304 FRIDTJOF NAN.SF.N. M.-.\. Kl. 



//■//(// is llic c.vU'iil of llic smallest area 7i'itliin zvhich isostatic moTemcnls 



may occur ^'^ 



This is a very difficult (Hicstion, wliicli we cannot answer at present. 

 It is ()])\i()us thai the (|U(sli(iii of time is here of mucli importance. The 

 smaller the area is witliiii wliii-li the e(|iiilil)rium is 'listurhefj, ihc jfjuger 

 il will lake before it can iie re-established, it may be possible that in tlie 

 course of a \cry lonj^' lime e(|uilibrium may be more or less attained 

 within (|uile small areas, although the process is so extremely slow that 

 it is not yet the case in many Icjcalities examined. 



W'Q have seen (])]). 253 f.) that in Norway and .Sweden there may be 

 <juite considerable differences in the uf)hea\al of the lan<l within small 

 distances of no more than a few kilometres. As, however, we do not know 

 tlie causes of these differences, they can hardly be used as proofs of a 

 great local ada])tal)ilily of the crust to isostasy. 



It ma\', however, be pointed out, that the comparati\ely small ice-cap 

 of Scotland, of the last glacial ])eriod, has caused a considera])le depression 

 and subsequent upheaval of the land, witliin an area with a diameter of 

 less than 500 kilometres. Tlie last glacial ice-cap of Iceland also caused 

 a depression and subsecjucnt upheaval of that island. 



T.ake Bonneville in the regicjn of the present Cjreat Salt Lake, in Utah, 

 with a diameter across of probably about 230 kilometres, caused a de- 

 pression and subsetjuent ui)liea\al of the flooded lanrl, although the depth 

 of the water above tlie present level of Great Salt Lake may have been 

 no more than 320 metres [cf. Gilbert, 1890I. The depression caused by 

 the load of water may possibly have been about 45 metres in the centra! 

 area of the lake. 



These facts indicate that considerable isostatic movements may take 

 place within areas no more than a few hundred kilometres wide, and 

 probal)ly even much smaller. 



Isostasy and Erosion. 



Our views regarding the ability of the earth's crust to attain its 

 level of isostasy must greatly influence our views as regards the morpho- 

 logical changes due to erosion on the continental surfaces. 



The rapidity with which the crust responds to the changes of pres- 

 sure caused by the deposition of sediments or by denudation is of essential 

 importance for the development of these processes and for their effect 

 upon the surface topography of the crust. 



Let us consider the probable effect of the isostatic crustal movements 

 upon the denudation of the land surface and especially upon the develop- 

 ment of peneplains. 



It is obvious that the time required for the planing down of a 

 mountainous region to a peneplaiii will be essentially increased by the 



