1921. No. II. 



THE STRANDFL.AT AND ISOSTASY. 



37 



Fig. 18. Shore-ledge 18 metres broad, cut in argilaceous shist near Fornebo Harbour. 



broad ledges have been formed in this manner, in recent time and at present 

 sea-level. Figs. 17 and 18 represent some parts of these shore-ledges. 

 They are cut in solid rock and are from 8 or 9 metres (Fig. 17) to 18 and 

 20 metres broad (Fig. 18). The rock consists chiefly of ordovician clay- 

 slates, alternating with bands of lime-stone, occurring in series of lenti- 

 cular nodules with intercalated shales [cf. Werenskiold, 191 1]. This 

 formation is intersected by numerous dykes of diabase. 



The general floor of these ledges is perhaps half a metre above 

 average sea-level. The tide is insignificant in the inner end of Christiania 

 Fjord, and the sea does not often rise sufficiently to flood the ledges. In 

 som places the outer part of the ledge, near its edge, may be somewhat 

 higher than the floor inside. In Fig. 17 the outer edge of the ledge is about 

 0.6 to 0.8 metre above average sea-level, while the floor inside is 0.2 to 0.3 

 metre lower. In some places rocky nabs occur near the outer edge rising 

 as much as i metre above the floor of the ledge (see Fig. 18). These pro- 

 jecting nabs and ridges near the edge of the ledges, are in some cases 

 formed by dykes of diabase, which has been more resistant to the erosion 

 than the argilaceous shists and lime-stone, but they may also, as in Figs. 17 

 and 18, consist of the same kind of rock as the inner part of the ledges. 

 In the nab in Fig. 18, there is relatively much lime-stone. 



These ledges have obviously been formed by shore erosion by frost, 

 in a similar manner as that described on the preceding pages. When the 

 fjord was covered by ice in the early winter, as it always used to be except 

 in late years when the fjord is kept open by ice-breakers, the ledges were 

 covered by thick layers of ice formed at high water. When this ice melted, 

 all fissures and depressions of the rock surface would be kept filled with 

 water, which would freeze and disintegrate the rock at each frost. 



