Exercise 3 – Glacial erosion and fjords
1) Roche moutonnées and glacial erosion
Figure 1 A shows an idealized roche moutonnées in plan view and Fig 1 B in profile. Roche moutonnées are
typically from 1 m to more than 50 m in length. Ice flow is towards the right.
Indicate on the figures:
1)
2)
3)
4)
Edge of plucked area
Plucked surface
Zone of striation
Zone of friction cracks
2) Erosion of Sognefjorden
Figure 2: The Sognefjord drainage basin. The track follows the deepest portion of the fjord. Modern glaciers are
drawn with light stippling (from Nesje and Whillans, 1994).
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Figure 3: Longitudinal profile of Sognefjorden along the track shown in Fig. 2. Summit levels and hanging
tributary fjord bottoms are indicated (Nesje & Whillans, 1994). Sediment thickness on the fjord bottom is not
shown in this profile. In your answers you can assume that the profile shows the bedrock surface, although
sediment thickness could exceed 200 meters.
1) What is the maximum depth of the Sognefjord, and what is the depth of the threshold?
2) How much glacial over-deepening has occurred along the fjord?
3) What is the maximum depth of the glacial trough of Sognefjorden measured from the fjord bottom to the
summits of the surrounding mountains?
4) Compare this depth to the depth of Grand Canyon in the United States.
5) How many meters of the maximum depth of the through is caused by glacial erosion, and how much could
have been caused by non-glacial erosion (rivers, weathering)? [Hint: If you assume river erosion could be a
contributing factor, postulate the existence of a low gradient river with a total fall of 300 m over a distance of
230 km].
6) How would you explain the threshold at the mouth of Sognefjorden? [Hint: Look at the map in Fig. 2].
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3) Erosion of Sogndalsfjorden
Figure 4: Quaternary map of Sogndal showing striations (isskuringsstriper), moraines (in black), and terraces of
sand and gravel (terasser av sand og grus).
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Figure 5: Longitudinal profile of Sogndalsfjorden. Hatched area are Quaternary deposits (Grønning, 1983). The
mouth of Sogndalsfjorden is hanging relative to the main fjord Sognefjorden.
1) Identify the larger basins and troughs along Sogndalsfjorden on both map (Fig.4) and profile (Fig. 5).
2) Locate the deepest trough seen in the profile in Figure 5 on the map in Figure 4. How do you explain why the
fjord is deepest here?
3) What is the height difference between the threshold at Fimreite and the deepest basin in mid-Sogndalsfjord?
4) Study the glacial striations on Figure 4. Are all striations parallel to the fjord and valleys? Explain.
5) What is the approximate age of the end moraines?
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