Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/206

168 168 AMtTND HELLAND ON THE EJORDS, LAKES,

senting the level of the sea at the end of the Glacial epoch, have not the same height? We must not conclude that the land in Western Norway has risen only about 100 metres since the end of the Glacial epoch, but in Eastern Norway 200 metres. A compari- son of the present glaciation of Greenland with that of Norway will show that this difference in the heights of the uppermost terraces may be occasioned in another way. Terraces are found in Green- land also at the mouth of rivers ; for example, there is a terrace with marine fossils near Claushavn, in Disko Bay ; its height, how- ever, is only 30 metres. Thus the level of Greenland was once 30 metres below the present one. We should then expect to meet with a terrace of this elevation in many places in the fjords ; but instead of this we find them filled with ice, which has prevented the formation of terraces. Wherever a river issuing from a glacier is now depositing the mud at its mouth a terrace is being formed at the present sea-level. Suppose, then, that North Greenland were to rise 20 metres ; the terrace at Claushavn would be 50 metres high, and those now formed at the mouth of the rivers would be 20 metres high, while in the ice-fjords there would be none. Thus a diffe- rence in the heights of the uppermost terraces does not necessarily indicate that the amount of rise in the land has been different in different places. The reason, then, that the terraces in Eastern Nor- way overtop those in the western fjords by 100 metres may be that the glaciers in the fjords of the latter prevented terraces being formed when the land was 200 metres lower than at present ; only when it had risen about 100 metres had the glaciers retreated sufficiently to allow terraces to be formed in the fjord-valleys ; in these the glaciers retired at different times, and the lowest terraces are found in the valleys from which the glaciers last retired. Looking at the

section of Lake Oifjordvand (fig. 4) the question naturally arises with regard to the terraces at the lower end of the lakes, how, if they are formed by the rivers from the loose detritus brought down the valleys, has this been transported across the lake? This has been explained by Professor Sexe*. The highest terrace in front of the lake clearly cannot have been formed when a thick glacier passed out into the fjord, nor can it have been formed when the glacier had retired from the lake ; for then the detritus would have filled up the basin. The terraces therefore must have been formed when the glacier just reached the end of the lake. This is evident from the fact that where detritus in front of a lake is more than 100 metres above the sea (?'. e. higher than its former level) it occurs not as a terrace, but as a. moraine. The geological structure of the highest terraces is the same as that of the moraines ; they are therefore only moraines made level by the waves of the sea.


 * Marker efter en Istid.