Keywords

4.1 Introduction

The Luxembourg Sandstone (unit li2 of the geological map of Luxembourg), which outcrops over ~350 km2, covers large parts of southern Luxembourg, also called Gutland (Fig. 4.1). This unit varies in thickness from a few to ~100 m and displays a regular joint pattern. Rivers have cut through the sandstone, forming impressive cliffs and bizarre landforms sculptured in the rock by erosion (Fig. 4.2). Rockfalls and landslides have played an important part in the landscape formation. Typical weathering forms involving dissolution and calcite crystallization are also observed on exposed surfaces. Rock overhangs, caves and open joints have been used as shelters in ancient times, while rocky promontories and plateaus were preferred settlement areas, with many castles still perched on them. Natural resources like water and building stones have been exploited by man since his early days, as attested by archaeological findings going back to the Palaeolithic (Le Brun-Ricalens and Valotteau 2005). Numerous artistic and touristic descriptions and sketches of sandstone landscapes in Luxembourg exist, but Lucius (1907, 1952) was among the firsts to give a scientific account of their forms and evolution (Fig. 4.3).

Fig. 4.1
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Simplified geological map and generalized cross-section of Luxembourg. li2 (light blue) Luxembourg Sandstone outcrop area. The red rectangles delimit the areas described in the text, namely Luxembourg’s Little Switzerland (Fig. 4.10) and Luxembourg City area (Fig. 4.18). © SGL 2015

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Fig. 4.2
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a Central part of the Wolfsschlucht gorge, opposing a subvertical escarpment on the uphill side of the gorge (right) and the ~30° inclined face of a slab tilted to the valley (left). In the background, needle monolith isolated from the escarpment and weakly tilted toward the gorge. b Sandstone cliff of the Bock promontory overlooking the Alzette valley in Luxembourg City. Remnants of the fortress with a defence gallery dominate the cliff. Together with the bridge, they formed the outer defence system of the fortress

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Fig. 4.3
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Early sketch featuring sandstone landforms in Luxembourg’s Little Switzerland, drawn by Lucius (1952), founder of the Geological Survey of Luxembourg

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The Lower Liassic sandstone forms with the underlying Triassic and overlying younger Jurassic sediments the Gulf of Luxembourg, a NE extension of the Paris Basin into the Rhenish Shield (Fig. 4.1). The sediments were deposited by seas transgressing progressively on a pre-Triassic peneplain cut into the Paleozoic rocks of the Variscan Ardenne and Eislek. The sandstone facies is diachronic and passes westward to younger formations (Bintz et al. 1973; Muller 1980). The Mesozoic cover has an overall thickness of ~1000 m. It was slightly folded into a wide synclinal structure with numerous minor undulations plunging gently to the southwest. The Luxembourg Sandstone nicely exhibits such fold structures in the study area. Lithologically, the alternation of harder and softer strata is remarkably expressed in the very typical cuesta landscape of Gutland and Lorraine (the Belgian equivalent of Gutland, see Chap. 23) (Fig. 4.1).

The Luxembourg Sandstone forms the Gutland’s main cuesta, which is uninterruptedly followed from Germany, through Luxembourg and Belgium, to France. Isolated buttes in front of the cuesta frequently correspond to minor synclinal undulations (Fig. 4.1). The plateau is more intensely dissected in the northeastern outcrop area, in Germany, where the thickness of the sandstone gets more and more reduced due to the SW-plunging direction of the strata and to Tertiary erosion, causing discontinuities in the cuesta relief (Hövel et al. 2015). The Sauer River crosses the sandstone area epigenetically, separating the Luxembourgian plateaus of the Mullerthal and their double cuesta to the southwest from the dissected plateaus around Ferschweiler, isolated by the Prüm and smaller creeks to the northeast. The N-flowing obsequent Alzette, a tributary of the Sauer, incised a deep valley into the sandstone and the underlying Triassic rocks at and downstream of Luxembourg City, with sandstone cliffs dominating gentler slopes developed in Keuper marls.

While the cuesta scarps are thought to be fairly stable Late Tertiary to Early Quaternary features (Busche et al. 2005; see also Chap. 23), the evolution of the valley side cliffs is more recent, probably still active during the last glacial period and, to some extent, even the Holocene. Through their control on mass movement efficiency, climatic conditions, groundwater flow and river transport determined the speed of cliff evolution. This evolution is illustrated hereafter in two picturesque and touristic areas of the Luxembourg Sandstone area, namely Luxembourg’s Little Switzerland and the site of Luxembourg City. Little Switzerland, a part of the Mullerthal region (Fig. 4.1), has a long touristic vocation due to its bizarre landforms, such as the escarpments of the Wolfsschlucht (wolf’s gorge, Fig. 4.2a), formed by a succession of large mass movement events. The City of Luxembourg (Fig. 4.1) has been built on top of incised sandstone plateaus above the Alzette and its small tributary, the Petruss. Man exploited this special topographic position, strongly fortifying the place over the centuries, shaping the landscape and exposing the up to 50-m-high cliffs.

