1921. No. II. THE STRAXDFLAT AND ISOSTASV. 1 43 



there is in Helgeland one almost perfectly horizontal plane of the sub- 

 merged platform at a level of some few metres below the present sea 

 surface — and there is at least one plane above the sea, measured h} 

 Sahl Strom on Heroi and Donna at a level of about 8 to lo metres 

 Whether there are more planes in this region is not quite certain. 



Træna. 



Far out to sea, northwest of Lovund and Lovundvær, separated from 

 their plateau by a deep sea, the Træn Fjord, up to 438 metres deep and 

 12 kilometres broad, is a submerged plateau with the island-group of 

 Træna (see Fig. 114). We may call this plateau the Træna Plateau. It is 

 31 kilometres long from SSW to NXF^, and about 9 to 12 kilometres 

 broad. Its islands are much scattered, and subdivided into several smaller 

 groups: the Træna Islands proper, in the southwestern part of the plateau, 

 with the biggest islands Sanda (with the high peak Trænstaven, 338 m.) 

 and Husoi, built up of pressed granite, gneiss-granite, Sandavær just 

 north of this group and built up of the same kind of rock, Sandoi (Sandoy, 

 of gneiss-granite) and to the north of it Torvær (young gneiss) in the 

 middle of the plateau, Dorvær to the northeast, built up of not pressed 

 granite, Arvær and Båsan or Rosoian (young gneiss) to the northwest 

 of Torvær and Dorvær, and finally Selvær in the northeastern part of 

 the plateau, built up, in its southern part of mica-schist with layers of 

 crystalline limestone, and in its northern part of gneiss [cf. Rekstad, 1912, 

 geological map]. 



The geology of the islands of the Træna Plateau has been described 

 by J. Rekstad [191 2]. John Oxaal [19 15] has given an interesting general 

 description of the island-group and its strandfiat. 



There are on these islands many evidences of the vigorous marine 

 erosion, or shore erosion, to which the land has been exposed. As such 

 evidence may be mentioned the precipitous sides of the mountains rising 

 abruptly above the strand flat, the many big caves, the frequently cirque- 

 like valleys ('botten' valleys) with almost vertical sides. 



The erosion by frost, especially along the shores, has obviously been 

 of the very greatest importance for the development of these formations. 

 When the shore-erosion by frost is greatly intensified during periods with 

 a severe climate, and is assisted by the violent wave action of a stormy 

 sea on an exposed coast, as in this region, this erosion will become 

 extremely effective, and will have a great ability to cut away the land. 

 Thus the shapes of the mountains of Træna are simplv explained: 

 the shore-erosion has cut away the land, but on the higher and more 

 resistant islands it has not been able to plane down the whole islands, 

 and the high mountainous parts with their precipitous sides, or shore 

 walls, especially on Sanda Island (pressed granite) and Buøi in Dørvær 



