1921. No. II. THE STRAXDFLAT AND ISOSTASV. 39 



resistant. The ledge illustrated in Fig. 17, was cut back till the erosion 

 stopped against the nearly vertical wall of the dyke of diabase to the right 

 in the picture. This wall rises about i .7 metre above the floor of the ledge. 

 The level upper surface of this dyke is about 2.2 metres above the sea, 

 and seems to be part of a shore-ledge formed before the last glacial period. 

 A little further inland there is a higher ledge at about 3.1 metres above 

 the sea, which is 40 to 50 metres broad, and partly cut in solid rock, 

 partly formed of loose material. 



In the region of Fig. 18 there was no dyke of diabase to stop the 

 shore erosion, and tlie shore-ledge has here been cut back about 18 metres 

 to the foot of the cliff of argilaceous shist, which in some places is nearly 

 vertical, 4 to 6 metres high and even 10 to 12 metres high. 



In one place (at Godthåb) there is a broad dyke of syenite-porphyry 

 along the shore, in which the shore-ledge has been cut, and it is to some 

 extent backed by a cliff of argilaceous shist. This porphyry is traversed 

 by numerous fissures and was easily disintegrated by frost. 



The effect of the shore erosion differs much with the height above 

 sea-level. At the lowest levels of the shore, where the rocks emerge only 

 at low water, the recent shore erosion has been insignificant. One may 

 even find rocks of argilaceous shist scoured by the glaciers, which have 

 kept their rounded polished surface, where the glacial striation is still 

 visible. The reason is obviously that the rocks at these low levels have 

 been protected by the water against the disintegration by frost, w^hile the 

 wave-erosion has had no appreciable effect. They are a convincing demon- 

 stration how the former process has been essentially more effective than 

 the latter one. 



The disintegrating effect of alternate thaw and frost may be most 

 vigorous just above average sea-level, where ice is formed at high-water, 

 but where the freezing of the water during frost is not too frequently 

 disturbed by submergence. The shore-ledges have therefore been developed 

 at this level.* 



In some places the ledges are, however, as much as a metre, or even 

 1.3 metre, above average sea-level, and are rarely submerged. It is there- 

 fore a question whether the shore may not have been slightly elevated 

 since their formation. If so it can, however, only be a fraction of a metre. 



These ledges on the Fornebo Peninsula demonstrate on a small scale 

 very clearly how the shore erosion by frost, assisted by the wave action, 

 works, and how it is able, in relatively short time, to cut fairly broad 

 shore-ledges. The planing effect of this erosion may differ much ac- 

 cording to the resistance of the rocks and other circumstances. The result 

 is an often very uneven rock surface, varying somewhat in height in dif- 

 ferent places. We cannot therefore expect that a plane of abrasion thus 

 formed, will ever become quite level. 



