224 PRINCIPLES OF STRATIGRAPHY 



which sank into the sea anno 860, was discovered in the last cen- 

 tury at a distance of 4,710 meters ofif the west coast of Katwijk, in 

 south Holland, while the remains of another structure swallowed 

 earlier by the sea now lie at a distance of a mile from the coast. 

 The dunes at Gravenzande, north of the Meuse mouth, which in 

 1726 had a width of 640 meters, had been entirely removed by the 

 sea near the end of that century, making an average retreat of the 

 coast for this section of 10 meters per year. 



On the west coast of Denmark, erosion of the coast proceeds at 

 a rapid rate, accompanied by subsidence. At Agger, near the west- 

 ern end of the Liimfjord, a strip of coast 141 meters in width dis- 

 appeared between the years 181 5 and 1839, making an annual re- 

 treat of more than 5.6 meters. Even more extended was the loss be- 

 tween the years 1840 and 1857, during which time the sea devoured 

 a strip 157 meters broad, the coast thus retreating at a rate of 

 more than 9.4 meters per year. In many places along the coast of 

 the North Sea submarine forests are found, with many stems still 

 erect, while numerous structures built in historic times are now 

 beneath the water. 



The cliffs of glacial sand, gravel and boulder clay which form 

 the coast of Yorkshire, England, for 36 miles from Flamborough 

 head to the mouth of the Humber furnish numerous examples of 

 rapid erosion by the sea. The process is generally a removal of 

 the loose material at the foot of the cliffs, by the waves, with a 

 partial undermining of the cliff, followed by the slipping of a large 

 mass into the sea. The cliff faces the opening of the Skagar 

 Rack on the opposite shore of the North Sea, 400 miles away, and 

 so is exposed to the full force of the waves of this turbulent epi- 

 continental sea during northeast gales. The ordinary rise of the 

 spring tides is 16 feet, the water reaching the foot of the cliff 

 throughout a great part of the length, in some places rising 2 to 3 

 feet above the foot. At low tide the beach is 150 to 300 yards 

 wide. "The waste of the cliff has been estimated at 2 miles since 

 the time of the Romans, a mile of which has gone since the Nor- 

 man Conquest." ( Wheeler-73 : 2.?^". ) Phillips (54) and others 

 have estimated a loss of 2^ yards annually, equal to 30 acres a 

 year over the 36 miles of coast, the average height being taken at 

 40 feet. On one part of the coast a recession of 215 feet occurred 

 in the 37 years between 1852 and 1889, according to the estimates 

 of Captain Kenny, while Captain Salversen estimated the erosion 

 on 12 miles of the coast to be 132 feet in 40 years, or a loss of 204 

 acres. From the record of old maps it appears that whole town- 

 ships have been devoured by the sea, while others have lost churches. 



