Geomorphology of the high-elevated flysch range – Mt. Babia Góra Massif (Western Carpathians)

A detailed geomorphological map of the Mt. Babia Góra Massif (1725 m a.s.l.), at a scale 1:10,000 is presented. A slope, glacial, periglacial and ﬂ uvial features were mapped on the base of the coupled ﬁ eld studies and LiDAR DEM analyses. This study underlines the complexity of rock slope failures (RSFs), in shaping the morphology of compact and isolated upland, exhibiting one of the highest incidences of RSFs yet recorded in Europe (29% for a whole massif, and 45% for the north face). A total of 212 RSFs were mapped of which 18 failures are large landslides (>0.25 km 2 ), among them is one of the largest known (2.6 km 2 and 150 × 106 m 3 ) in the Polish Flysch Carpathians. The mapping and Schmidt-hammer results shed light on the problem of glacial relief of the massif, documenting the remnants of glacial deposits beyond the tongues of landslides in the Szumi ą ca Woda valley.

Because of the forested nature of the Flysch Carpathians, gravity-induced landforms are covered by vegetation and thus regional-scale landslide inventories are quite recent and based on systematic field mapping (e.g. SOPO Project; Mrozek et al., 2014) and LiDARbased mapping. Recent mass movements inventories led to the discovery of approximately 60,000 objects in the Polish part (Mrozek et al., 2014), and over 13,600 landslides in the Czech part of the Outer Western Carpathians (Pánek et al., 2019). Although the presence of slope failures on the Mt. Babia Góra has been noted for over 100 years (Alexandrowicz, 1978;Łajczak, 2014;Rehman, 1895;Zapałowicz, 1880;Ziętara, 2004;Ziętara & Ziętara, 1958), the lack of highresolution topographic data significantly constrained landform recognition and mapping. Deep landslide scars and trenches are clearly visible above the tree line, however, entire landslide distribution and extent were widely undetected due to their occurrence on steep slopes on inaccessible mountain terrain covered with dense forest or shrub pine cover. Recently, new features of landslide relief were discovered using LiDAR data in selected test areas (Kłapyta & Kolecka, 2015;Łajczak et al., 2014).
Due to high altitude and location close to the northern fringe of the Carpathians, in a periglacial corridor between the Fennoscandian ice sheet and the ice masses of the Alps (Heyman et al., 2013), there were potentially suitable conditions for local glaciation in the Babia Góra during the Quaternary. The presence of glacial landforms and sediments has been vigorously discussed. The early concept of the presence of an extensive ice cap during the Riss and cirque glaciers during the Würm glaciation (Sawicki, 1913) was largely rejected by further studies on the basis of the landslide genesis of many scars and hummocky boulder deposits (Alexandrowicz, 1978;Starkel, 1960;Ziętara, 1962;Ziętara & Ziętara, 1958). Nonetheless, the glaciation hypothesis was still accepted by some authors (Klimaszewski, 1952;Książkiewicz, 1963;Niemirowski, 1963), but has evolved into two concepts according to which glacial landforms are (i) older and subsequently modified by large scale paraglacial mass movement processes (Książkiewicz, 1963(Książkiewicz, , 1966Niemirowski, 1963;Wójcik et al., 2010), or (ii) younger than large Neogene-early Quaternary age landslides and that some of their scarps have been glacially remodelled (Łajczak, 1998, 2014).
The aim of this study is to present the first detailed geomorphological map of the entire Babia Góra Massif, based on combination of extensive field landform inventory mapping and LiDAR data analysis. These data allow for the first time to create a comprehensive picture of landform assemblages in this area with the special attention to the geomorphic imprint of RSFs. A combination of geomorphological mapping with relative age dating and sedimentological analysis sheds new light on relations between mass movement and glacial features in the Babia Góra Massif.