4.2 Geological and Geomorphological Background

Rock properties and mineral composition are paramount in the understanding of weathering and erosion processes. Mineralogy is a primary control on the nature of the weathering products, while geotechnical properties help predict the rock’s reaction to physical and chemical weathering and their susceptibility to mass movements. Mineralogically, the bulk of the Lower Liassic Luxembourg Sandstone is made of quartz grains and variable proportions of carbonate (CaCO3) matrix (Fig. 4.4). Quartz grains are well sorted in the fine to medium sand fractions. Clay minerals are only present in small amounts, forming with carbonates and small percentages of quartz up to decimetre-thick marly interlayers. Accessory minerals are essentially iron oxides and hydroxides. In confined settings and under the permanent water table, pyrite is stable and the rocks are gray in colour.

Fig. 4.4
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Mineralogy of li2 (Lower Liassic) sandstones (siliceous and calcareous) and marls. The li2 unit consists of ~45% sandstones, ~45% calcareous sandstones, ~2% marls and ~6% sandy marls. Secondary minerals are mainly iron oxides and hydroxides, and pyrite. The thin orange arrow describes the weathering trajectory from bluish sandy marl (yellow dot, see also Fig. 4.8) to yellowish loose rock

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Two contrasted sandstone types are equally represented in the formation, namely sandstone and calcareous sandstone (Fig. 4.5). The yellowish sandstone contains less than 10% carbonate matrix. It has a very high porosity (up to 30%) and permeability (up to 10−2 m/s) yielding an excellent filtration capacity and a high-quality aquifer. With up to 60% calcitic matrix, the lighter coloured calcareous sandstone is much denser, having almost zero porosity and a very small permeability. Uniaxial compression strength is high but variable, possibly in relation with the degree of latent cracking state. Crack formation affects only the calcareous sandstones. The compression strength of the porous sandstones is low.

Fig. 4.5
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Geotechnical properties of the sandstones and calcareous sandstones. Owing to their high porosity, the sandstones are much lighter and less resistant to compression than the calcareous sandstones

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The Luxembourg Sandstone as a whole is cut through by a nearly vertical, sub-orthogonal network of primary joints with a metre- to decametre-wide spacing. These joints define large blocks or slabs and influence strongly the layout of the drainage system. Joints and fissures are mostly closed on the plateaus but may be widely opened by dissolution in lower lying zones of water infiltration or by unloading along the plateau edges, guided by the evolution and opening of the valleys (Fig. 4.6). A secondary fracture system is associated with the faults. These large-scale fracture and joint patterns are independent of the networks of small joints developed in the calcareous sandstones and often opened by dissolution.

Fig. 4.6
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View of the sandstone cliffs in the White Ernz valley at Larochette, with Larochette castle in the far right. Numerous large open joints perpendicular to the valley axis cut through the massif, the cliff surface following the second main joint direction

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Figure 4.7 shows generalized cross-sections through the slopes formed by the Luxembourg Sandstone and the underlying Triassic strata. The lithological profile is characterized by alternating steep slopes on hard rocks (sandstone, limestone, dolomite) and gentler slopes on soft rocks (marl, claystone). Thick packages of hard rock form cliffs and escarpments, whereas smaller ones support minor steps in the landscape. Each hard rock layer shows a high but thickness-dependent degree of fracturation, thus ensuring a fracture permeability that allows and guides waterflow even in low-porosity rocks. One notes that the Alzette valley north of Luxembourg City and the Sauer valley in the Mullerthal area are developed in similar stratigraphical and lithological positions, with valley sides consisting of Luxembourg Sandstone cliffs dominating gentler low slopes on Lower Liassic and Upper Triassic marls and, in the case of the Sauer valley, a bottom cliff cut in Middle Triassic dolomites (Fig. 4.1).

Fig. 4.7
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Schematic cross-sections through the Luxembourg Sandstone and the underlying Triassic rocks. a Theoretical “lithological” slope profile and b actual geomorphological profile, including regolith and debris cover. The slope is shaped chiefly by stone and rockfall from the cliffs and creep over the gentler slope sections. In the Sauer valley (Little Switzerland), the Muschelkalk dolomites form a cliff at the base of the slopes while the sandstone escarpment makes the edge of the plateau. Together, they constitute a so-called double cuesta. The Alzette valley north of Luxembourg exposes only the upper third of this profile. GW groundwater

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As sketched in the geomorphological profile (Fig. 4.7b), slope profiles evolve by physical and chemical weathering and soil and rock creep. Fracturation and rock and block fall produce screes. Dissolution of carbonates converts marly rocks to clay, and sandstones to sand. The regolith thickness varies from very thin on, e.g. the Steinmergelkeuper (Middle Keuper, alternation of dolomitic marl and dolomite) to several metres on marly and gypsiferous material. Depending on the geomechanical properties, slope angles vary from ~25° on marls to 45° for screes and 60° for altered cliffs. In relation with the succession of harder and softer layers, creep accumulates colluvial wedges at the foot of each steepened reach of the slopes. In the landscapes of the Luxembourg Sandstone, hillslopes appear frequently disturbed by landsliding, especially where the underlying highly sensitive Rhaetian (Upper Keuper) claystones are in contact with groundwater.

