Collection of rocks from the Simplon Tunnel

Map simplon tunnel Switzerland

The Simplon Tunnel connects Italy with Switzerland. When it was completed on 24 February 1905 it measured 19.803 km and was the longest tunnel in the world. The tunnel afforded a unique view of the deeper parts of the south-central Alps and the structure was reappraised in light of the new theory of basement nappes.

The rocks in this collection are registered as BM1906,7(1-385) and were purchased from Messrs Grebel, Wendler & Cie on 26 January 1906. They had been dressed at source to a standard 11 cm by 8 cm size and arrived with a typed identification list in French and German compiled by Prof H. Schardt, the tunnel geologist (list now stored in the Earth Science Library Special Collection). The rocks, which can rarely be seen in situ, are all metamorphic rocks covering the full range from slates to gneisses.

Nappes in Switzerland

The original theory of basement nappes held through to the early 1970's when it was refined by Milnes (1973) to give the arrangement shown above. The collection correlates well with this work and it is worth placing it within the context of the book's conclusions. Milnes recognised three broad stratigraphic units: 'cover', 'transitional' and "basement". The cover consists of sedimentary rocks that were deposited onto the latter remnant metamorphic and granitic crystalline basement. The transitional rocks are those that show characteristics of both cover and basement and are found within the Lebendum nappe unit.

Impure marble containing galena

Impure marble containing galena, BM.1906,7(351)

COVER

Chiefly calc-schist and marble with locally important associations of dolomitie-anhydrite schists or schistose gneisses; they are considered to be of Mesozoic age metamorphosed to amphibolite facies grade during the Alpine orogeny. The marble is often impure and one example contains galena and weak grossular garnet, whereas the purple-tinged anhydrite in the dolomite-anhydrite schists can often have a striking appearance.

TRANSITIONAL.

The basement rocks areĀ found in all the nappes except the Lebendum nappe.

Psephite showing remnant conglomerate clasts in middle-left

Psephite showing remnant conglomerate clasts in middle-left, BM.1906,7(327)

Although the rocks in this unit can sometimes resemble those of the other two, there are two lithologic members which are unique: a brown-weathering non-calcareous to calcite-bearing schist (the 'scisti bruni'), and the grey-weathering psephites and psammites. The brown schists (eg. BM.1906,7(352-4)) are a calcareous quartz schist with all-important reddish-brown biotite. A good example of psephite (old terminology for a metamorphosed conglomerate) can be seen below.

BASEMENT

Feldspar-quartz mica schist of the Monte Leone nappe

Feldspar-quartz mica schist of the Monte Leone nappe. This one contains disseminated pyrite, BM.1906,7(176)

The Monte Leone and Berisal Nappes have feldspar-quartz mica schists, sometimes garnetiferous, and amphibole schists. The Antigorio Nappe has the very distinctive 'Antigorio Gneiss', a feldspar-quartz-biotite (muscovite-chlorite) granitoide gneiss to schistose gneiss. At times it is possible to see the original porphyritic igneous texture in this rock.

For further information contact: David Smith

Antigorio gneiss

'Antigorio gneiss' from the Antigorio Nappe - showing relict porhpyritic texture. BM.1906,7(270)

Return to Petrology collections.