596 KOSSiTER [chap. 16 



The longest series of sea-level observations, at Swinemiinde in the Baltic, 

 were only commenced in 1811. Direct evidence of the most suitable kind is, 

 therefore, strictly limited. However, as glacial eustasy is of significance in 

 certain geophysical problems (see section 5 of this Chapter), such results as 

 have been obtained are of interest. 



Three major attempts have been made in recent times to compute secular 

 variations in level ; all have used data from as many tide-gauge stations as 

 possible, in the form of annual means. But whereas Gutenberg (1941) and 

 Egedal, Disney and Rossiter (1954) used data for as many years as possible, 

 Munk and Revelle (1952) limited their investigation to the consecutive decades 

 1905-15 and 1915-25. Both Disney and Rossiter showed that simultaneous 

 readings should be used when possible, as secular changes are by no means 

 constant in time at any one place, but to do so can impose limitations on the 

 amount of data that can be studied. Ignoring the results for North America and 

 Scandinavia, the regions most affected by postglacial uplift of the land (see 

 below), Gutenberg averaged his results for all areas to arrive at a figure of 

 11 cm per century rise in sea-level during recent decades. Munk and Revelle 

 concluded that 10 cm per century must be considered a maximum value for 

 this era. These estimates will be referred to later (see page 605). 



C. Changes in the Distance of the Tidal Bench Mark from the Centre of the 



Earth 



Reference has been made immediately above to one of the main causes of 

 movements in the "fixed" reference mark for sea-level measurements, that of 

 isostatic adjustment in the earth's crust following deglaciation. It is generally 

 agreed that isostatic compensation need not be complete, immediate or 

 continuous. Nevertheless, there can be little doubt that in terms of hundreds 

 of years isostatic compensation is responsible for a considerable postglacial up- 

 lift. Moreover, changes in loading of the earth's surface when persisting over 

 thousands of years produce plastic flow in the earth's crust. Whilst direct 

 methods exist for determining some of these movements, such as observations 

 of lake levels and precise spirit levelling over large land areas at intervals of 

 time of the order of 50 years (see, for example, publications of the Finnish 

 Geodetic Institute), the possibility exists that observations of sea-level can 

 also be used to determine them. An independent estimate must first be made 

 of the effects of glacial eustasy, and amongst others Gutenberg (1941) has 

 attempted this for North America and Fennoscandia. Fig. 1 illustrates his 

 results for Fennoscandia. 



Apparent variations in sea-level may also arise from sudden movements in 

 the earth's crust, but if, as is most often the case, they are of local origin, they 

 can easily be detected by comparisons with readings from other gauges. 



Lastly must be mentioned the yielding of the crustal layers of the earth to 

 the tide-generating forces. These are most conveniently discussed below. 