Methodology
Geomorphological mapping encompassing 80% (∼57 km 2 on a Main Map) of the Babia Góra Massif (72 km 2 ) was made by integrating both detailed field landform inventory and remote sensing techniques. The boundaries of the massif were defined according to Łajczak (2016) (Figure 1(b)). Due to its shape (9.4 × 4.1 km), the Main Map excludes external, low-relief part of the massif with single occurrence of mass movements (Figure 1(b)). These landforms were included in the calculation of total area affected by RSFs. Detailed field mapping at 1:5000 scale was carried out over a five-year period (2013-2018) and was complemented by analysis of high-resolution (1 × 1 m) airborne LiDAR-derived Digital Elevation Model (DEM) and colour orthophotos taken by the Centre for Geodesy and Cartography of Małopolska council in 2002, with a pixel size of 25 cm.
All geomorphological features were identified and manually digitised employing on-screen mapping using LiDAR-derived hillshade, slope and contour (contour interval of 2 m) as a base map created in Arc-GIS software. Mapping was also supported by comparison with previous geological and geomorphological maps (Alexandrowicz, 2004;Golonka & Wójcik, 1976;Książkiewicz, 1971Książkiewicz, , 1983Teťák et al., 2016;Wójcik et al., 2010), which were scanned, georeferenced, and critically analysed by comparing them with field results and remote sensing data.
Mapped landforms have been divided into four different morphogenetic systems: slope, glacial, periglacial, and fluvial. The Rock slope failure (RSF) classification of Jarman (2006) and Jarman and Harrison (2019), embracing rockslides, rock avalanches and rock slope deformations, was adopted in the mapping. Because flow-like forms are integral part of the rockslides, they were mapped as a one type. A threshold size of 0.25 km 2 for large landslides was adopted according to Jarman (2006), whereas the lowest identified size was 0.0003 km 2 . RSFs morphology was defined by the occurrence of typical diagnostic features for mass movements as: scarps, tension cracks, irregular 'hummocky' terrain with bulges and toes and slide or backward rotated blocks (Břežný & Pánek, 2017;Van Den Eeckhaut et al., 2012).
Because moraines are hardly distinguished from landslide deposits based solely on surface morphology, additional field analyses including clast shape and roundness measurements for 50 clasts at each site  (Benn & Ballantyne, 1994) together with Schmidt hammer (SH) relative age dating (Shakesby et al., 2006) were performed in the Szumiąca Woda valley, where previous research indicated the presence of glacial landforms. The SH measurements followed the methodology of Matthews and Shakesby (1984). Thirty hammer impacts were recorded per boulder from a total of five boulders at each of 25 measurement sites, and the mean rebound value was calculated. The rebound values (R-values) tend to decline in response to rock-surface weathering and hence they reflect the time for which the rock surface has been exposed to subaerial processes (Kłapyta, 2013;Matthews & Shakesby, 1984).
The main map layout is at 1: 10,000 scale when printed on A0 sheet. The background shaded relief map and 25 interval contours generation were derived from the DEM and performed in ESRI ArcGIS, where also further landform recognition and interpretation were carried out (ESRI, 2016). The digitised vector files for the base data were converted from ArcGIS format and importing into CorelDRAW, where final map processing was performed.

Study area
The Babia Góra Massif is the highest mountain range in the Outer Western Carpathians (Diablak, 1725 m a.s.l.; 49°34'23''N; 19°31'46''E), located close to the northern fringe of the Carpathians, at the border between Poland and Slovakia. The study area is a part of the thrust-and-fold belt of the Flysch Carpathians formed by the Magura unit, which consists of ca 2 km thick sequence of Upper Cretaceous-Eocene flysch rocks dipping gently (ca 20°) towards the S and SW (Książkiewicz, 1971(Książkiewicz, , 1983 (Figure 2). Flysch bedrock is affected by joint and fault sets running in NE-SW, SE-NW and ENE-WSW directions (Aleksandrowski, 1985;Łajczak, 2014). These discontinuities define the boundaries of the Massif and the large rock mass movements (Figure 1(b)). According to recent findings (Jankowski et al., 2019;Jankowski & Margielewski, 2014), the local topography of the Babia Góra Massif was associated with Late Miocene orogenic collapse and gravitational displacement of large blocks southward from the Babi Góra Massif along normal faults. As a result of significant unloading, the core zone of the Massif located at the footwall of the normal fault became intensive isostatically elevated (Jankowski & Margielewski, 2014). A deep tectonic zone bounding the southern slope of the Babia Góra is underlined by the zone of triangular facets and linear concentration of mineral springs along the supposed fault zone (Łajczak, 2016) (Figure 1(b)).
