OUGSME field trip to the Province of Liège

23 January 2010

Text Peter Blackie

OUGSME members attending the AGM in Brussels on 23 January visited some of the geological features of the country around the River Amblève 20 km south of Liège, Belgium.

The quarry
Figure 2: The quarry at Chambranles and some of the features mentioned in the text.

Access to the quarry is through a 60 m long cutting which is now quite overgrown. By today’s standards the cutting is very narrow at 2m wide, and would not permit lorries or mechanical equipment to pass, however, exploitation ceased in about 1945 when other, larger quarries became more economical.

Today it is only visited by geological students and the occasional hiker.

The quarry face confronting one on entering is a vertical wall of sandstone laid down in the Famennian period of the late Devonian, 375-359 Ma, when Belgium and England were south of the equator. According to Ek the quarry face became vertical as a result of folding during the Variscan (end Silurian to Late Carboniferous) when, a series of terrains were compressed together as the Rheic Ocean closed (Hunter - 2001). The land would have been above our heads.

The wall of sandstone displayed several layers often only a centimetre or two thick. Features seen included what looked like a series of small cow-pats 30 to 40 cms in diameter (Figure 3), which Ek interpreted as being made by sea or river erosion of sand which had been partially consolidated by calcium carbonate, these lay above a bed of clear, even ripple marks (Figure 4) indicating a shallow water beach.

cow-pat features
Figure 3: Cow-pats features (Terry Warrington)

ripple marks
Figure 4: Ripple marks (Neil and Eileen Lawley)

A bed of fossils, probably bivalves, was also visible (Figure 5). Note that the sandstone itself had a black matrix, indicating the presence of organic matter, and indeed some plant fragments were discovered.

Above the sandstone was an 8m thick layer of limestone which had been quarried away but which was visible at the ends (see plan in Figure 3). Access required a certain amount of scrambling over fallen blocks of stone. This bed was interpreted as being laid down during a marine transgression.

A further bed of sandstone stood "above" (i.e. southwards of) the limestone. Still Famennian. Remains of branches were clearly visible embedded in the rock face (Figure 6).

bivalves
Figure 5: Bivalves (Neil and Eileen Lawley)

branch fossils
Figure 6: Branch fossils (Peter Blackie)

The morning was rounded off by an exposé by Ek, using the buildings of Martinrive as examples, of the use of Famennian sandstone and "pierre bleu" (Carboniferous limestone) in traditional Waloon construction. One could tell the wealth of the original owners by the relative proportions of the two, given that "Pierre bleu" is both more handsome and more costly than sandstone!

REFERENCES:
Hunter A. (2001) “The geological history of the British Isles” Open University, Milton Keynes.
Macar, P. and Ek, C. (1965) “Un curieux phénomène d’érosion Famennienne: Les "pains de grès" de Chambranles
.
(Ardenne belge) Sedimentologie 4, 53-64.

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