Geomorphology 341 (2019) 153–168

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Geomorphology

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Cirques in the Sierra de Guadarrama and7 Somosierra Mountains (Iberian Central System): Shape, size and controlling factors

Javier Pedraza a,⁎,RosaM.Carrascob, Javier Villa b, Rodrigo L. Soteres c, Theodoros Karampaglidis d, Javier Fernández-Lozano e a Department of Geodynamics, Stratigraphy and Paleontology, Faculty of Geological Science, Complutense University of Madrid, C/José Antonio Novais, 12, 28040 Madrid, Spain b Department of Geological Engineering and Mining, Faculty of Environmental Sciences and Biochemistry, University of Castilla-La Mancha, Avenida Carlos III s/n, 45071 Toledo, Spain c Institute of Geography, Pontifical Catholic University of Chile, Ave. Vicuña Mackenna, 4860 Macul, Santiago, Chile d Department of Geoarchaeology, National Research Centre on Human Evolution, Paseo Sierra de Atapuerca, s/n, 09002 Burgos, Spain e Department of Earth Sciences and Physics of the Condensed Matter, Faculty of Sciences, University of Cantabria, Avenida de los Castros s/n, 39007 Santander, Spain article info abstract

Article history: The Guadarrama and Somosierra mountain ranges form the eastern sector of the Iberian Central System and Received 12 February 2019 hosted numerous glaciers during the Late Pleistocene (MIS2). Glaciation was of low intensity with glaciers of Received in revised form 29 May 2019 small sizes, strongly controlled by the climatic context and the topography. This study analyses the shape, size, Accepted 29 May 2019 distribution and location of 96 cirques existing in these mountain ranges. In addition to the standard morphomet- Available online 2 June 2019 ric parameters and controlling factors (altitude, aspect and lithology) used in most studies, additional factors

Keywords: were considered here in relation to the pre-glacial relief and fracture network. The data were obtained and proc- Cirque morphometry essed using ArcGIS 10.4 and relations between the parameters and controlling factors were evaluated using sta- Controlling factors for cirque formation tistical methods. The results indicate that most are simple cirques, tending to isometry, with low vertical incision Topoclimate capacity, considerable variation in size and predominantly east-facing. In the context of the Iberian Peninsula and Iberian Central System other European mountains, these cirques are among the most isometric, the lowest in height and present the least overdeepening. The development of these cirques has generally been the result of random combination of various factors. Thus: (i) the largest cirques are located at intermediate altitudes, with the headwall located on the main divides, at former torrential valley heads or at the headwalls of fracture corridor valleys, and are north-facing; (ii) the longest cirques are located at former torrential valley heads, on metamorphic bedrock (i.e. schists, slates) and on uniform slopes. Finally, the prevailing eastern aspects are explained by topoclimatic factors and are in agreement with previous studies, which have proposed a Circulation Weather Type (CWT) model throughout the Iberian Peninsula during the Last Glacial Period, similar to its current configuration. © 2019 Elsevier B.V. All rights reserved.

1. Introduction varying origins (i.e. fluvial, mass movement, periglacial, volcanic), but this interpretation cannot be generalised (Haynes, 1968; Graf, 1976; 1.1. Cirque morphology research: a brief overview Turnbull and Davies, 2006; Sanders et al., 2013). Debate on the morphological evolution of cirques has mainly focused In glacial geomorphology, the term cirque (cirque ou amphithéâtre; on genetic processes (Brown, 1905; Evans, 1997; Barr and Spagnolo, Charpentier, 1823,p.24–25) refers to a specific landform (Evans and 2015)suchassubglacialerosionby rotational slipping (Lewis, 1949), lead- Cox, 1974; Benn and Evans, 2010), generally convergent with a trun- ing to deepening or downwearing processes (White, 1970), and extra- cated elliptic paraboloid. glacial erosion from periglacial freeze–thaw, which causes headwall re- Interpreting the origin and evolution of glacial cirques (hereafter treat or backwearing (Johnson, 1904). Given the plucking process cirques) tends to be complicated due to the multiple factors that control appearing to operate equally on cirque walls and floors, the subglacial or- their formation, such as: (1) typology of the pre-glacial relief; (2) geolog- igin of cirques must also be considered (Hooke, 1991; Richardson and ical structure; (3) glacial history; and (4) regional climate (Unwin, 1973). Holmund, 1996; Gordon, 2001; Cook and Swift, 2012). Other topics The primary origin of cirques is often located in pre-glacial hollows of highlighted in this debate include the erosive capacity of glaciers to limit mountain relief (‘glacial buzzsaw’ hypothesis; Mitchell and Montgomery, ⁎ Corresponding author. 2006; Foster et al., 2008; Egholm et al., 2009; Anders et al., 2010; E-mail address: [email protected] (J. Pedraza). Thomson et al., 2010)ortheuseofcirquefloor altitude as a paleoclimatic

