Description of the Sognefjord drainage basin
The Sognefjord is the largest fjord system in Norway. penetrating more than 200 km inland from the coast of western Norway (Fig. 1). There is a correspondance between the fracture systems in the bedrock and the trend of the main fjord and its branches. These fracture systems have been important for guiding erosion.
The longitudinal profile of the Sognefjord (Fig. 2) shows one main basin with a relatively flat bottom bounded to the west by a high threshold. The main fjord, starting in the eastern part at Årdal (190 km), becomes abruptly deeper westwards to reach depths of about 800m below the present sea level where it coalesces with the Lustrafjord. The maximum depth (1308m) of the Sognefjord is at Vadheim, further out. The fjord bottom then rises to the Solund area, and the sea bottom extends westwards at depths of 100-150m.
The outer Sognefjord has few tributary or distributary fjords. The inner part, however, has five branches (Fjærlandsfjord, Årdalsfjord,Nærøyfjord, Aurlandsfjord andLustrafjord) (Fig. 1). These tributary fjords to the Sognefjord all "hang" above the bottom of the main fjord, and some of the branches have minor basins and thresholds.
The mountains along the Sognefjord rise gradually eastward from about 500m in the coastal region to altitudes above 2000 m in Jotunheimen (Fig. 1).
The highestmountain adjacent to the Sognefjord is Bleia (1721m), and the largest relief along the fjord of 2850m is found here. However, the average relief along the fjord is about 2000m.
Quaternary glacial erosion in the Sognefjord drainage basin
The Sognefjord is presumed to follow a preglacial (original / paleic ) river system. In many places the paleic surface is preserved more or less unaltered and the paleic surface and the present landscape commonly occur together (Fig. 3). The consistent and gradually rising summit level eastwards along the Sognefjord (Fig. 2) may therefore be regarded as remnants of the paleic surface. However, the preglacial valley floor is difficult to reconstruct accurately along the present fjord (Fig. 2). and this introduces some uncertainties when calculating glacial erosion.
Assuming that consistent summit levels and the wide, high-lying valley represent the paleic, pre-Quaternary land surface, the present relief represents the total Quaternary erosion and denudation. Both glacial and fluvial erosion are selective and follows zones of bedrock weakness, but at different rates. Therefore, the relative amounts of glacial and fluvial erosion must have varied greatly from site to site. The total Quaternary erosion and denudation is quantified by subtracting the present landscape from the reconstructed paleic surface.
Excluding the Quaternary sediments at the bottom of the Sognefjord, the volume difference between the reconstructed paleic surface and present topography is 7610 km3.