Nordvestfjord inner reaches

 

 I rediscovered the above B/W photo in my offioce the other day -- it's one of the old Geodesic Institute photos that we used in our 1962 expedition to the Scoresbysund area of East Greenland.  I have speculated about this before, but the mlost striking feature of the photo is the extraordinary "break"on the fjord side  between  a lower relatively gentle slope of highly ice-scoured bedrock and an upper section which we can refer to as a dissected plateau edge.  If we like, we can refer to this junction as a "trim line" because it must separate a lower heavily glaciated landscape from an upper zone which was at one time ice-free.

As we move down the fjord this lower slope with a modest gradient gradually disappears, to be replaced by vertical (and in places overhanging) cliffs, especially on the outside of bends, where the intensity of glacial erosion has been at its greatest.  This is related to a gradual increase in glacier discharge as one passes from the upper trough to the lower or outer trough.  Those areas of steep fjord sides should be the ones where trough depth is at its greatest -- indeed there are water depths of over 1500m as one approaches the outer fjord threshold, but the deepest continuous stretch in the fjord long profile (with a depth of over 1400m) is a 30 km stretch which coincides with relatively gentle fjordsides as in th photos above.  That's a bit of a puzzle........

What is the glaciological explanation of this phenomenon?  This is not your classic U-shapesd fjord or outlet glacier trough cross profile. And why do we not see this "inner fjord bench" in Sognefjord in Norway,  and in many of the other big fjord systems of the Northern Hemisphere?  Are we seeing evidence here of the gradual transition, in a brutalised dendritic fjord system, from areal scouring to highly concentrated linear erosion?  Is this all expalained by reference to glacier thermal regime, with a transition from cold-based ice to warm-based ice?

Two other possibilities.  The lower, gentle, slope segments might be remnants of an ancient fluvially -influenced landscape, possibly dating back to pre-glacial times"  I don't like that theory, since it does not adequately explaiin the sudden break of slope at the "trim line".  The other possibilioty is that the "trim line" is a geologically controlled feature. coinciding with the junction between relatively hard rocks and relatively soft ones.  To be investigated...........

Here is another photo of the same upper reach of Nordvestfjord, taken from above the snout of Daugaard-Jensens Gletscher.  It also shows the relatively gentle gradients of the fjordside slopes.

See these articles:

Nordvestfjord: a major East Greenland fjord system
J. A. DOWDESWELL, C. L. BATCHELOR, K. A. HOGAN & H.-W. SCHENKE

2015, Geol Soc of London

HARBOR, J. M. 1992. Numerical modelling of the development of U-shaped valleys by glacial erosion. Bulletin Geological Society of America, 104, 1364-1375

For the contrast between the inner fjord and the outer fjord, see these photos of the west side of Nordvestfjord, taken from near Syd Kap and the flank of Pythagoras Bjerg:

..... and this one, which is seriously spectacular.  I haven't been able to discover where exactly it was taken, but it reminds me of the fjord wall near "Hell's Bells" (as we called it), between Syd Kap and the diffluent trough occupied by the twin lakes of Holger Danskes Briller.

The fjord walls onjn the west side of this outer zone are steeper than in the inner zone, and are sustantially more broken up as a result of complex interactions between the main Nordvestfjord glacier and abundant tributary glaciers flowing from ice caps and from smaller "alpine" glacier catchments.