https://doi.org/10.1016/j.geomorph.2019.05.024 0169-555X/© 2019 Elsevier B.V. All rights reserved. 154 J. Pedraza et al. / Geomorphology 341 (2019) 153–168 indicator equivalent to equilibrium line altitude (paleo-ELAs) (Sugden, surficial Quaternary deposits of fluvial, glacial and periglacial origin 1968, 1969; Olyphant, 1981a; Traczyk, 2004; Evans, 2006a, 2009). (GEODE, 2004)(Figs. 1 and A1). Cirque landforms are depressions that promote snow accumulation/ GU is an uplifted block mountain, whereas SO is closer to a tilted block transformation and are therefore useful as paleoclimatic indicators mountain. In GU, the main ridge runs 112 km in NNE-SSW and NE-SW di- (Peterson, 1968; Peterson and Robinson, 1969; Derbyshire and Evans, rections, alternating with minor segments in an ENE-WSW direction. 1976; Graf, 1976). A relationship has been shown between the distribu- Some of the ancient pre-glacial erosion summit surfaces reach heights tion of these glacial forms and the regional storm track, the radiation bal- of around 2200 m asl, culminating at 2428 m asl (Peñalara Peak). In SO, ance and some local parameters such as the effects of shading or morning/ the main ridge runs 40 km in an ENE-WSW direction alternating with afternoon (Evans, 1977, 2006a, 2006b; Olyphant, 1977; Mitchell, 1996; minor segments in a NNE-SSW direction and the summit surfaces are Chueca and Julián, 2004; Mîndrescu et al., 2010; Barr et al., 2017; Araos lower than those of GU, reaching around 2000 m asl and culminating at et al., 2018). Nevertheless, correlating the morphometric parameters of 2274 m asl (El Lobo Peak). Other characteristics differentiating the SO cirques with climate has certain limitations (Barr and Spagnolo, 2015)be- area include the presence of low- or very low-grade metamorphic lithol- cause their development and location are also controlled by non-climatic ogies and an abundance of folding structures (Figs. A1 and A2). factors, including morphostructure, lithology and pre-glacial topography These inland mountains are located in the Mediterranean region of at various stages of development (Haynes, 1968; Peterson and the Iberian Peninsula and consequently present a continental Mediter- Robinson, 1969; Sugden, 1969; Graf, 1976; Olyphant, 1981a; Evans, ranean mountain type climate (Köppen-Geiger Climate Classification 1994, 2006b; Brook et al., 2006; Hughes et al., 2007; Bennett and Dsb and DsC; AEMET-IPMA, 2011). This continentality factor is slightly Glasser, 2009; Sanders et al., 2012, 2013; Delmas et al., 2014, 2015). more pronounced in SO than in GU. Bearing these hypotheses in mind, cirques should be classified as a These lithological, morphostructural, topographic and climatic dif- complex landform whose development is controlled by multiple factors ferences between GU and SO, merit separate analyses to enable a com- and which generally tends to develop an allometric morphology parison between both sectors of the ICS. (Olyphant, 1981a, 1981b). 2.2. Glacial morphology and chronology 1.2. Approach adopted in this study: aims and scope Slope glaciers (glaciares de ladera; Pedraza and Fernández, 1981; This study is focusing on cirques in the Guadarrama (GU) and Pedraza et al., 1996) and cirque glaciers predominated in GU and SO Somosierra (SO) mountain ranges forming part of the Iberian Central Sys- alike (Fig. 2), while valley and plateau-type glaciers were restricted to oc- tem (ICS). Most of the glaciers in these mountains are located on the east- casional areas (Carrasco et al., 2016b). During the maximum ice extent in ern slopes of the orographic alignments, a configuration that is considered these mountains (local MIE), the estimated equilibrium line altitude an indicator of snow drift between slopes (Fernández-Navarro, 1915; (ELA) was 1926 m (AAR-0.6; Carrasco et al., 2018). The maximum alti- Obermaier and Carandell, 1917; Fränzle, 1959; Sanz-Herráiz, 1978, 1988; tude reached by the ice was 2413 m asl in GU and 2267 m asl in SO, the Palacios et al., 2012). It has also been suggested that non-climatic factors longest glacier flowline was 4.3 km in GU and 2.1 km in SO and the min- influenced the location of these cirques, specifically pre-glacial fluvial mor- imum glacier front altitude was 1350 m asl in GU and 1537 m asl in SO. phology and fracture networks (Fernández-Navarro, 1915; Sanz-Herráiz, Early 20th century studies of the glacial features of GU and SO 1978, 1988; Sanz-Donaire, 1979; Pedraza and Carrasco, 2006). However, assigned relative ages for the geomorphic record corresponding to the these hypotheses are based on general reviews of the ICS discussing cirque Riss and Würm glaciations in the Alps. However, this chronology was location in relation to aspect or fracture network (Guerra-Zaballos and amended in the mid-20th century to reflect a single glaciation hypoth- Sanz-Donaire, 1985, 1987; Carrasco et al., 2016a) and therefore require esis, equivalent to the Würm stage (see Pedraza and Carrasco, 2006; validation using rigorous analytical methods. This circumstance contrasts Oliva et al., 2019). Recently, an evolutionary sequence has been defined sharply with the considerable advances made in research on the chronol- for these paleoglaciers based on absolute chronology techniques. This ogy, evolution and regional correlation of ICS paleoglaciers (Vieira et al., sequence is in agreement with the one established at regional level in 2001; Palacios et al., 2011, 2012; Domínguez-Villar et al., 2013; Pedraza the ICS, which is divided into three main stages (Carrasco et al., et al., 2013; Carrasco et al., 2015, 2016b; Bullón, 2016). 2016b; Oliva et al., 2019): (i) the local MIE, the chronology of which is The main aim of this study is to conduct a comprehensive analysis of under discussion because data for three paleoglaciers place two of the influence of different factors on the typology (shape and size) and them within the MIS2, contemporary with the global LGM (around spatial distribution of cirques in the GU and SO mountains, using a 26 ka BP; Palacios et al., 2012; Carrasco et al., 2016b), and the other in novel methodology. Besides the factors of altitude, aspect and lithology the MIS3, prior to the global LGM around 35 ka BP (Bullón, 2016); (ii) normally considered in previous studies, we also include other new fac- a re-advance and stabilisation stage, ending around 17/15 ka BP tors, at the same analytical level, such as tectonic structure and previous (Carrasco et al., 2016b); and (iii) a deglaciation stage, starting after morphology of the terrain. Given that these cirques are among the the previous stage and ending around 11/10 ka BP (Palacios et al., southernmost examples of Pleistocene glacier activity in the northern 2012). hemisphere, examining them in detail will contribute to a better under- standing of past glacial dynamics in an area of climatic transition. 3. Methodology 2. Geological and geomorphological setting All palaeoglaciers in the eastern sector of the ICS were analysed: 79 2.1. Regional context cirques in GU and 18 cirques in SO. Mapping the glacial geomorphic fea- tures and the main fracture alignments, were compiled by means of pho- GU and SO comprise the eastern sectors of the ICS, an intraplate tointerpretation and field work in order to obtain: (i) the topographic, mountain range originating from the Alpine orogeny (mainly in the geometric and, in some cases, genetic indicators of the landforms (cirques Neogene) by the reactivation of Late Variscan fractures. Its limits and and associated morphologies); and (ii) the fracture network (Figs. A1 and layout are controlled by fracture networks, essentially running NE-SW, A2). Photointerpretation was conducted using high-resolution stereo-3D NNE-SSW (and conjugates), E-W and N-S (Casas-Sainz and de Vicente, images provided by the Spanish National Geographic Institute (Spanish 2009). Its basement is formed of granite and metamorphic rocks initials: IGN, http://www.ign.es) and ArcGIS 10.4 was used to calculate (Variscan and pre-Variscan lithologies), locally covered by a sedimen- and represent morphometric features and generate slope maps of the ter- tary succession of Cretaceous, Paleogene and Neogene lithologies and rain and cirques. J. Pedraza et al. / Geomorphology 341 (2019) 153–168 155

