174 AMUND HELLAND ON THE FJORDS, LAKES,
other forces beside the quantity of ice and water have determined the depth reached by erosion ; but generally it may be said that valleys draining large districts of rainfall have a large transverse section. If a glacier fills a tributary valley, and is thinner than that in the main valley, the depth to which it erodes its bed must be less than the depth of the main valley. Hence many tributary valleys must debouch high above the bottom of the main valley. Instances abound of tributary valleys debouching thousands of feet above the beds of the main valleys, along the steep sides of the fjords of Western Norway. As mentioned above, we are unable to draw an exact line between the work of rivers before the Glacial epoch and that of glaciers during it.
The following, then, is the history of the configuration of Norway. The land is formed of rocks different in composition and origin, for which reason the country from the first has had a very uneven surface. On this thousands of rills and rivers began their work, and previous to the Glacial epoch eroded valleys more or less deep and broad. During it the glaciers followed, on the whole, the course of these, enlarging and shaping them, and excavating the fjords and lakes.
The occurrence of many large deep fjords in Western Norway is a consequence of their glacial formation and of the climate of the country ; for on the west coast the rain- and snow-fall is very con- siderable, and the land slopes rapidly to the sea. Hence the thick- ness and velocity of the glaciers were probably very considerable here. Further, they seem to have remained longer on the west coast. If the fjords and lakes of Norway are formed by glacier- erosion they must be confined within the limit of glaciers. As these, however, once extended over the whole country, it has everywhere fjords and lakes. If at the end of the Glacial epoch the glaciers halted for long intervals at various places, we ought to find marks of erosion corresponding with these limits. We have already shown that we do so find them. When thick glaciers descen- ded into the fjords and constantly deepened their beds, very peculiar relations of depth would be caused in the fjords. The glacier of the main fjord was constantly increased on its way by supply from tributary fjords, in consequence of which the depth or breadth of the fjord must have been increased. As the glacier proceeded further down the fjord the loss from melting would exceed the supply from the sides ; and thus its erosive power, and consequently the depth of the fjord, would decrease ; but while the glaciers deepened the bottom of the fjords, the depth of the sea in front of them would be dimi- nished, as all the detritus would be deposited there.
Our theory, then, requires : — (1) the depths in the fjords should be greater than in the sea ; (2) the depth of the fjord should increase towards the inner part and decrease towards the mouth; (3) great masses of boulders, gravel, and mud should be found in the sea in front of the fjords. These requirements are satisfied by the observed facts. It is well known that great depths occur in the Norwegian fjords. While soundings of from 400 to 500 metres are very frequent in the