Landforms within the study area are strongly influenced by geological structure (Łajczak, 2013, 2014). The main 10 km long ridge of the Babia Góra Massif of coherent Magura-type thick sandstone cap that overlie strongly folded, weak claystone/siltstonedominated flysch of sub-Magura beds at the northern, and chaotic complexes (tectonic melange) at the southern foreland (Jankowski & Margielewski, 2014). Steep (40°-78°) northern anaclinal/escarpment slopes form typical cuestas with local rock walls and concave profiles that are presently affected by shallow rockslides, rock falls, modern and relict debris flows and snow avalanches Łajczak, 2016). Southern dip (cataclinal) slopes with a shallower gradient (20°-40°) and convex-concave profiles show translational rockslides (Łajczak, 2014) that are strongly influenced by geological structure (Figure 2(b)). With respect to historical and modern activity of gravityinduced slope deformations (Łajczak & Migoń, 2002;Midowicz, 1974), the study area shows higher frequency events compared with adjacent parts of Western Carpathians with the rigid thick-bedded flysch. The western part of the Massif of the Mała Babia Góra (1515 m a.s.l.), features deeply-incised valleys with headwaters located just below the main ridge, which contrasts with the eastern part (Diablak 1725 m a.s.l.), dominated by relatively shallow and short valleys with low-elevated headwaters located at 900-1200 m on the northern, and 1200-1400 m on the southern slopes (Łajczak, 2014).
Due to the topographic barrier Babia Góra is one of the wettest locations in the Western Carpathians, with high orographically induced annual precipitation exceeding 1450 mm and maximum daily intensity of 234 mm at the northern slopes (Obrębska-Starklowa, 2004). The predominant wind direction is NW in summer and SW during winter (Obrębska-Starklowa, 2004). Forests reach up to approximately 1370 m a.s.l.  and dwarf pine up to 1650 m a.s.l. Although the mean annual air temperature in the summit area is below 0°C (Obrębska- Starklowa, 2004), no evidence of contemporary permafrost exists (Dobiński et al., 2016). The Babia Góra National Park contains a Polish part of the Massif and has been also declared as Biosphere Reserve by UNESCO in 1977.

Slope and slope landforms
In the study area, slopes and slope covers are strongly influenced by the flysch bedrock and thus in this map, two types of slopes were distinguished according to the extent of thick-bedded Magura sandstones and claystone/siltstone dominated Sub-Magura beds ( Figure 2). Blocky and debris slope covers are typical of rigid thick-bedded sandstone, whereas muddy covers come from weak claystone formations. Solid bedrock is only locally exposed along the main ridge, in rocky landslide scarps and along river channel undercuttings and in rock steps up to 4.5 m in high forming waterfalls ( Figure 3).
Geomorphological mapping revealed relict and modern debris flow tracks on the northern slopes of the Babia Góra Massif. A total number of 24 fossil and 3 modern tracks (Figure 4(a)) were identified below Mt. Diablak and Gówniak within the altitudinal belt 1200-1680 m. The mean length of individual fossil debris flow tracks is 630 m and maximal length reaches approximately 1 km. They occur on steep slopes (20°-34°) with local maximal inclination up to 70°. Debris flow tracks from 3 to 4 m deep gullies (Figure 4(b)) in the erosional zone, and fossil debris accumulation up to ∼3 m thick and ∼5-35 m wide in the deposition zone which form lateral levees, that pass downslope into torrent cones. Modern tracks were triggered by heavy rainfall in 1997, 2002 and 2004 (Łajczak et al., 2015) and the longest track reaches 760 m (Łajczak & Migoń, 2002;Matyja, 2006) (Figure 4(a)).