Fig. 1. Geographic location of the Guadarrama and Somosierra Mountains in the Iberian Peninsula and Iberian Central System.

The basic and derived morphometric parameters of the cirques corresponding maps (Geomorphological and Fracture Network), based on analysed here are shown in Table 1. These parameters, together with ele- criteria equivalent to other analysed factors (Tables 3 and 4,Figs.A1andA2). vation, mean slope, and mean aspect were calculated using the Auto- The information obtained using ACME and that corresponding to the mated Cirque Metric Extraction (ACME) GIS tool (Spagnolo et al., 2017) controlling factors was coded and implemented in Excel to enable statistical and the calculations were implemented using the Digital Terrain Model analysis using the Real Statistics add-on (www.real-statistics.com). The (DTM) available at IGN (http://www.ign.es). procedure applied by García-Ruiz et al. (2000) for the Central Pyrenees Besides quantitative data, our analysis included various qualitative (Spain) was used for this analysis, as follows: (i) tables and diagrams datasets. In addition to lithology, normally used in this type of research, with basic descriptive data; (ii) matrix of correlations between parameters; we have also included data on previous morphology (landform type, topo- (iii) tables showing analysis of variance (ANOVAS) for each factor; and (iv) graphic features and torrential basins) and fractures (direction and density). table grouping cirques into morphometric subsets according to affinities Altitudinal values were determined according to morphological (proximity of each element to the centroids of each group or cluster). criteria using data obtained from the general geomorphological map (Table 2, Fig. A2). For aspect, standard procedures were followed, dividing the values into eight orientations: N, NE, E, SE, S, SW, W and NW (see, 4. Results and discussion Evans, 1977; Evans and Cox, 1995; Barr and Spagnolo, 2015)(Fig. 3). The lithological map was summarised from the National Geological Map 4.1. Cirque morphometry (series MAGNA) to scale 1:50,000 (GEODE, 2004) using morpholithological criteria (Figs. 4 and A1). The classifications established for the previous mor- The mean values for the basic morphometric parameters, length (L), phology of the terrain and the fracture network were derived from the width (W), height (H) and planar area (2D), are 556.45 m, 529.77 m, 156 J. Pedraza et al. / Geomorphology 341 (2019) 153–168

Fig. 2. A) Panoramic view of the main sectors of the Sierra de Guadarrama with the Sierra de la Cuerda Larga (Hierro, 2.381 m asl) in the foreground and the Montes Carpetanos (Peñalara, 2.428 m asl) in the background, showing different summit surface levels and paleoglaciers (photo: C. García-Royo, 05 March 2008). B) Los Pelados-El Nevero summit surface in the Montes Carpetanos (Sierra de Guadarrama). This area was occupied by a plateau glacier in the Last Glacial Period. The cirque headwall of the former slope glacier of Los Hoyos is shown in the foreground. A reconstruction of both paleoglaciers, has demonstrated their interconnection during the Glacial Maximum (Carrasco et al., 2018). C) The Cebollera shallow cirques (Peña Cebollera Vieja, 2.128 m asl) in the Sierra de Somosierra. D) Porriñoso-Peñacabra cirque on the eastern flank of the Los Pelados-El Nevero plateau, showing the structural factors controlling cirque morphology: longitudinal fractures favour lengthening while orthogonal fractures govern cirque thresholds. E) Slope-cirques (right to left, Hoyo Cerrado and Hoyo Borracoso) formed on the eastern side of the Macizo de Los Pelados-El Nevero (Montes Carpetanos, Sierra de Guadarrama). The image shows the summit surface divided onto two levels and the elongated morphology of the cirques.

212.76 m and 29.78 ha, respectively, for GU and 487.06 m, 461.39 m, cirques are among the narrowest; (iii) they are also the lowest; (iv) 219.17 m and 20.85 ha, respectively, for SO (Table A1). A comparison the GU cirques are among the intermediates, while SO cirques are with the values obtained for other Iberian Peninsula mountain ranges among the smallest. showed that (Fig. 5, Table A1): (i) GU cirques are among the longest, Circular cirques predominate in GU and SO alike, with mean values while SO cirques present values closer to the rest; (ii) both GU and SO for the circularity ratio (CR) and the length/width (L/W) ratio close to J. Pedraza et al. / Geomorphology 341 (2019) 153–168 157

Table 1 Basic and derived morphometric parameters of glacial cirques used in this study. Parameters extracted with Automated Cirque Metric Extraction (ACME) GIS tool (Spagnolo et al., 2017).

