I92I. No. II. THE STRAXDFLAT AND ISOSTASY. 303 



Uf much importance are the two (juestions of the time required for 

 the isostatic readjustment of the crust, and (:)f tlie size of the area within 

 wliicli it can take place. 



Hoz<' long a time docs the earth's crust require to reach its )iez\.' isostatic 

 level after a disturbance of its equilihriion/ 



As the internal friction in the plastic or mobile substratum, under 

 the rigid crust, is, at any rate, extremely great, whatever the state of 

 tliis matter may be, we may expect that when the substratum is exposed 

 to stress it will take a long time before this friction is gradually overcome, 

 and motion is started. In addition to this there is naturally also an 

 enormous resistance to overcome in the rigid crust itself, before it can be 

 depressed or upheaved. Hence the isostatic movements of the crust will 

 always show a great deal of lag; 



For this reason many geologists have assuiued that the establish r.ent 

 of isostasy at the earth's surface will require extremelv long geological 

 periods. I think, however, that our study of the strandfiat and the raised 

 shore-lines of Norway may prove that the time required for the attain- 

 ment of approximate equilibrium is vcr}- much shorter than is generally 

 believed. 



Although it may be very difficult to estimate the length of time 

 elapsed since the ice-cap of the last glacial epoch actually began to de- 

 crease, there are now many careful researches l)y Gerard de Geer and his 

 school in Sweden as well as b}' others, especially in North America, which 

 will help to estimate the length of the late-glacial and postglacial periods. 

 According to the results of De Geer's investigations, it is not more than 

 13,000 vears since the margin of the retreating ice-sheet stood in Southern 

 Scania. Even if we take other estimates we cannot possibly assume the 

 time since then to have been more than 16,000 or 20,000 years. Purirg 

 this period nearlv the whole of the late-glacial and postglacial upheaval 

 of Fenno-Scandia has been accomplished. A very considerable part of 

 the upheaval has even been accomplished in about half tliat time, since 

 the ice-margin had retreaterl to Northern Sweden, 8,000 years ago. 

 e. g. the lanrl near Eottenviken has been upheaved about 270 metres 

 during that space of time. 



We know that along the coast of Norway the upheaval of the land 

 was practically completed at least before the beginning of the Christian era. 

 Hence we may assume that along the Norwegian coast the upheaval of 

 the land and the re-establishment of equilibrium was completed during 

 a period of probably 11,000 years, and at any rate of not more than 

 18.000 years. This is a remarkably short period, and seems to indicate 

 that tlie plastic substratum of the earth's crust is more mobile, than many 

 geologists are prepared to allow. 