While the sandy facies gradually goes into the underlying Marls of Elvange (li1 in Fig. 4.1) at the base of the Luxembourg Sandstone, the passage to the overlying Marls and Limestones of Strassen (li3) is more abrupt and often marked by an erosion surface. Fed by direct infiltration from the top of the plateaus, groundwater circulating through either fractures or matrix porosity accumulates at the base of the reservoir layers and flows in dip direction to outlets, drains or to deeper layers. Outflow of water is either diffuse or concentrated as springs. Many large sources mark out the base of the sandstone, one of the largest, with a discharge of >3000 m3/day, being located close to the hamlet of Mullerthal. Small perched groundwater bodies may temporarily form on top of the marly interlayers (Fig. 4.8).

Fig. 4.8
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Fresh outcrop of the Luxembourg Sandstone at Reuland, mostly made of strongly fractured calcareous sandstones underlain by an impervious bluish layer of marl locating a perched water table in the sandstones. On top of the outcrop, a thin regolith smoothens the slope

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The Mesozoic sediments of the Gulf of Luxembourg have been uplifted and tilted, and underwent regional erosional levelling and river incision during the Cenozoic. The ancestor of the modern drainage system evolved on an erosion surface, still preserved on the top of the cuestas, that Lucius (1948), Louis (1953), Demoulin (1995) and Löhnertz (1994, 2003), Löhnertz et al. (2011) date to Tertiary times while Le Roux and Harmand (2014) argue for a Cretaceous age. As evidenced by the courses, independent of the dip direction of the strata, of the Moselle and its (sub)tributaries Sauer and Alzette, this river system evolved epigenetically. Fluvial incision into the surface probably began sometime at the end of the Neogene but especially accelerated at ~0.8 Ma, when an active mantle plume under the Eifel region is assumed to have caused strong and fast regional uplift (Meyer and Stets 1998; Garcia-Castellanos et al. 2000; van Balen et al. 2000). The cuesta landscape started then to form. There is consensus that the position of the main cuesta escarpments, at current altitudes of ~400 m, has changed only slightly since the early times of their formation (Tricart 1949; Busche et al. 2005; Liedtke et al. 2010). Narrow valleys with steep cliffs and flat valley bottoms developed in the first stages of incision into the sandstone, as long as rivers were powerful enough to remove the material delivered by the hillslopes. In case of insufficient energy, the coarse material of rockfalls accumulated on the valley floor as debris cones, stabilizing the cliffs and preserving V-shaped transverse profiles of the valleys. However, as soon as the underlying marls and claystones were exposed by further incision, slide processes tended to widen the valley floors. Following the hypothesis of Louis (1953) and Löhnertz (1994, 2003) for rivers in the nearby S Eifel, the observation that the valley of the smaller Alzette is broader than that of the larger Sauer might perhaps indicate that a pre-Quaternary (Paleogene?) Alzette valley had been incised, then filled up and, finally, re-excavated in more recent times.

4.3 Luxembourg’s Little Switzerland and the Wolfsschlucht (Wolf’s Gorge)

Luxembourg’s Little Switzerland (LLS) is part of the larger Mullerthal region (Fig. 4.1). Due to their majestic cliffs, bizarre rock formations and impressive weathering structures, its sandstone landscapes are touristically promoted since the last quarter of the 19th century. First hiking trails were installed as early as 1879 toward the famous cascade of the Schiessentümpel, near the hamlet of Mullerthal (Fig. 4.10), or 1881 from Echternach to the impressive gorges of the Wolfsschlucht (Figs. 4.2, 4.9 and 4.10) (Massard 2012).

Fig. 4.9
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Another view of the Wolfsschlucht’s western part. The plateau is on the left, limited by the subvertical escarpment, the sandstone slabs in the right are tilted downslope. The interlayering of thicker calcareous sandstone and thinner sandstone is enhanced by systematic differential erosion

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Fig. 4.10
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a Geology, draped on the DEM of the Mullerthal region (see location in Fig. 4.1). Below the Luxembourg Sandstone (li2) that forms the main plateau are Lower Liassic marls and limestones (li1) and Keuper (ko, km) and Muschelkalk (mo) rocks. The sandstone is in turn locally covered by marls and limestones (li3-4). Fluvial terraces of the Sauer are noted ‘dt’. 1 Wolfsschlucht, 2 cut-off meander of the Sauer at Thull, 3 Raiberhiel and Siewenschleff (Fig. 4.14), 4 Schiessentümpel cascade (Fig. 4.15). Open and filled blue circles sources and wells. The green dotted rectangle locates the geomorphological map excerpt shown in Fig. 4.11. b Schematic cross-section along the black line crossing the northern part of the sandstone plateau in a. The long profile of the eastward flowing Sauer is projected onto the cross-section. Map colours conform to the international colour code. © SGL, ACT