Parameter Acronym (unit) Description. From ACME and according to Gordon, 1977; Aniya and Welch, 1981; Olyphant, 1981a; Evans and Cox, 1995.

Length L (m) Length of line within the cirque polygon, intersecting the cirque threshold midpoint, splitting the polygon into two equal halves. Width W (m) Length of line perpendicular to the length line, intersecting the length line midpoint.

Height H (m) Difference between minimum elevation of cirque floor (Emin) and maximum elevation of cirque headwall (Emax). L/W ratio L/W Defines the planimetric cirque shape as indicator of trend to elongation (ratio N 1), widening (ratio b 1) or circularity (ratio ~ 1). L/H ratio L/H Both ratios are indicators of vertical cirque development and measures of cirque incision (deepening processes). Lower values W/H ratio W/H indicating greater incision Circularity ratio CR Circularity is defined as the ratio between cirque perimeter and perimeter of a circle with the same area. High values indicate low circularity; values closer to 1 indicate high circularity. Perimeter P (m) Lenght of contour line of the cirque polygon Volume V (hm3) L × W × H Planar area 2D (ha) Area of cirque projected on a horizontal plane Surface area 3D (ha) Real cirque surface area 3D/2D ratio 3D/2D Indicator of cirque floor depth.

Elevation (altitude) Emax (m asl) Maximum elevation (altitude) of cirque headwall

Emin (m asl) Minimum elevation (altitude) of cirque floor

Emean (m asl) Value of mean contour line of cirque elevations. Slope mean (degrees) Mean slope value for the delimited cirque area Aspect mean (degrees north) Mean aspect value in degrees north (0–360° interval) Plan closure (degrees) Cirque plan closure, providing a quantitative evaluation of cirque planar shape.

1; nevertheless, these cirques are more prone to elongation than those the substratum, fracture networks and pre-glacial topography (old ero- in other Iberian Peninsula mountains (Figs. 2 and 5, Table A1). The cor- sion surfaces). Consequently, these factors should be taken into account relation between L/W and CR with cirque size (planar area, 2D, surface in an analysis of cirques morphometry at the same level as altitude, ori- area, 3D, and volume, V) is very weak in all cases (absolute values of r entation and lithology. within the range 0.08–0.16 and 0.08–0.36, respectively), being negative The correlations for GU and SO (Table A2) show that cirque height in GU and positive in SO (Table A2). This indicates that CR is not closely (H) increases with size (2D, 3D and V) in a proportion similar to length associated with cirque size. The mean values for length/height (L/H) and width (L and W). Taking into account the evolutionary significance and width/height (W/H) ratios are high in both mountain massifs of these indicators (Olyphant, 1981a; Evans, 2003, 2006a, 2009, 2010; (3.08 and 2.85 in GU, 2.29 and 2.24 in SO), and considerably higher Křížek and Mida, 2013; Mîndrescu and Evans, 2014; Barr and than most values obtained for other mountain ranges in the Iberian Pen- Spagnolo, 2015), it seems probable that SO cirques underwent isometric insula (Table A1, Fig. 5). According to general observations development whereas GU cirques present a trend towards allometric (e.g., Embleton and Hamann, 1988; Hughes et al., 2007; Barr and development, with wall recession being more pronounced than bed Spagnolo, 2015), the data obtained suggest that GU and SO cirques are deepening (overdeepening). However, this exerted little influence, shallower and have little vertical incision. given that the average W/H ratio is N2.5 in both GU and SO, indicating The development of cirques over time tends to be allometric, with that the overdeepening process was unimportant and erosion generally wall recession predominating over bed deepening (Olyphant, 1981a; Evans, 2006a, 2009, 2010; Mîndrescu and Evans, 2014; Delmas et al., 2015). However, in the case of SO and GU, the short duration of glacia- tion limited this evolutionary allometric effect, and could also be one of the factors that prevented the development of more cirques of all sizes. A comparison with cirques in other regions (Křížek and Mida, 2013; Delmas et al., 2014; Barr and Spagnolo, 2015; Evans and Cox, 2017) showed that only those in the High Tatras (Křížek and Mida, 2013)are similar to the GU cirques and only those in the Franco-Spanish Eastern Pyrenees (Delmas et al., 2014) to the SO cirques (Fig. 5). However, due to their geographical location and intensity and duration of glacia- tion, both the High Tatras and the Franco-Spanish Eastern Pyrenees are very different to GU and SO. Nevertheless, all these mountains pres- ent notable lithological and morphostructural similarities, suggesting that cirque development might be determined by crystalline rocks in

Table 2 Altitudinal classes and correlation with morphotopographic features (see Fig. A2).

Altitudinal Code Associated topographic features (approx.) class (m asl)

N2400 1 Main divide Level 1: summit surfaces of highest massifs and their slopes 2400–2200 2 Main divide Level 2: summit surfaces of intermediate massifs and their slopes 2200–2000 3 Main divide Level 3 and secondary divides: the most general summit surfaces and their slopes. 2000–1800 4 Uniform or complex slopes 1800–1600 5 Intermediate surfaces b1600 6 Uniform or complex slopes (avobe 1000 m asl), piedmont Fig. 3. Distribution of glacial cirques according to aspect. The numbers in parentheses surfaces and intermountain plains. correspond to the aspect class number. 158 J. Pedraza et al. / Geomorphology 341 (2019) 153–168

