Traversing the Swiss Alps on the Haute Route—the middle section
Continuing eastward on the Haute Route is a roller-coaster trek of long uphills and downhills, with great expectations about what type of landscape will appear after topping the next pass and then what little village will lie in the valley at the bottom. In general, lodging in the valleys is in small hotels; in some parts, the route stays high and the lodging is in mountain huts (called cabanes in French and hūttes in German) that usually have dormitory-style beds. Here is a photo of the Mont Fort cabane that is located above the famous ski town of Verbier (for us, day 4 on the route).
Continuing east from Mont Fort, the route stays high (2500–3000 meters elevation) and crosses great expanses of raw rock with frequent steep, exposed bouldery stretches. On our trek in early September, we encountered snow and chilly temperatures on the route between cabanes Mont Fort, Prafleuri, and Dix. Here is the view northward after crossing the first of three passes between Cabanes Fort Mont and Prafleuri. We left at dawn because of an inclement weather forecast; luckily, the snow waited to start until we were just arriving at Cabane Prafleuri.
Along this part of the route, which goes through the interior, highly-deformed Western Alps, we found ourselves continually diverted by amazing rock exposures. We saw many small-scale structures, such as those below, that gave clues to the larger structures of the region. Notice that these small-scale folds are recumbent—that is, they are laying on their side so that one half of each fold is completely overturned.
Below is a particularly attractive fold with Haute Route trail marker for scale. This rock has been transported by a glacier.
The geology of the Alps is extremely complex, but the Cenozoic history (past 65 million years) is enough to chew on here. At the beginning of the Cenozoic Era, there was an ocean where today we see a vast mountain range. In this ocean, which separated the African and European continents, there was a subduction zone where oceanic crust was being consumed beneath continental crust. Starting about 35 Ma, this subduction zone was caught up in the Alpine Orogeny where it was uplifted, folded and faulted. We saw abundant evidence of this sequence of events during the day we walked between cabanes Prafleuri and Dix.
Below is the view that created us on the snowy morning after we crossed the pass south of Cabane Prafleuri and prepared to traverse along Lac des Dix (a reservoir) to Cabane des Dix. The Glacier des Cheilon is visible in the distance; it is rapidly retreating up the valley. Cabane des Dix (not visible) overlooks it from the western (right side) slope.
At the south end of Lac des Dix, we were excited to find we were eating our bread and cheese on the highly diagnostic rock type called eclogite. Eclogite begins life as mafic igneous rock (basalt or gabbro); as eclogite it contains the characteristic minerals pyrope (a type of garnet) and omphacite (a type of pyroxene) that indicate metamorphism at very high pressures but relatively low temperatures considering the depths implied by the high pressure. The only place on Earth where these conditions are found is in subduction zones, where cold ocean crust with overlying cold and watery marine sediments sinks to depths of hundreds of kilometers. The crust is there subjected to very high pressures but since the geothermal gradient is less than at other places on the planet (the average being 25–30 degrees Celsius / kilometer of depth), the temperatures are relatively low. Here are photos of the eclogite rock with its abundant red garnet minerals (Swiss army knife for scale of course!).
Upon the approach to Cabane Dix, we were treated to an exceptional exposure of serpentine rock, which forms by hydration of ultra-mafic rocks (peridotite) at the base of oceanic crust 5–10 kilometers beneath the sea floor. This serpentine was then subducted, along with the upper layers of oceanic crust, before it was later uplifted to the surface during the collisional events that formed the Alps. Underlying Cabane des Dix we also saw slices of gabbro (mafic igneous rock that is part of ocean crust) and chert (marine sediments that form from clay and microfossils of single-celled radiolaria). The first photo is Jay on the serpentine outcrop with Cabane Des Dix in the background. The second photo is serpentine with slickenlines that show the direction of shearing (fault movement—glove for scale).
An interesting aspect of the Alps is that it was the first mountain range to be extensively studied by geologists and terms these early geologists coined became parts of the geologic lexicon. For example, there are many German words that we use to describe mountainous regions; for example: horst, graben, klippe, fenster, flysch, and molasse. Similarly, glaciers were first described in the Alps and some of the terms we still use reflect this: arête, cirque, roche mountinnée, esker, and moraine.
An important structural term that originated in the Alps is nappe, a recumbent fold that results from the intense compression especially common in areas of continental collisions where one limb of the fold becomes completely overturned. This extreme compression can cause layers to be carried many kilometers from their place of origin. The central part of the Haute Route is in what is called the Penninic Alps or the Penninic Nappes, because all of these rocks have been folded and carried far northward by compression associated with the Alpine Orogeny. The cross section below (from http://earthwise.bgs.ac.uk/index.php/Western_Grampians_Complex,_Grampian_Caledonides) is from Great Britain but shows what a nappe structure looks like. The yellow and pink layers on the right side of the upper and lower diagrams show this structure particularly well. Consider how long these layers would have been before they were compressed and shortened so much.
Link to this web site to see a good figure showing the evolution of the Alps in the region of the Haute Route during the Cenozoic: http://www.evolene-geologie.ch/geology/mountains-194.html
Here is a photo of Cabane Dix (sitting on its hunk of ocean crust—right side of photo) from the east after we had crossed the Glacier des Cheilon and were climbing the Pas de Chevre (Goat Pass) to access our next stop in the valley village of Arolla. Despite inclement weather, this was our favorite mountain hut. It is operated by the Swiss Alpine Club and run by a fabulous group of young people.
The metamorphic rock in this region makes a particularly lovely setting for wild flower rock gardens.