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Geologically, the LLS is located close to the northern limit of the Luxembourg Sandstone outcrop area and its bordering cuesta. The frontslopes of a doubled cuesta makes here the southern side of the Sauer valley (Figs. 4.10 and 4.11). The N-flowing Black Ernz, White Ernz and Lauterborn creek incise the sandstone plateau locally covered by residual Marls and Limestones of Strassen (li3 in Fig. 4.1). The network of smaller creeks mostly traces the primary joint system of the sandstone. The attitude of the 60- to 90-m-thick sandstone is determined by the SW-plunging Weilerbach syncline and its smaller undulations interrupted by a few mainly N-trending faults (Fig. 4.10). The underlying Marls of Elvange (li1) and Triassic strata are exposed in the valley of the Sauer and in the lower course of the Black Ernz, with a well-expressed small cliff in Middle Triassic dolomites near Grundhof (Fig. 4.10). About 100 m of soft marls separate this cliff from the proper frontslope of the sandstone cuesta. Springs mark the base of the sandstone body where the strata locally dip toward the escarpment.

Fig. 4.11
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Excerpt of the geomorphological map (1:100,000) showing the importance of the structural component in the landscape and geomorphology of the Echternach region (Désiré-Marchand 1985). mo upper Muschelkalk. km middle Keuper. li2 Luxembourg sandstone, Lower Lias. li3 Strassen marls and limestones, Lower Lias. am modern alluvium

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Figure 4.11 shows an excerpt of the geomorphological map of Luxembourg by Désiré-Marchand (1985) near Echternach. The split frontslope exposed along the southern side of the Sauer valley features here the cuesta of the Luxembourg Sandstone as a double cuesta (not drawn as such on Fig. 4.11). North of the Sauer, the Ferschweiler plateau around Ernzen is also limited to the east by an important escarpment. East of the White Ernz and south of the Thull cut off meander (Figs. 4.10 and 4.11), the top of the cuesta culminates at altitudes of 400–410 m as a preserved remnant of the Tertiary topography. Residual soft Marls and Limestones of Strassen on the dipslope of the cuesta attest that this erosion surface levelled indifferently rocks of contrasted resistance. The weaker rocks were then eroded chiefly during the Quaternary.

4.3.1 Cliff and Gorge Formation

The Wolfsschlucht is a system of escarpments and gorges that extends parallel to the valley side of the Sauer and breaks apart the upper part of the frontslope of the cuesta north of the Erelchen plateau (Fig. 4.12). The gorges are up to 200 m in length, in the mean ~20 m wide and similarly deep. The most spectacular eastern part is 50 m long and up to 40 m deep. The uphill side of the system is characterized by almost vertical cliffs following the primary joint direction, while the downslope gorge walls are formed by large sandstone slabs inclined by ~30° (Lucius 1907) (Figs. 4.2 and 4.9). Blocks and slabs are also limited laterally by major joint surfaces approximately perpendicular to the primary one.

Fig. 4.12
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a Historical topographic map (1:20,000, 1979) of the Erelchen Plateau, south of the Sauer valley at Echternach, locating the Wolfsschlucht (“Gorge du Loup”, along the red-coloured hiking trail) at the top of the cuesta escarpment. Yellow lines delimit three large landslide scars (A, B, and C) © ACT. b Aerial photograph of the same area, taken from the WNW. One notes how landslide B displacement pushed the Sauer channel northward © Rol Schleich

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While the upper escarpments are discontinuous and scattered with debris cones, the lower parts of the slopes to the Sauer are generally steep, irregular, and interrupted by benches, a morphology that unequivocally betrays a slope evolution by rotational and translational landsliding affecting the whole hillslope and escarpment. Field evidence and the overall topography on maps and DEMs allow identification of three large interconnected landslides (Fig. 4.12). These landslides are ~300 m in width on average. Scarp size and drillings through more than 30 m of alternately sandy and marly regolith in the slipped bodies point to very deep-seated surface of rupture. They probably were successively active for a long period of time, with last movements possibly in historical times as suggested by a major episode of slope sliding that forced a ~40 m northward displacement of the Sauer Holocene channel in the central zone (B, Fig. 4.12). In LLS in general and the plateaus north of Echternach in particular, most large movements are assumed to have taken place mainly during the last Glacial through destabilization of the escarpment foot by active solifluction in the marls and clays (Römer 2002; Reinheimer et al. 2010). The rockfall that induced the Irrel waterfall in the Prüm valley, ~5 km north of Echternach (Fig. 4.11), has been dated to 15,000–13,000 years BP (Hill, in Schröder-Lanz 1984; Dittrich et al. 1997).