Fig. 4. Lithological classes and percentage of cirques for each class. Sierra de Guarrama (A) and Somosierra (B). tended to involve widening of the accumulation basin. This could be due Gómez-Villar et al., 2015). Nevertheless, they are much smaller than to the topography of most of the glaciers, insofar as their location on those obtained for Alpine mountains, e.g. the Pyrenees or Maritime Alps, steep slopes (20%–30%) undermined the stability of the snow mantle, where the ranges are 1300–1700 m (Serrano, 1998; García-Ruiz et al., thereby hindering the development of ice thick enough to cause signif- 2000; Federici and Spagnolo, 2004; Delmas et al., 2014; Lopes et al., 2018). icant basal erosion. In both GU and SO, the main summits have mean altitudes of Finally, using a clustering process based on morphometric parame- 2000–2300 m asl (depending on the massifs) and are therefore very ter affinities, we obtained the three groups shown in Table 5. These pro- close to the estimated altitudinal limit for the development of glacial vide an indicator of the level of morphological convergence or processes in the ICS, which ranges between 1900 and 2000 m asl divergence between GU and SO cirques. (Pedraza and Carrasco, 2006). These figures agree with the values ob- tained for the mean minimum cirque floor altitude (mean Emin) 4.2. Spatial distribution of the cirques according to altitude and aspect. In- (1897.3 m asl in GU and 1898.4 m asl in SO) and with estimated regional fluence of these factors on cirque morphology ELAs for the Glacial Maximum (MIS2) in the GU (ELA in the Macizo de los Pelados-El Nevero, AAR-0.6 = 1926 m asl and AABR-BR-1.5 = 4.2.1. Altitude 1911 m asl; Carrasco et al., 2018) and other area in the ICS (ELA in the The total height range (or rise) for cirque development from maxi- Sierra de Béjar, AABR-1.5 = 2010 m asl; Carrasco et al., 2015). mum Elevation Maximum to minimum Elevation Minimum (maximum The largest cirques in GU present mid-altitude values (altitude class Emax to minimum Emin) is 916 m for GU and 542 m for SO (Fig. 6, 3, Table 6), whereas in SO, it is the cirques located at higher altitudes (al- Table A1). These ranges are comparable to those described in some titude class 2, corresponding to the highest summits in SO, Table 6, other mountain sectors in Spain whose structure and topography are sim- Fig. A2) that are the largest (greater L and W) and deepest (greater ilar to those of GU and SO (Alonso, 1994; Ruiz-Fernández et al., 2009; H), with the largest surface area and volume (greater 2D, 3D and V).

Table 3 Classes of previous (pre-glacial) terrain morphology (see Fig. A2).

Morphological units Code Cirques location on:

I. LANDFORM TYPE 1 Rectilinear uniform slopes, normally corresponding to fault scarps or fault lines Data indicating the snow cover accumulation and stabilisation capacity. Capacity scarps. minimum on uniform slopes, maximum on mesetas. 2 Complex slopes with gradient changes and steps of structural or erosive origin. Location reference: the cirque. 3 Valley headwalls of structural or mixed structural-erosive origin: fracture corridors, synclinal or monoclinal structures or inter-spur valleys. 4 Mesetas or high plateaux. II. TOPOGRAPHIC FEATURES 1 Main divide (more effective the larger the divide). Indicators of slope interconnection and likelihood of originating snow drift 2 Secondary divide. Limited effectiveness for transfer as this is normally a sharp divide, processes. especially in SO.

Location reference: the Emax of cirque. 3 No divide reached. Over-supply difficult from ice transfer or snow drift. III. PREGLACIAL TORRENTIAL BASINS 1 Large cones associated with a single torrential headwater. Determined by indirect indicators associated with alluvial cones. 2 Large cones associated with various torrential headwater. Indicators of possible conditioning factors of pre-glacial morphology in cirque 3 Medium cones associated with various torrential headwater size and shape. 4 Small cones associated with a single torrential headwater. Location reference: the cirque. 5 Small cones associated with various torrential headwater. 6 River channels not associated with significatif alluvial deposits in the area where cirques are found. J. Pedraza et al. / Geomorphology 341 (2019) 153–168 159

Table 4 Classes of location and density of fractures.

A. Location of large fractures in relation to cirque axis L B. Fractures density measured by: n° de fractures/cirque 2D area

Code Description Code Description (value)

1 Main fault coinciding with L axis. Possible effects of elongation conditioned by fracture 1 Minimum (0–0.16) 2 Fracture systems in parallel or tending to parallel L axis. (None coincident). May facilitate widening and generate compartmentation 2 Medium (compound cirque) (0.16–0.32) 3 Orthogonal or oblique to longitudinal L axis. May generate transversal or oblique steps prone to over-deepening 3 High (0.32–0.48) 4 Association classes 1, 2, 3 4 Maximum (0.48–0.64) 5 Association classes 1, 2 6 Association classes 1, 3 7 Association classes 2, 3 8 No large fractures affecting cirque. Only possible minor joints or stratigraphic layers

This agrees with the values obtained from correlations (r value ≤ 0.18 in (absolute values of r 0.02–0.04) and medium low in SO (absolute values GU and r value ~0.5 in all cases in SO; Table A2). for r 0.2–0.3). This is also reflected in the variance (Table 6): the L/W The correlation between the parameters that define cirque shape ratio in GU is very similar in all classes and the value for circularity re- (CR and L/W ratio) and altitude (Table A2) is generally very low in GU mains almost constant at 1.1; however, in the SO, cirques at lower

Fig. 5. Regional correlations for morphometric parameters. Comparison between mean values for the main parameters of GU and SO cirques and those of other regions. The figure includes representative examples from the Iberian Peninsula and other European regions, used here as a fundamental basis for correlation and discussion. LEGEND: Iberian Central System (Spain): GU-Guadarrama, SO-Somosierra (in this paper). Cantabrian Mountains (Spain): SI-Alto Sil and MC-Macizo Central (Gómez-Villar et al., 2015); W-Picos de Europa and SS-Sierras del Sur (Ruiz-Fernández et al., 2009). Pyrenees (France-Spain): PC-Upper Aragón and Gállego river basins (García-Ruiz et al., 2000); AR-Aran valley and BO-Boí valley (Lopes et al., 2018); PO- Eastern Pyrenees (Delmas et al., 2014). Alps (France-Italy): MA-Maritime Alps (Federici and Spagnolo, 2004). Eastern and Southern Carpathians (Romania): RO-Romanian Mountains (Mîndrescu and Evans, 2014). Western Carpathians (Slovakia-Poland): HT-High Tatras (Křížek and Mida, 2013). 160 J. Pedraza et al. / Geomorphology 341 (2019) 153–168

