Sands off the Thames estuary, and other similar banks in the North 

 Sea have been formed by tidal currents and are a threat to ships. 



Imperceptibly, year by year, world-wide changes in sea level are 

 taking place and affecting nature's work where the sea meets the 

 land. As world climate seems to be growing milder, glaciers are 

 melting and returning water to the oceans, with the result that sea 

 level is rising at about one to two millimeters a year in many places. 

 In some regions, such as Holland, which have a tendency to sink, 

 the rate is greater and all the more menacing because the land is 

 already below high tide level. In other regions, the Baltic Sea, for 

 example, where the land has been steadily rising since it was freed 

 of its load of ice from the most recent ice age, the sea level is falling 

 and more land is appearing. Scotland, too, is recovering, and has 

 well-marked raised beaches on the west coast. The east and south 

 coasts of England, however, are sinking, and the rising sea level is 

 shown by the submerged forests found on many beaches. In the 

 North Sea the sea level has risen by about 1 80 feet or more during 

 the last ten thousand years. The present shore line, then, is very 

 recent geologically. 



Having looked briefly at the character of the waves, it is now 

 time to follow them onto the shore — together with the wind and 

 tide — to see how they attack and alter the coast, eroding some 

 areas and building up others. 



The sea, in its fiercer moods, can be an awe-inspiring sight, 

 especially when it hurls itself against cliffs and sea walls. The de- 

 struction it causes depends partly on the rocks themselves and 

 partly on how the sea breaks against them. If the rocks have many 

 cracks, each time the sea breaks against them the air trapped inside 



At Beachy Head, on England's south coast, 

 sections of the soft chalk cliffs tumble into 

 the sea each year; but in Cornwall (above), 

 on the west coast, slate cliffs stand 

 up to the endless pounding of the waves. 



227 