The landslide A of Fig. 4.12 is characterized by many large springs (with a total discharge rate of about 600 m3/day) and diffuse outlets emerging at various altitudes in the middle of the slope, which might indicate that this slide remains the most prone to reactivation. In the upper part of landslide zone A, spectacular escarpments with numerous tilted blocks are observed (Fig. 4.9). However, the larger slide B is responsible for the most impressive central part of the Wolfsschlucht (Fig. 4.2), which opened as a result of gliding, tilting, toppling and backtilting movements of rock slabs.

To the SE of landslide C (Fig. 4.12), talus slopes with alternating scree cones with elements up to several m3 in size and stony regolith embedding isolated blocks bury the base of the sandstone escarpment. Locally, blockfalls may be directly related to the toppling and disintegration of sandstone monoliths. Large monoliths may also move longer distances downslope by falling, bouncing and rolling, as exemplified by the 100 m3 sandstone block lying in the Sauer River at Weilerbach, ~2 km to the NW of the Wolfsschlucht. There, chaotic slopes scattered with many blocks attest recent escarpment collapse.

Along the Erelchen plateau escarpment, the contact between the vertical sandstone face and the scree cones locally occurs through an up to 10-m-wide bench, which is key to the understanding of the mechanism of whole slope evolution (Fig. 4.13). Rockfall-induced accumulation of blocks at the base of the sandstone cliff loads the upper part of the slope cut in the underlying Elvange marls and Rhaetian clays highly prone to landsliding. The combination of such loading with episodes of increased infiltration through the Luxembourg Sandstone induces high hydraulic pressures in the underlying limestones and Rhaetian sandstones, which in turn cause multiple rotational landsliding in the marls and clays up to the base of the sandstone escarpment, inducing flattening of the upper part of the slipped body and preparing further sandstone slab toppling (Fig. 4.13). As a result, slightly displaced slabs leave 1- to 2-m-wide open joints with parallel walls, such as the Devil’s Crevice (“Brèche du Diable”, Fig. 4.12) or the Siewenschleff at Berdorf (Fig. 4.14). Thick calcareous tufa accumulated in the now dry Devil’s Crevice, originating in water percolation through the Marls and Limestones of Strassen preserved locally on top of the plateau and infiltration in the formerly not wide open joint.

Fig. 4.13
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Cross-sections showing the evolution of slope failure and gorge opening at the height of landslide B and Wolfsschlucht (see Fig. 4.12). Successive individual slides assumed to have evolved retrogressively (see text) are represented. At the base of the slope, the Sauer channel was displaced by ~40 m. At the top, tilt or backtilt motions of destabilized individual sandstone slabs along the cuesta edge are determined by the location of their respective centre of gravity with respect to the underlying shear surface

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Fig. 4.14
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a Siewenschleff (seven narrow passages). Horizontal displacements of several decimetres to 2 m are observed in the two primary joint directions. b Raiberhiel (robbers’ cave). Narrow cave formed by backtilting above a rotational slide motion. Inclined stratification on the backtilted slab evidences the rotation angle. Both photographs taken near Berdorf (see Fig. 4.10, point 3)

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While many slabs are tilted toward the valley by up to 30° (Lucius 1907), others are backtilted, creating triangular caves like the Raiberhiel (Fig. 4.14b). In the Wolfsschlucht, depending on the position of their respective centre of gravity with respect to the circular rupture surface that propagated from the downslope marls and clays below the base of the sandstone escarpment, many tilted slabs lean against backtilted blocks (Fig. 4.13). This occurs when the curved shear surface ends up in a vertical joint of the sandstone, causing rotation and up to 45° backtilt of the outward slab and destabilizing and tilting the next one.

Downslope, multiple rotational landsliding occasionally evolved into translational movements that reached the foot of the valley side, as attested by the 500-m-long reach of the Sauer that was displaced by ~40 m toward the inner side of the broad channel meander just upstream of Echternach (Figs. 4.12 and 4.13). In case of compound landslides, the usual hard-to-answer question is whether the system evolved progressively or retrogressively (i.e. down- or uphill). Here, several observations point to the latter, namely (1) the relative freshness of many rock fragments in the scree cone with respect to more altered material in regoliths elsewhere in LLS and (2) the overall freshness and length of the escarpments, which both point to the young age of the escarpments, and (3) the increased incision and lateral erosion of the Sauer after the Thull meander was cut off at the Tardiglacial-Holocene transition (Coûteaux 1970), which probably induced the destabilization of the foot of the slope, whereas 6-m-thick alluvial deposits stabilize it now.

