558 PRINCIPLES OF STRATIGRAPHY 



cay, extending without interruption from the mouth of the Adour 

 to that of the Garonne, a distance of over 240 km. They are ar- 

 ranged in parallel rows, ten being the maximum number, covering 

 a strip of coast from 4 to 8 and in some places even 10 km. wide, 

 and approximating an area of 120,000 hectares (296,520 acres). 

 Their height ranges up to 90 meters, making them the highest 

 known coastal dunes. It is true that there are dunes with a height 

 of from 120 to 180 meters on the west coast of Africa between Cape 

 Bojador and Cape Verde, but these appear to be desert dunes which 

 have reached the coast from the Sahara under the influence of pre- 

 vailing northeast winds. 



The country bounded seaward by the great dune belt of the 

 Biscayan dunes is a low, marshy tract known as the Landes, which 

 by many geographers is believed to be subsiding, and the shore of 

 which is retreating at a rapid rate. Near Houtin, in the northern 

 part, the sea advances at a rate estimated at not less than 2 meters 

 per year, while great difficulty is experienced by St. Jean-de-Luz, 

 south of Biarritz, at the southern end of the dune region, from the 

 encroachments of the sea. 



On the south coast of the North Sea along the entire coast of 

 Belgium (Flanders) and the Netherlands, to the mouth of the 

 Elbe, extends a nearly uninterrupted dune belt, which altogether 

 forms a length far exceeding that of the coast of the Landes. These 

 dunes are not as high as those of the Biscayan coast, averaging 15 

 to 20 meters, with the highest, near Petten, attaining only 35 meters. 

 The coast here is known to subside at a rate where measured of i.i 

 meters per century, and as further indicated by the gradual deep- 

 ening of the Zuidersee, formerly a swamp, but now an embayment 

 of the sea. 



The dunes of the Danish coast of the North Sea are of especial 

 interest on account of their rapid inland advance. This section, 

 together with the coast of Schleswig-Holstein, forms the second 

 largest dune region of Europe, covering an area of 67,000 hec- 

 tares (165,557 acres), many of the dunes reaching a height of 30 

 meters. The retreat of this coast has been discussed in an earlier 

 chapter, but an example may be repeated here to show the rate of 

 dune advance. In 1757 it was found necessary to tear down the 

 church of the village of Rantum on the Island of Sylt on the west 

 coast of Schleswig-Holstein, because it was reached by the advanc- 

 ing dunes. In 1791 or 1792 the entire dune chain had passed over 

 the church ruins, these then lying on the shore which had advanced 

 to this point. Sixty years later the site of the church was 700 feet 

 from shore, the depth there being 12 feet. The second church has 