Table 5 morphology of the terrain, and more specifically, of the landform type Types of cirques identified by CLUSTER analysis of morphometric parameters in GU and and topography. SO. The group 1 are the largest cirques in surface area and the longer, highest and widest. The scant correlation between altitude and morphometric parame- Both in GU and SO are elongated, but very shallow in the former and deeper in the latter. The group 2 are the intermediate cirques in surface area, length, height and width. These ters (low or very low r values, Table A2) in SO and GU strongly suggests are those less elongated but shallow. The group 3 are the smallest cirques in surface area that altitude is a determining factor in cirque distribution, but while it and length, height and width. These are shallow and very elongated cirques. contributes to the configuration of some of their morphometric vari- Group Guadarrama Somosierra ables (fundamentally length, L), it is not a determining factor in their development. (1) (2) (3) (1) (2) (3)

N° of cirques 7 36361143 4.2.2. Aspect E 2197.71 2134.06 2069.00 2255.00 2100.29 2152.67 max Most cirques in both GU and SO are located on the eastern sides (72% Emin 1775.43 1985.47 1832.81 1757.00 1887.14 1998.33 H 422.29 148.58 236.19 498.00 213.14 154.33 and 88% of cirques in GU and SO, respectively) with a predominantly E L 1213.52 374.01 611.12 971.00 467.43 417.33 aspect (class 3): 30 cirques in GU (38%) and 9 in SO (49.5%). Few are lo- W 1184.04 341.17 591.15 754.00 474.57 302.33 cated on the western slopes (10% and 0% of cirques in GU and SO, re- CR 1.12 1.09 1.12 1.17 1.11 1.13 spectively) but there are some on the northern sides (26.4% and 27.5% L/H 3.31 3.20 2.92 1.95 2.24 2.68 L/W 1.14 1.15 1.11 1.29 1.02 1.37 of cirques in GU and SO, respectively). A noticeable contrast was ob- W/H 3.28 2.90 2.72 1.51 2.35 1.95 served between GU and SO as regards the number of cirques located V 627.22 21.21 89.03 364.60 49.24 20.20 in the northern (NW, N, NE) and southern (SE, S, SW) quadrants, Area 2D 117.76 11.02 31.43 69.64 19.43 11.21 whereby the latter location predominates in GU (35.4% sunny aspects Area 3D 126.98 11.86 34.08 80.82 21.20 11.61 3D/2D 1.09 1.07 1.09 1.16 1.09 1.05 and 26.4% shaded aspects), while the opposite occurs in SO (27.5% shaded and 22.0% sunny aspects) (Fig. 3, Table 6). In the Iberian Peninsula mountains, the most common cirque as- pects are N and NE (Santos-González, 2010). This is also the established altitudes (altitude class 4) are the most circular, while those at higher al- pattern in the majority of mountains in the northern hemisphere and titudes (altitude classes 2 and 3) present slightly allometric growth has been associated with the lower efficiency of solar radiation in from elongation. these aspects (shading or morning/afternoon effects; Evans, 1977; These results are consistent with the more general argument Evans and Cox, 2005). Regionally, the E aspect is generally the second established for the influence of altitude on these processes, which is most common location for cirques and may become the predominant considered to be high in the distribution of the cirques, but low as aspect in ancient massifs with flat summits, such as GU and SO. This cir- regards control of their geometry (Evans and Cox, 1995; Křížek and cumstance has been attributed to snow drifting processes (Evans, 1977, Mida, 2013). However, the low mountains of the Iberian Peninsula 2006b), which have proved very efficient in glacier development sometimes showing a clear gradient in glacial erosion from the lower (Mitchell, 1996; Purves et al., 1999; Coleman et al., 2009; Mîndrescu to the highest summits (Ruiz-Fernández et al., 2009; Santos-González, et al., 2010; Delmas et al., 2014; Radbourne, 2015). 2010; Valcárcel et al., 2012). The data obtained from studies carried Given that, to a large extent, mountain glaciers constitute dendritic out in the Cantabrian Mountains show that in some sectors, altitude drainage systems (ice-drainage basins; Björnsson, 1986; Hardy et al., clearly affects cirque development (L, W, H and area increase in parallel 2000; King, 2009), the arrangement of the main ridges in mountain with altitude), while in other sectors this factor has little effect (Ruiz- chains with low structural complexity is a primary conditioning factor Fernández et al., 2009; Santos-González, 2010; Gómez-Villar et al., for cirque location (Alonso, 1994; García-Ruiz et al., 2000; Křížek and 2015). Furthermore, as has been observed in the Pyrenees (Lopes Mida, 2013; Gómez-Villar et al., 2015; Palma et al., 2017). Both GU et al., 2018), the positive gradient in glacial erosion may be inverted and SO belong to this group of mountains, and their main alignments from a specific altitude. This is consistent with the results obtained for generally tend to run NE-SW, ENE-WSW, and to a lesser extent E-W. GU and SO, and suggests that cirques located at intermediate altitudes The secondary alignments are orthogonal or almost orthogonal, pre- are better developed. As analysed in the corresponding sections, this dominantly running NW-SE, WNW-ESE, and to a lesser extent N-S. process may be associated with the influence of the previous This relief structure facilitates a wide variety of cirque orientations on

Fig. 6. Box-and-whiskers plot of maximum (Emax), minimum (Emin), mean (Emean) and mean of Emean (MEm) cirque elevation (m asl). J. Pedraza et al. / Geomorphology 341 (2019) 153–168 161

Table 6 Analysis of variance. Mean values of the morphometric variables of cirques according to altitudinal and aspect classes. See Tables A3 and A4 for the statistical significances (ρ) and others data.