Sandstone cliffs and their rim of scree cones and stony regolith are also found in all small narrow valleys, which have however not yet incised the underlying marls and clays and consequently do not display the lower lying gentle slopes. Though still active currently, e.g. through dissolution of the carbonate matrix, weathering processes affected every sandstone outcrop especially during the cold phases of the Pleistocene, opening joints and liberating blocks by the combined action of freeze-thaw cycles and vegetation roots. Currently, most of the material fallen at the cliff foot cannot be removed by the creeks, which are able to transport only sand and gravel. Many rivers (e.g. Black Ernz, Alzette, Lauterborn creek) but not all (e.g. White Ernz) display prominent knickpoints in their long profile at, or upstream of, the point where they cross the contact between the Luxembourg Sandstone and the underlying weaker rocks. In the Black Ernz, the Schiessentümpel cascade marks the base of such a knickpoint near the hamlet of Mullerthal (Figs. 4.10 and 4.15). While the knickpoint is cut into the sandstone, the channel bottom downstream of it exposes the basal marls, causing many large springs to flow out at the channel’s level. Upstream of Mullerthal, the Black Ernz valley shows a strongly asymmetric transverse profile, which might to some extent be related to the slight synclinal undulation cut obliquely by the river in this area (Fig. 4.10b). The river flows along the western valley side, where it removed all debris delivered by the hillslope except the largest sandstone blocks, which clutter the river bed and form the picturesque cascade.

Fig. 4.15
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Schiessentümpel cascade in the Black Ernz valley near the hamlet of Mullerthal (see Fig. 4.10, point 4). Large sandstone blocks collapsed from the cliffs crowning the valley sides cannot be removed by the current river and form the cascade

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4.3.2 Overview of Weathering Structures in the Sandstone

Many weathering structures and microforms have developed in the body and on the free faces of the Luxembourg Sandstone, partly in relation with valley incision. They also depend on the contrasted carbonate content between the yellow sandstone and the lighter coloured fractured calcareous sandstone. The sandstones show grain size variations that induce changes in porosity at the layer scale. As for the calcareous sandstones, despite their very low porosity, they may develop high fracture permeabilities. They are also characterized by the greatest structural variability, alternating continuous layers of variable thickness and lenses or rounded masses aligned or scattered within non-calcareous sandstones. Chemical weathering mainly occurs through carbonate dissolution by water circulating in the outcropping sandstones and at the base of the sandy regolith. The carbonate dissolution front, undulating in function of the joint density, generally lies a couple of metres beneath the regolith. Secondary calcite precipitation occurs within the weathering structures or as tufa deposits in open joints and in creek channels. Pyrite oxidation occurs in non-dissolved gray sandstones above the water table, inducing carbonate dissolution and the formation of secondary gypsum, which is subsequently leached by percolating waters.

4.3.2.1 Large Open Fissures

Large open irregularly walled fissures of the primary network are seen everywhere in the sandstone outcrop area (Fig. 4.6). Runoff concentrating in the topographical depressions of the plateau surface infiltrated the fissures and opened them progressively by dissolution of the carbonate matrix (up to more than 40% of total volume) and removal of the sandy residue by lateral flow toward the valleys. This mechanical emptying of the fissures was probably most efficient during the Pleistocene periods of active river incision, in general warm/cold transitions (e.g. Vandenberghe 2008; Demoulin et al. 2012), which allowed for sufficient hydraulic gradients. Nowadays, these deep open joints are sealed by a weathering mantle, which collapses sometimes during rainy periods.

4.3.2.2 Dissolution in Calcareous Beds

Dissolution along the fissures of the calcareous sandstones is very common, leaving all kinds of figures from wide open fissures giving the appearance of a set of teeth to the layer (Fig. 4.16b, c) to broadly spaced pillars in the final steps of a layer’s dissolution and, locally (e.g. in the upper part of the Wolfsschlucht escarpment), to keyhole-like voids indicating groundwater flow under pressure. The latter imply a groundwater level above the observation level, thus suggesting formation at a very early stage of valley incision and scarp formation. Similar structures found at different altitudes all over the sandstone area might document the incision history of the creeks into the sandstone (Adamovič et al. 2015).

Fig. 4.16
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Typical weathering structures covering the sandstone rock faces in Luxembourg’s Little Switzerland. a Differential weathering and erosion between interlayered resistant calcareous and weaker porous sandstones, b and c fissures opened by dissolution in the calcareous sandstone layers, d holes created by surficial dissolution and liberation of calcareous sandstone nodules out of porous sandstone layers, e honeycomb weathering structures and f rock overhang

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4.3.2.3 Elliptical Cavities in the Sandstone

When calcareous sandstone nodules are embedded in porous sandstones, percolating water concentrates at the outer surface of the impervious nodules and dissolves the calcitic matrix of their outer rim, isolating progressively the nodules from the surrounding rock. If liberation is complete, the nodule may fall out, leaving a cavity in the sandstone (Fig. 4.16d). This very common process mostly observed on exposed joint walls is active even today in zones of diffuse infiltration of water close to exposed surfaces.

