itself may change its level. This can be brought on by localized 

 landslips, subsidences, or faults, or it may involve slow land move- 

 ments over hundreds of miles of coast. Over the last twenty years 

 surveys made by the United States Coast and Geodetic Survey 

 show that the coast line from Florida to Massachusetts is sinking 

 at the rate of about .02 feet every year. At this rate, in a thousand 

 years the coast would be twenty feet beneath the sea, but the 

 geological process is not so simple. 



Usually it is impossible to distinguish between short-term eu- 

 static changes and true land movements during the same period, 

 because the only measurable quantity is the relative position of 

 land and sea. Over short periods, say a thousand years, the change 

 in level of a given site is usually due to local land movement ; it is 

 risky to try to attribute such a change to small eustatic changes. 

 But over longer periods, tens of thousands of years, the eustatic 

 changes mount up so they can be measured. We can, for example, 

 find raised and sunken beaches tens and hundreds of feet above or 

 below the present sea level. 



From 14,000 to 4000 b.c, at the end of the last glaciation when 

 the polar icecaps were melting, the sea rose about three hundred 

 feet all over the world. When this happened, water flowed far 

 inland, flooding the coastal river beds and forming long inlets and 

 estuaries. The first great sea voyages that we know of were made 

 soon after the sea had stopped rising ; at that time there were many 



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