Rock avalanches ( Figure 5, Table 1) occur in the upper section of the Babia Góra northern slope (1300-1500 m a.s.l.), with the highest slope gradients (40°-70°). These failures have striking rocky cavities and debris lobes of disintegrated, openwork angular debris ( Figure 6(d)). The morphology, blocky character  of the material and similar SH R-values (Supplementary data 2) indicate single and rather sudden origin of these failures.
Rock slope deformations (RSD; Agliardi et al., 2001;Jarman, 2006;Pánek & Klimeš, 2016) were recognised in the ridge-top area of Babia Góra Massif, above scarps of large rockslides, marking retrogression to the crest. RSD are characterised by diffuse margins (Jarman & Harrison, 2019) and occurrence of fissures, tension cracks, ridge-top trenches with crevice type caves, steep like arranged uphill and downhill-facing scarps ( Figure 6(e,f)), which resulted from deep-reaching slope extension, opening of joints and short horizontal displacements. A total of 20, small (up to  86 m in length) crevice caves were recognised in the Massif. A total of 125 RSFs occur on the north-exposed escarpment slope, where the highest overall landslide area (12 km 2 ) and RSF density (4.6/km 2 ) are also found ( Table 1). The northern slope area features the highest local relief and slope gradients (40°-70°) and suitable structural conditions for sliding. This is well expressed in the exceptionally high incidences of slope failure (44.9%), which is more than two times larger when compared to south dip slope (Figures 4 and 5; Table 1). The elevation belt 920-1500 m a.s.l. is preferred for mass movements on the northern slope, showing values above the mean (Figure 7(b)), however, the most pronounced cluster of RSFs is at the altitudinal range 950-1150 m, where more than 70% of the area is affected by mass movements (Figure 7). RSF scarps on the northern slope (38%) cluster between 800 and 1040 m which corresponds to the rockslides on the valley-sides and in the headwaters (Łajczak, 2014).
In contrast, on southern/dip slopes, RSFs cluster in the higher elevation belt between 1200 and 1650 m, where more than 40% of the area is affected by mass movements ( Figure 5). The largest concentration of RSFs occurs in headwater areas at 1200-1300 m and 1400-1470 m (Łajczak, 2014) (Figure 7). The cluster at 1500-1650 m corresponds to relatively shallow but extensive structural translational rockslides and rock slope deformations along gentle dipping bedding planes (<20°).
Average RSF size is 0.10 km 2 , however, the size distribution of RSFs in the Babia Góra Massif is dominated by large landslides (>0.25 km 2 ), that cover 60% of the total area affected by RSFs ( Figure 5, Supplementary data 1) and represent 8.5% of their total number (Table 1). A total of 18 large landslides could be classified as largest scale slope deformations, with volumes larger than 5×10 6 m 3 (Fell, 1994) (Supplementary data 1). Large landslides are relatively uniformly distributed on the anticlinal and cataclinal slopes, however, on the northern slope, they comprise up to 61% of the total landslide area, as well as three of the largest RSFs in the study area (Table 1, Supplementary data 1). Six examples of complex RSFs (Cruden & Varnes, 1996) were distinguished in the study area ( Figure 5, Table 1), and display evolution from deformation to sliding and collapse/flow. This are the largest (4.6-164.3 × 10 6 m 3 ) gravity-induced slope deformations in the Babia Góra Massif, covering the middle and lower portions of the northern slopes. Among them are one of the largest known failures in the Polish part of the Outer Western Carpathiansthe Markowe Szczawiny landslide complex (2.6 km 2 area, 164.3 × 10 6 m 3 in volume) (Table 1; Supplementary data 1). The landslide complexes shows an exceptionally large height with an elevation difference of approximately 880 m between the back wall and leading edge of the accumulation area, and extensive maximal sliding distance of approximately 3.2 km, which is larger than the longest known landslide in the Polish part of the Outer Western Carpathians (Mrozek et al., 2014).