Code (class)a Guadarrama Somosierra

(1) (2) (3) (4) (5) (6) (7) (8) (1) (2) (3) (4) (5) (6) (7) (8)

Altitudinal classes N° of cirques 1 17 45 16 ––––03114– ––– L 435.65 529.04 605.52 455.10 –––––660.75 453.45 378.67 – ––– W 506.80 517.83 577.85 408.67 –––––554.00 444.27 400.67 – ––– H 215.00 246.12 204.36 200.81 –––––289.75 206.36 172.00 – ––– CR 1.11 1.10 1.11 1.08 –––––1.15 1.12 1.09 – ––– 2D area 17.59 27.00 35.48 17.46 –––––35.95 17.63 12.49 – ––– L/H 2.03 2.19 3.67 2.45 –––––2.51 2.21 2.30 – ––– L/W 0.86 1.09 1.14 1.16 –––––1.27 1.06 0.96 – ––– W/H 2.36 2.09 3.37 2.24 –––––2.02 2.25 2.48 – ––– Aspect classes N° of cirques 7 7 30 20 7 1 0 714931000 L 639.94 481.44 573.83 461.29 440.42 532.71 – 864.76 971.00 471.75 419.22 545.33 500.00 ––– W 628.78 436.33 536.58 459.54 455.97 395.43 – 788.65 754.00 351.25 483.44 400.00 595.00 ––– H 182.57 149.29 221.20 208.55 156.86 209.00 – 338.71 498.00 190.75 199.11 213.33 252.00 ––– CR 1.11 1.12 1.10 1.11 1.11 1.06 – 1.09 1.17 1.13 1.13 1.07 1.09 ––– 2D area 43.74 17.28 29.30 20.18 23.23 16.29 – 66.29 69.64 14.43 18.72 17.83 25.89 ––– L/H 4.17 4.06 2.70 2.42 3.03 2.55 – 4.65 1.95 2.55 2.15 2.61 1.98 ––– L/W 1.06 1.20 1.12 1.07 1.16 1.35 – 1.28 1.29 1.36 0.89 1.37 0.84 ––– W/H 3.81 3.80 2.62 2.39 2.77 1.89 – 3.49 1.51 1.85 2.56 1.98 2.36 –––

a See Table 2 for altitudinal code classes and Fig. 3 for aspect code classes.

the eastern and western slopes. However, GU and SO cirques are pre- These results are interesting in terms of large cirques in GU and SO dominantly located on east-facing slopes (70% and 80% of cirques in that present shaded aspects. Current data show lower radiation bal- GU and SO, respectively). ances in these orientations compared to others and a greater perma- At present, the prevailing winds in GU are westerly (NW, W and to a nence of year-round snow (Palacios and Sánchez-Colomer, 1997; lesser extent SW) and are responsible for significant snow drift from the Palacios et al., 2003, 2012; Fernández-Cañadas, 2014; Durán et al., western (windward) to eastern (leeward) sides. This phenomenon has 2017). This could explain the larger size of cirques located on N and been verified from direct wind measurements in studies on annual NW slopes, due to the effects of shading or morning/afternoon. How- snow cover (Palacios and Sánchez-Colomer, 1997; Palacios et al., ever, since these cirques are located at the headwaters of ancient torren- 2003; Fernández-Cañadas, 2014; Durán, 2015; Durán et al., 2017). In tial basins, the previous morphology of the terrain must also have spite of significant climate variations during the Last Glacial Cycle, the influenced their development. circulation weather types (CWT) in the Iberian Peninsula were similar The above suggests that cirque distribution is strongly determined to those at present (Ludwig et al., 2016). Although a NW storm track by aspect. However, as in the case of altitude, the influence of this factor predominated due to the more southerly position of the polar fronts on cirque size and shape is more selective and limited, impliying that (Florineth and Schlüchter, 2000), W and SW situations were main- the W aspect is a totally restrictive factor. In N-S alignments, where tained (Ludwig et al., 2016, 2018; Naughton et al., 2016). The preva- cirques can develop on both slopes, most are usually located on the east- lence of westerly winds has also been verified in loess formations in ern slope, with predominantly E aspects. In contrast, very few cirques the Southern Spanish Meseta, formed during Heinrich Stadials 3, 2 are located on the western slope, and none present a W aspect. This con- and 1 (Wolf et al., 2018). This similarity between current and Last Gla- trasts with the other standard alignment, namely E-W, in which cirques cial Cycle storm tracks has also been detected further north, in studies are located on both the southern and northern slopes. of cirque distribution in Britain and Ireland (Barr et al., 2017). Thus, topoclimate, i.e. climatic influence of the relief, can explain 4.3. Physiographic location of cirques according to lithology, previous mor- cirque distribution in GU and SO. This argument coincides with that phology of the terrain and the fracture network. Influence of these factors established to explain predominant cirque orientation in other Iberian on cirque morphology Peninsula mountain ranges, such as the Pyrenees (E-facing; Delmas et al., 2014) and the Cordillera Cantábrica (predominantly NE-facing; 4.3.1. Lithology Santos-González, 2010). Three different types of geological substrata underlie GU and SO The largest cirques in terms of surface area, volume, width, height cirques (Figs. 4 and A1, Table 7): (i) homogeneous, composed of a spe- andlengthoccurinNorNWaspectsinGUandNorSinSO(Table 6). cific lithology (granites or orthogneisses; classes 1 and 2, respectively); This finding is slightly inconsistent with r values, which show a low (ii) heterogeneous, composed of various lithologies, but corresponding or very low correlation between aspect and cirque size indicators to the same geological formation characterised by lithological alterna- (absolute values of r ≤ 0.4) (Table A2). Cirque shape indicators sug- tion (paragneiss-schists and slate-phyllite with quartzite intercalations, gest that the influence of aspect was very low in both GU and SO, classes 3 and 4, respectively); and (iii) highly heterogeneous, formed of with practically no variation in CR values and only minimal variation various lithologies corresponding to different geological formations in the L/W ratio for all classes (Table 6). Vertical incision indicators in (classes 5 and 6, respectively). Substratum rock type (i) is characteristic GU and SO showed values higher than 1.95 for L/H and 1.51 for W/H. of GU and type (ii) is characteristic of SO, while type (iii) is present in Thus, very high values were obtained in GU for L/H and W/H ratios in both areas. all N aspects. Although there are no NW aspect cirques in SO and only There is complete agreement that lithology exerts a very low influ- one N aspect cirque, the NE aspect also shows the second highest L/H ence on cirque shape because L/W and CR values are similar for all li- ratio. This indicates that no aspect in either GU or SO facilitates thology classes (around 1.10) and r values are minimal (Tables 7 and overdeepening processes, and given that the L/H ratio is always A2). The r values for L/W (0.10) and CR (0.07) in SO show a very low higher than the W/H ratio, N-facing aspects are more prone to elon- correlation with cirque shape/lithology (Table A2). This is in accordance gation processes. with CR, which maintains practically the same value for all lithological 162 J. Pedraza et al. / Geomorphology 341 (2019) 153–168

