Looking at Luxembourg
By Phil Marston - Photo by Kirsty Crocket
The Luxembourg Sandstone, on which Luxembourg City stands, was laid down in the Jurassic, about 170 Ma when the area was at the margin of a warm, shallow sea. Unfortunately, that climate wasn’t much in evidence when 12 of us gathered shivering in the Old Town on a cold, misty, January Sunday, just after the AGM.
We were at the bottom of a steep-sided valley, cut through the sandstone by the Pétrusse River, just above its confluence with the larger Alzette. It was early and there were few people about; probably all sensible Luxemburgers had headed south for the ski slopes or the sunshine. The ground, even in the car park, was slippery with ice and the gilded spires of the cathedral merged into the murk of the low grey cloud that loomed just a few hundred metres over the city.
Luxembourg City Tour
Our guide for the morning was as brisk as the weather, but much warmer. Alain Faber is the Curator of Palaeontology at Luxembourg’s National Museum of Natural History, and he had generously agreed to show us around the ‘sentier géologique’, the city’s geological path. This route follows both sides of the Pétrusse River from the Pont Adolphe to the ‘al Bréck’ or Old Bridge, and is marked by ten wayboards, explaining the formations and fossils that are found in the valley. One board shows how local stone has been used to build the mighty Pont Adolphe which, when it was built in 1903, had the largest stone arch of any bridge in the world, at 84 metres high. Now though, this bridge is in trouble. A hundred years of weathering has badly eroded the sandstones forming the arch and a major restoration campaign is being planned in this centenary year. Like many city bridges, the Pont Adolphe now carries a weight of traffic beyond the imagining of its builders. At rush hour, there can be 30 buses on the bridge at any one time, a sobering thought if it is pointed out to you when you are standing under the thing, even on a Sunday morning.
In the Lower Lias the area where Luxembourg City now stands was under shallow marine water in the north eastern part of the Paris Basin and formed the so-called Gulf of Luxembourg, where sedimentation was characterised by the formation of a locally retrograding sedimentary prism.. Erosion from the uplifted Ardennes and Eifel regions to the north provided the sediments for the ‘Grès du Luxembourg’ (Luxembourg Sandstone) the lower levels of which date from the early Hettangian. The rock is fine to medium grain sandstone with a calcite cement and the degree of cementation varies, so the strata range between massive calcite-cemented sandstones and lenses of sandy limestone. These lenses are evident in the sides of the Pétrusse valley and Alain pointed out the numerous faults in the formation. The Grès du Luxembourg is highly porous and an efficient aquifer and the faults give it a permeability which can be seen in the many springs feeding the Pétrusse River all along the valley. The present river level is at the bottom of the sandstone formation, at the top of the underlying Helmsingen Marl which underlies the Grès du Luxembourg.
This is an area where history is intimately bound up with geology. The Pétrusse and Alzette rivers eroded the local sandstone to produce the bluffs on which fortified settlements like the Bock, the city’s oldest and proudest fort, could be built. Now, the Bock and its surrounding fortifications look like they have grown naturally out of the rock and the Chapel of St. Quirin, where we started our tour, is actually built into the sandstone on the valley side, above the springs.
When the city was under siege by the French in 1794, the inhabitants were well supplied with water from a central well sunk into the sandstone aquifer. Alain showed us a secret entrance, now barricaded by an iron door, through which the besieged inhabitants brought in food and snuck out on raiding parties against the surprised French troops. But in the end, even though the geology provided water, the French found and cut off the food supplies and starved the Luxemburgers out.
This is an intriguing short tour in mid-winter, so it should be beautiful in spring and summer. Some of the wayboards illustrate bedding and fossils in the adjacent rock and together they provide an excellent overview of the local geological formations and their incorporation into the history and construction of the city.
Don’t forget to visit the museum itself, which is at the start of the 'sentier geologique' and from where you can pick up a free explanatory leaflet. You can get more information at the museum’s website at www.mnhn.lu
I would like to express our grateful thanks to Alain Faber and to his family, who allowed us to prise him away from a warm hearth to a cold river valley on a Sunday morning. Our visits and field trips are only possible because of the knowledge and enthusiasm of people like Alain.