Sognefjord and some of the other west Norwegian fjords, shown on a false-colour satellite image. White has been replaced by red. The red-coloured (snow-covered) area is essentially high land -- plateaux and mountains -- and also areas (like Jostedalsbre) currently occupied by ice caps and glaciers. During glacial episodes this is the area which supplied ice into the westward-flowing outlet glaciers. The green coastal area is lower and much more broken up by connecting valleys, islands and sounds -- this is the area where diffluence occurred, with ice escaping from the main trough and spilling over interfluves, and in the process losing its erosive power.
This is an interesting article, although it does not investigate the precise mechanisms which have operated at depth in the Sognefjord outlet glacier in its heyday......
Geomorphology, 9 ( 1994 ) 33-45
Erosion of Sognefjord, Norway
Atle Nesje, Ian M. Whillans
The above figures illustrate some of the factors that might have influenced the morphology of the fjord. I do not think that either structure of geology have played major roles either in the orientation of the trough, or its width, or its depth. On the other hand all authors seem to agree that it is immensely ancient, created along the route of an even more ancient fluvial route on an uplifted erosional surface called the Paleic Surface. Many glaciations have contributed to the present form of the trough.
The lowest of the three diagrams is particularly interesting, since the authirs (Nesje and Whillans) have entered the floor depths of the subsidiary troughs, which join the main trough well below sea level as hanging valleys. As we can see, the biggest subsidiary troughs, which carried the largest glaciers, and which brought big supplements to the discharge of glacier ice in the upper reaches of the fjord, are Ardalsfjord, Laerdalsfjord, Aurlandsfjord, Sogndalsfjord and Fjaerlandsfjord. Aurlandsfjord was the biggest of all, with a bed at about -1000m. Interestingly enough, none of these supplements has resulted in a spactacular drop in the floor of the main fjord; rather, they have simply led to a continuing and rather gradual bed lowering until the deepest point on the fjord floor is reached at Vadheim. In the outer reaches of Sognefjord, Risnesfjord brought in a lot of ice, with a bed at around -500m, but that led to no deepening at all of the main fjord trough. And then, almost immediately downstream, there is a sharp break of slope and the reverse slope up to the fjord threshold starts. That's very sudden, and very spectacular, and a backslope with a gradient of 1 in 10 (a thousand metre rise over 10 km distance) cannot simply be explained by reference to diffluence.
This steep reverse slope is not just a spectacular geomorphological feature; it clearly represents a very dramatic glaciological change of state in subglacial conditions.