74 Distribution of Chief Forms of Vegetation 



6. Salt-marshes are deyeloped on the tidal mud of 



protected bays and estuaries, and their 

 vegetation vegetation, like that of fen, is land-forming. 



If the sea is kept out and the land drained 

 salt-marsh makes excellent pasture and often excellent 

 arable land. 



7. Sand-dunes are developed wherever on the coast 

 there is a constant supply of blown sand derived from the 

 sand thrown up above the average limit of high tide. The 

 ultimate form of sand-dune vegetation in this country is 

 generally grassland, sometimes heath. The British sand- 

 dunes, though they often bear shrubs, are rarely colonised 

 by trees, perhaps because of the scarcity of suitable seed- 

 bearing trees in the neighbourhood. 



8. Shingle beaches are formed from pebbles thrown 

 up by high tides. Considerable areas exist at Dungeness 

 in Kent, Orf ordness in Suffolk, and Chesil Bank in Dorset- 

 shire, while various smaller areas occur at numerous other 

 places along the coast. The shingle-beach vegetation is 

 quite characteristic. Much of it is very closely allied to 

 sand-dune vegetation ; in other cases where it lies on silt, 

 to which the tide has access, it bears many salt-marsh 

 plants. 