Periglacial and glacial landforms
The landscape of the highest Babia Góra massif has been affected by periglacial processes. Blockfields and blockstreams cover ca 1.85 km 2 of the study area and represent cold-climate periglacial landforms developed in Magura-type sandstones (Figure 8(a)). Block covers are 1-2 m thick, of in situ block mantles with ca 30% of fine matrix (Łajczak & Włoch, 2004;Ziętara, 1989Ziętara, , 2004 and are mostly concentrated on northern slopes (72% of the total area) in the altitudinal range 1425-1725 m. Blockfields are relict (Klimaszewski, 1948(Klimaszewski, , 1952Łajczak, 2013), however, evidence of current build up by mechanical weathering of sandstone was also documented (Jahn, 1958). On the high gradient slopes, elongated blockstreams were formed as the result of slow creep.
Step-like cryoplanation terraces with gently sloping treads and rubble-covered scarps related to outcrops of sandstone beds were identified along the main ridge area between Mt. Kępa and Mt. Kościółki. Small (1-2 m high) relict pronival ramparts and shallow nivation hollows have been identified in the highest part of the Massif, around Mt. Diablak (1725 m) (Figure 8(b)).
Glacial sediments in the Babia Góra Massif were distinguished by sedimentological analyses (clast shape and roundness; Figure 9(b)), presence of large (>2 m) sub-rounded boulders (Figure 9(e)), as well as SH measurements (Figure 9(a,c), Supplementary data 2), which indicate clear differences between moraines and adjacent landslide and fluvial deposits. Moraines are preserved exclusively on the northern slopes of Mt. Babia Góra beyond the limits of landslide tongues in two discontinuous locations in the upper (1375-1160 m) and lower part (915-990 m) of the Szumiąca Woda valley (Figure 8(a,e)). The first area shows a glacial cirque with low-relief moraine cover with large (up to 4-5 m) sandstone boulders (Książkiewicz, 1963;Łajczak, 1998) resting on a matrix-supported diamicton with sub-rounded and rounded clasts (Figure 9 (b)). In the lower part of Szumiąca Woda valley moraines are traced as isolated patches of till without moraine ridges, which were eroded by river channel avulsion and buried by proximal outwash fans. Erosion is an effect of local site conditions, especially exposure of glacial deposits to high energy fluvial and mass movements (Kirkbride & Winkler, 2012). A 5 m high morphological step in the longitudinal valley profile is formed by large boulders (Figure 9(e)) lacking in adjacent landslide deposits. SH-rebound values indicate an old relative age of the moraines (R = 30-32), similar to periglacial block covers (R = 30) and slightly older than pronival ramparts (R = 33), but significantly   older than landslide boulders (R = 34-44) (Figure 9(a, c), Supplementary data 2).

Fluvial landforms
Impressively large fans were mapped beyond mass movement landforms at the foot of the Babia Góra Massif. They form gently inclined (5°) surfaces of well-rounded Magura sandstone clasts (5-10 cm in diameter) with a clayey-sandy matrix. Exceptionally large fans occur widely across the northern slope of the study area attaining 200 m in cone height and 1200 m in radius. An outsized fan (3.7 km in radius and 210 m hight) was also identified in the Bystrá valley on the southern foreland (out of the map). A double system of fans located at 60 m (Sulowa Cyrhel level, Middle Pleistocene) and 15-20 m (Słonowy level, Late Pleistocene) above the present floodplain was identified (Kłapyta & Kolecka, 2015). Sediment fans represent the oldest mapped landforms in the study area with the lowest recorded R-values between 27 and 29 (Kłapyta & Kolecka, 2015) (Figure 9(c)). The distal part of the Słonowy level fans passes directly into 15-10 m high river terraces (Late Pleistocene; Książkiewicz, 1963). The genesis of cone-shaped fans in the Babia Góra Massif may be complex. Prolonged incremental growth by fluvial and debris-flow processes is evidenced by layered fan sediments on the southern slope of the study area. In contrast, massive diamicton in the Cylowy fan suggests that it formed from massive slope failure and therefore represents a landslide-related fan sensu Sorriso-Valvo (1988). This is supported by its low fan/catchment area ratio (0.88) typical of outsize fans (Jarmann et al., 2011), derived from catastrophic slope failures. A large fan in the distal part of moraines in the Szumiąca Woda Valley could be related to glaciofluvial accumulation. The youngest set of river terraces (Rybny level, 5-6 m high) was formed during the Holocene. A total of 20 landslide lakes (largest 450-1500 m 2 in area) and two peat bogs were recognised in the Massif (Łajczak, 2016).

