1 in 1000.

Spring tides on this part of the coast rise 19 feet.

Between Gravelines and Dunkirk the shore is low, and a flat sandy beach extends out half to three-quarters of a mile. At Dunkirk the sand drifted by westerly gales is arrested by the harbour jetties, which project 500 yards beyond the shore, forming a large sandbank, the top of which is not covered at spring-tides.

Off the coast between Calais and Dunkirk are numerous shoals of black and grey sand, which run parallel with the shore and extend out several miles from it. They are generally long and narrow, and converge in direction towards the Straits of Dover. These sandbanks are all very steep on the shore side, with a gentle slope towards the offing. At some banks there is a depth of from 19 to 20 fathoms at less than half a cable from the southern edge.

« AnteriorContinuar »