WAVE EROSION 223 



Point, a cliff of Carbonic rock, and Abbotsham, 12 miles distant; 

 the boulders have been rolled "for a distance of from 10 to 15 

 miles and piled up into banks of from 100 to 150 feet wide and 20 

 feet high, the top being from 6 to 9 feet above high water." 

 (Wheeler-73: 1"/.) 



The progress of wave erosion on a cliff of uniform exposure 

 depends in a large measure upon the relative hardness or resistance 

 of the rock in question. Unconsolidated material is readily re- 

 moved, even in sheltered places. Berkey (4) has estimated from 

 observations extending over a year that the cutting back of a cliff of 

 glacial sand and fine gravel near Port Jefferson, at the head of a 

 narrow enclosed bay on the north shore of Long Island, New 

 York, proceeds at the rate of 10 feet per hundred years. On the 

 exposed outer shore of Cape Cod, the unconsolidated glacial sands 

 and older clays are cut back at a rapid rate, as shown by the neces- 

 sity of repeated removal of the lighthouses at the three Nauset 

 Lights. 



The Island of Sylt, on the west coast of Schleswig-Holstein, fur- 

 nishes an instructive example of rapid erosion. The sand dunes 

 which protected this island from the sea began to move eastward, 

 in the middle of the eighteenth century, and the sea followed. The 

 church of the village of Rantum had to be taken down, and in 

 thirty years the sand hills had passed the site, and the waves had 

 swallowed the foundations of the church. Fifty years later the 

 former site of the church was nearly 300 yards from the shore. 

 (Andresen-2.) 



Many striking examples of the advance of the sea on a low 

 coast are recorded in the district of the Landes on the west coast of 

 France. This region between the Gironde and the mouth of the 

 Adour, is believed by many to be subsiding, while others hold the 

 advance of the sea as due to wave and current erosion entirely. 

 The retreat of the coast near the Garonne is estimated at not less 

 than 2 meters per year ; the lighthouse of Cordonan, formerly situ- 

 ated on the coast, is at present separated from the mainland by an 

 inlet 7 kilometers in width. On many portions of the coast the 

 dunes are washed away by the constant advance of the sea. 



On the south coast of the North Sea subsidence with encroach- 

 ment of the sea through erosion is abundantly illustrated. 'The 

 Zuidersee, originally a marsh, then a coastal lake, finally became an 

 arm of the sea, and is constantly increasing in depth, being navi- 

 gable to-day by much larger vessels than in the former centuries. 

 In many places the sea now covers sites of villages which flourished 

 thirty years ago. A structure built by the Romans under Caligula, 