Discussion and conclusions
The Map documents the complexity of mass movements in a mountain flysch area of moderate elevation. The Babia Góra area has undergone wholesale slope modification and downwasting by having one of the highest incidences of slope failure yet recorded in Europe (29% for a whole massif, and 45% for the north face). The total area covered by RSFs is much higher than calculated for the other part of the Outer Western Carpathians (Figure 1) Pánek et al., 2019). In the British mountains the maximum area affected by RSF reach 10-20% in the selected massifs (Jarman, 2006;Jarman & Harrison, 2019). Conversely, in some valleys in the Italian Alps, extraordinary concentration rock mass-wasting could reach 38% of the area (Fioraso, 2017).
Although the large RSFs constitutes first-time failures, sensu Soldati et al. (2004), they gave no evidence of a single giant rockslide on the northern slope suggested by Alexandrowicz (1978) and Ziętara (2004), that was to produce a 8 km long scarp, 200 m thick debris mass and 1 km southward retreat of the main ridge. RSF ages are unknown in the study area, however, morphological relations and SH relative age dating (Figure 9), suggest that they have evolved during or just after deglaciation and have significantly degraded glacial and periglacial relief. The remnants of moraines and glacial cirque between Diablak and Kościółki ( Figure 9) indicate local glaciation of the north face of the Massif during the last glaciation (most probably the LGM) (Książkiewicz, 1963;Łajczak, 1998, 2014Niemirowski, 1963). However, the small scale of glaciation (0.87 km 2 area, and 2.2 km glacier length) and limited glacial erosion preclude a paraglacial origin of mapped RSFs. Similarly, the fluvial pattern with low-elevated headwaters and relatively weak stream incision do not support a 'parafluvial' (Jarman et al., 2014) genesis of RSFs. Recent findings of post-LGM tectonic rupture along the southern margin of the Tatra Mountains (Pánek et al., 2020) indicate that the high-magnitude earthquakes (Mw > 7) could be possible triggers for some RSFs in the Western Carpathians. Seismic triggering related to regional-scale glacial isostatic adjustment due to the retreat of Scandinavian ice sheet (Pánek et al., 2020) is thus not ruled out as a mechanism of some RSFs in the study area. Postglacial evolution of the Babia Góra massif is possible coupled with the long-term/post-Miocene evolution of this area, which was characterised by gravitational displacement of large blocks leading to significant unloading and 'en block' uplift of the core zone of the Massif. The mechanism of footwall elevation (Wernicke & Axen, 1988) of the Babia Góra Massif along normal faults was recently proposed by Jankowski and Margielewski (2014) and Jankowski et al. (2019). In this context, the high incidences of slope failures in the study area could be related to the long-term response to uplift and thus perhaps a 'paratectonic driver'. Subsequent mass movements are recurrent features within older landforms that produced RSFs concentrated in headwater areas and on steep slopes of the cuesta, where rock avalanches were formed. SH R-values suggest its Holocene age. In order to determine timing and possible causes of RSFs in the Babia Góra Massif further research with geophysical prospecting, kinematic analysis and cosmogenic nuclide dating should be undertaken.

Software
The software used was ESRI ArcGIS 10.6.1 to georeference, digitise, visualise the aerial photographs and generate the geomorphological map. CorelDRAW X8 was used to final map processing.