4.3.2.4 Honeycomb Weathering

Honeycomb structures (Fig. 4.16e) are also frequently found. Surfaces showing narrow ridges ringing shallow depressions are common, while zones of almost spherical cm-size hollows are less frequent. It seems that shallow depressions are often precursors of the spherical hollows. Honeycomb, absent in the Wolfsschlucht area, is particularly abundant on the escarpments near Nommern, NE of Mersch (Fig. 4.1). Occurrences are limited to the porous sandstone, where honeycombs follow the layering and associated grain size variations. Their formation remains unclear, possibly involving the formation of a crust by evaporation of outward-diffusing water and calcite recrystallization, then crust perforation and disintegration of the subcrustal sandstone in regularly spaced hollows and, finally, re-induration of the hollowed surface. The process could be repeated cyclically, as described by Robinson (2007). Salt action is thought to take often an active part in honeycomb formation (Robinson 2007). In the case of the Luxembourg Sandstone, salt may come from pyrite oxidation in fresh gray sandstones liberating sulfate ions or from residual gypsum leaching. Löhr (2012) dates honeycomb formation by archaeological evidence to early Postglacial times, though evidence from modern periglacial environments (Weise 1983) suggests it might also date back to colder periods.

4.3.2.5 Differential Weathering and Erosion

Differential weathering and erosion often nicely enhance sedimentary structures like cross bedding and layering (Fig. 4.16a). Less resistant sandstone layers are in depression while the hard calcareous sandstones stand out. Beside mechanical erosion by wind and rain, chemical dissolution, aided by the presence of moss, acts mainly where humidity is sufficient in the more porous sandstones.

4.3.2.6 Crust Formation

Crusts are quite common in the City of Luxembourg but rare in the Mullerthal region. Thin, often black crusts are produced by crystallization of various atmospheric salts incorporating dust particles. They are mostly limited to porous sandstones, the surface of which they protect. Once the crust is removed, the freshly exposed rock surface is very prone to abrasion of sand particles loosened by the dissolution of the carbonate matrix.

4.3.2.7 Rock Overhangs in Creek Valleys

Large regular rock overhangs with rounded concave forms are observed in many creek valleys at different heights above the valley bottom (Fig. 4.16f). They are believed to have developed essentially by lateral fluvial erosion and abrasion of the soft sandstone by the bed load during high discharge events. At the same time, the calcareous sandstone was broken up and plucked along small fissures, locally creating small cascades. However, other overhangs might have formed as a result of either freeze-thaw cycles or weathering enhanced by the soil moisture (Cílek and Žák 2007; Robinson 2007; Römer 2002).

4.4 Luxembourg City in Its Natural Landscape

The City of Luxembourg (Fig. 4.1) has been founded around 963 AD on the rocky Bock promontory of the plateau dominating the river Alzette and its small tributary, the Petruss. From the 12th century onward, it was progressively fortified to one of the most important fortresses of Europe, called “Gibraltar of the North” (Fig. 4.17). Spanish and even older construction works were largely expanded by the Marquis de Vauban under the French occupation in the late 1680s. He employed up to 15,000 people, exploited many quarries and, besides overground works, constructed about 4 km of underground galleries. Older works were maintained and completed during later occupations. While important fortifications had to be demolished in 1867 following a clause of the Treaty of London, interest arose in keeping witness remnants, notably for touristic reasons. From the late 19th century, the city expanded in different phases on the plateaus and the floor of the deeply incised valleys. The remnants of old quarters and fortifications are registered in the UNESCO World Heritage List since 1994.

Fig. 4.17
figure 17

a Excerpt of an historical city map of Luxembourg City (Gronzka 1942) by R. Gronzka, showing the Alzette and Petruss valleys and the cleaned-up cliffs surmounted by the fortress walls. b Schematic geological cross-section through the Alzette and Petruss valley and the plateaus of Luxembourg City (bold red line in a) showing the importance of both surface and underground defence works. w fortress well; c casemates: 17 km of underground galleries were spared of 23 km before dismantling; a alluvium; li2 sandstones and subordinate marls (see Figs. 4.4 and 4.5); li1 limestone and marl; ko clay-and sandstone, km3 dolomitic marl and dolomite. Dashed lines reconstruct initial slopes, showing the importance of valley remodelling. Remnants of the fortification walls, buried under 19th century filling along the plateau SW of the Petruss, are in dark gray

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The City of Luxembourg extends mainly on the undulated SW-plunging strata of the Luxembourg Sandstone (Fig. 4.18b), covered only to the west and southwest by progressively more continuous and thicker marls (li3-4 and lm in Fig. 4.1). By contrast, Lower Liassic marls and Keuper sediments underlying the sandstone are observed north of Luxembourg City, in the valleys of the Alzette and its larger tributaries. The orientation of the small valleys is largely determined by the primary joint system of the sandstone outcrop area, which is limited by a fault in the south.

