Nordvestfjord middle reaches

 

 I rediscovered the above B/W photo in my office the other day -- it's one of the old Geodetic Institute photos that we used in our 1962 expedition to the Scoresbysund area of East Greenland.  I have speculated about this before, but the most 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 middle 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-shaped fjord or outlet glacier trough cross profile. And why do we not see this "middle 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 explained 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 explain the sudden break of slope at the "trim line".  The other possibility is that the "trim line" is a geologically controlled feature. coinciding with the junction between relatively hard rocks and relatively soft ones.  I have looked at the geological map for the area, and there is no obvious geological boundary -- all of the rocks in the area are described as belonging to the basement complex -- crystalline or metamorphic rocks, and granite intrusions influenced by Caledonian orogeny.  However, in some parts of the East Greenland fjord country thick basalts lie on top of sedimentary and metamorphic rocks, providing at least a partial explanation for the bench on the fjordside, with steep slopes above and gentler slopes below.

In a significant research article, Bonow and Japsen (2021)  attribute many of the features of the fjordland landscape to the existence of two peneplains -- with an upper surface coinciding with the extensive plateaux which support multiple small ice caps today, and a lower peneplain which reveals itself in fjordside "benches" such as those desctibed above. 

Bonow & Japsen 2021: GEUS Bulletin 45 (1). 5297. https://doi.org/10.34194/geusb.v45.5297

The authors say:  The low-relief Upper Planation Surface (UPS; c. 2 km above sea level) cuts across basement and Palaeogene basalts, indicating that it was graded to base level defined by the Atlantic Ocean in post-basalt times and subsequently uplifted. The UPS formed prior to the deposition of mid-Miocene lavas that rest on it, south of the study area. In the interior basement terrains, the Lower Planation Surface (LPS) forms fluvial valley benches at c. 1 km above sea level, incised below the UPS. The LPS is thus younger than the UPS, which implies that it formed post mid-Miocene. Towards the coast, the valley benches merge to form a coherent surface that defines flat-topped mountains. This shows that the LPS was graded to near sea level and was subsequently uplifted.

Here is another photo of the upper reach of Nordvestfjord, taken from above the snout of Daugaard-Jensens Gletscher.  It also shows the relatively gentle gradients of the fjordside slopes and the deeply scoured nature of the whole of the ice-free landscape.

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 middle 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 on 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.

PS.

There is amazing new 2025 satellite coverage of this area available via Bing Maps. Just discovered it.  
Here we see the same area as featured at the head of this post.

Asymmetric cross profile of Nordvestfjord trough are clearly seen in the middle section.

The is the relationship between the upper planation surface and the lower one, according to Bonow and Japsen:

This photo shoes the various elements in Gåseland.  The photo below shows Nordvestfjord, in its middle section:

The section shown in the photo at the head of this post is in the distance, to the right of top centre.  The authors clearly see the plateau supporting the small ice cap as a part of the UPS, and the lower fjordside slopes as part of the LPS.  If there is one criticidm I have of the paper by Bonow and Japsen, it is that they are too preoccupied with "inherited features" and ancient landscapes, and do not pay sufficient  attention to glaciology and glacial erosional features.  Of course, every landscape tells a multitude of stories........