WAVE EROSION 225 



houses and the greater part of their land. "Kilnsea church fell in 

 1826-36, and the village was removed. Nearly the whole of this 

 parish has been washed away during the last century. Aldborough 

 church is far out at sea and Thorpe parish has been reduced from 

 690 to 148 acres. Of Ravenser and Ravenserodd, once a seaport 

 town at the mouth of the Humber, not a vestige is left." (Wheeler- 

 Jl : 222.) Wheeler has estimated that the total quantity of material 

 falling from this cliff each year is 1,615.680 cubic yards, of which 

 969,408 cubic yards is alluvial matter carried away in suspension. 

 This occurs only for about two hours before and two hours after 

 high water of spring tides, or about four hours 260 tides a year. 

 Thus each tide would carry away about 3,728 cubic yards. (73 : 

 225-) 



The progress of erosion of steep rocky coasts is strikingly illus- 



'./v/ 



Fig. 35. Diagrammatic view of the Island of Helgoland (direction S. W. 

 by N. E.), showing the great erosion platform, especially in the 

 southwest side, where the beds dip into the island. The rock- 

 is Banter Sandstein) (Triassic) much faulted. (After Walther.) 



trated by the numerous sea stacks found on nearly every coast, and 

 so familiar from the illustrations in the basaltic cliffs of Nova 

 Scotia, the chalk cliffs of Yorkshire and the Isle of Wight in Eng- 

 land, the Old Red Sandstone cliffs of North Britain, the Zechstein 

 cliffs of the Sunderland coast, the Buntsandstein of Helgoland, etc. 

 On these coasts the erosion is seen to progress at an almost visible 

 rate, changes in outline being perceptible often from year to year. 

 Perhaps the most noted example is the island of Helgoland, the 

 position of which, off the mouth of the Elbe, makes it such an 

 important strategic possession for the German Empire that they 

 obtained it in 1890 from England by relinquishing Zanzibar. In 

 1570 this island extended eastward across the present dune, where 

 it formed the Wittecliff of Muschelkalk, which had the height of 

 the present Helgoland. Destruction of these limestone banks by 

 the Helgolanders made it possible for the great flood of 1712 to 

 separate the entire mass of Muschelkalk and chalk from the main 

 mass of Helgoland. In the nineteenth century the last remnant of 

 this mass was a Httle island covered by dunes, which was threat- 