Fig. 4.18
figure 18

a Geology, draped on the DEM of the Luxembourg city area (see location in Fig. 4.1). See Fig. 4.10 for the stratigraphic legend. Sandy marls and limestones (lm) are in green. Scattered undifferentiated Cenozoic deposits and weathering products (d) are found on the plateaus. Open and filled blue circles sources and wells. Light yellow squares main quarries. Compare the extent of modern Luxembourg (gray areas) with the area enclosed in the fortifications (thin red lines) © SGL, ACT. b Schematic cross-section along the black line in a showing the undulated structure of the Mesozoic cover, cut by an E-striking fault near Hesperange. The long profile of the northward flowing Alzette is projected onto the cross-section

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The difference in rock resistance to weathering and erosion is particularly visible in the incision of the Alzette valley. The valley floor is ≥1 km wide north and south of the sandstone outcrop area whereas it is <0.3 km where the river flows on the sandstone. South of Hesperange (Fig. 4.18a), a very broad valley with flat hillslopes is cut in thick friable Middle Liassic marls. North of Luxembourg City, the sandstone cliffs dominating slopes similar to those described in the Sauer valley at Echternach (Fig. 4.7) limit the width of the valley to ~2 km. These cliffs are almost continuous down to the Mersch area, where they go into the escarpments of the WSW- to SW-striking sandstone cuesta (Fig. 4.1). Topographical map analysis and geotechnical surveys evidenced numerous large old landslides on the marly slopes, supposed to have been active at the end of the last Glacial but now stabilized by Holocene fluvial accumulation at the foot of the slopes. Landsliding occurred along shear surfaces in the Rhaetian claystones, in a context of steeper slopes and higher and much more variable groundwater level and hydraulic pressures than today. Currently, only small, though numerous, landslides are triggered by human activity on these slopes.

At Luxembourg City, the Alzette and Petruss have dissected the sandstone body into several wide plateaus (Fig. 4.18). The Alzette valley width between cliffs varies from ~50 m at the entrance of the river in the sandstones, north of Hesperange, to ~300 m. The Petruss, a small tributary whose valley is however of similar width, joins the Alzette at an altitude of 245 m, ~50 m below the historic centre of the town located on the top of the plateau. One may still observe how the fortress was made more impregnable by building up to 12 m high walls above the high subvertical cliffs along the two valleys (Fig. 4.17). The size and extent of the fortress walls are spectacularly depicted on numerous watercolor paintings by the English Romantic landscape painter J.M.W. Turner (Fig. 4.19a), as is also the importance of the dismantling activities of the fortress in the 19th century by a lithography of P. Blanchard (Fig. 4.19b). The box valley (Kastental) shape of the valleys, with steep vertical cliffs and flat valley bottom (Fig. 4.17b), has been sculptured by man, who removed the regolith at the foot of the cliffs to straighten the steep valley walls. Cliff shaping always followed the sandstone’s primary joint system in order to satisfy the military aim of avoiding dead angles. The valley floor of the Alzette was widened not only for military reasons but also to allow construction of the medieval craftspeople quarter of Grund. An old 50-m-deep well in the central plateau of Saint Esprit reached the groundwater table and provided the fortress with drinkable water in case of siege. The quasi-absence, except locally in the upper part of the cliffs, of typical weathering features similar to those described in LLS underlines the extent of the cleaning up by the fortress builders. Now, moss and lichen extensively cover favourably orientated rock faces mainly of the porous sandstone layers. More generally, first signs of differential erosion already reappear and encrusting is common. In contrary to Vauban’s time, when no vegetation was tolerated on the cliffs and walls, bushes and small trees are now allowed to grow on the cliffs for environmental reasons, causing however more frequent stone or rock fall than formerly. Continuous cliff cleaning up, stability inspection and, occasionally, stabilization works are thus necessary to ensure long-term protection.

Fig. 4.19
figure 19

a. WSW-looking view from Fetschenhof on the fortifications of Luxembourg City, drawn around 1839 by J.M.W. Turner. Watercolor, gouache and ink on blue paper. The central Bock promontory leads to the Plateau of the city and, in the left background, the Plateau Saint Esprit. Left middle ground the Rham Plateau, separated from the Plateau of the city by the Alzette valley. Right middle ground the Kirchberg Plateau (Courtesy and collection of the National Museum of History and Art, Luxembourg). b. Lithography illustrating the dismantling of the Luxembourg fortifications in the second half of the 19th century, by P. Blanchard

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4.5 Conclusion

The picturesque landscapes of the Mullerthal region with their long touristic vocation in Luxembourg’s Little Switzerland have preserved their natural character and nicely expose all geomorphological landforms typical of sandstone areas. The valleys incised in the plateau of the Lower Liassic Luxembourg Sandstone display an all the more large variety of rock sculpturing features as this Formation alternates beds of porous sandstone and calcareous sandstone. The region is accordingly characterized by a high diversity of weathering structures and erosion forms at all scales, which participated in, and therefore allow the reconstruction of, its Quaternary landscape evolution dominated by valley incision and gravitational mass movements. These easily accessible landscapes in the heart of West Europe are the quintessence of sandstone geomorphology. As such, they have high scientific and educational values that deserve to be preserved and valorized. This has recently come in addition to ecological and biodiversity considerations to decide the creation of the natural park Mellerdall. We also seized the opportunity of the Luxembourg City site to highlight how man always took advantage of the terrain characteristics to develop and protect his oldest settlements but also how he concurrently became a major factor of landform evolution.