Table 7 Analysis of variance. Mean values of the morphometric variables of cirques according to lithology classes. See table A5 for the statistical significances (ρ) and others data.

Code (class)a Guadarrama Somosierra

(1) (2) (3) (4) (5) (6) (1) (2) (3) (4) (5) (6)

Lithology N° of cirques 33400420003735 L 324.98 500.84 ––618.00 –––378.67 476.00 361.67 642.80 W 315.52 481.84 ––583.88 –––400.67 425.86 471.00 541.80 H 159.33 169.32 ––251.74 –––172.00 209.29 169.67 291.00 CR 1.07 1.12 ––1.10 –––1.09 1.13 1.13 1.11 2D area 7.77 24.65 ––35.51 –––12.49 18.41 15.96 32.19 L/H 2.07 3.83 ––2.54 –––2.30 2.34 2.19 2.29 L/W 1.11 1.15 ––1.12 –––0.96 1.20 0.76 1.22 W/H 1.95 3.46 ––2.42 –––2.48 2.02 2.94 1.96

a See Fig. 4. classes. However, L/W ratios are lower than 1 in predominantly gneiss 4.3.2. Previous morphology of the terrain materials (classes 3 and 5) and N1 in predominantly slate materials (classes 4 and 6). This suggests that gneisses are more susceptible to 4.3.2.1. Landform type. As shown in Table 8, 79.8% cirques in GU and 39% cirque widening processes, whereas slates are more susceptible to cirques in SO are located on uniform or irregular slopes with steep gra- cirque elongation processes. This notion is also supported by the values dients. These slope cirques were formed on both sides of main and sec- obtained for W/H ratios, which are higher for gneiss than for slate ondary divides (Fig. A2), most of which are fault-line scarps presenting (Table 7). a stair-stepped topography; thus, at times these appear to be staircase There is also agreement on the low influence of lithology on cirques (Gordon, 1977). Other valley-side cirques (Trenhaile, 1976; overdeepening processes, because granites (class 1) are only detected Mîndrescu and Evans, 2014) appear on valley slopes of a mixed as the least favourable type for these processes (L/H and W/H ratios structural-fluvial origin, orthogonal to the main alignments. A smaller close to 2, Table 7). According to the r values for parameters 2D, 3D number of cirques in GU (10 cirques, 12.6%) and a larger number in and V (lower than 0.19 in all cases in GU and lower than 0.48 in SO, SO (11 cirques, 61%) occurring in these mixed valleys are valley-head Table A2), variations in cirque size for the different lithologies should cirques (Trenhaile,1976; Mîndrescu and Evans, 2014) with a clear ten- be minimal. However, cirques formed on class 5 (mixed lithologies, pre- dency to form trough-type cirques (Gordon, 1977; Benn and Evans, dominantly orthogneiss) and class 6 (mixed lithologies, predominantly 2010). Lastly, there is a small number of plateau cirques (6 cirques, slates) are much larger (Table 7). 7.6%) on flat summits in GU. The above data suggest that lithology exerts a similar influence on CR and L/W ratio for both GU and SO present a value of around 1 for the shape and size of cirques in GU and SO. The most significant differ- all landform type classes, indicating that this factor exerts a very low in- ence is a slight tendency to widening on crystalline rocks (granitoids fluence on cirque shape (Table 8). This finding is somewhat inconsistent and gneisses), unlike the tendency to lengthening on low-grade trans- with r values for CR (around 0.4), but in full agreement with the values formation metamorphic rocks (alternating slate-phyllite-quartzite). for L/W (r = 0.01 in SO and r = 0.11 in GU, Table A2). This finding contrasts with the significant differences detected in the In GU, the largest cirques in terms of L, W and 2D area are located at Eastern Pyrenees (Delmas et al., 2014) between granites-gneisses- valley heads (class 3) and the smallest on uniform slopes (class 1). In SO, migmatites (isometric cirques with non-random distribution) and there is a general gradation of sizes, although the larger cirques are lo- schists (allometric cirques with random distribution). cated on both types of slope with very similar sizes (classes 1 and This homogeneity in the behaviour of lithologies presenting very dif- 2) and the smaller cirques at the valley heads (class 3) (Table 8). This ferent susceptibilities to modelling processes may be due to the geolog- significant difference between GU and SO may be due to the effect of re- ical evolution of GU and SO: these ancient massifs have been subjected lief types on each mountain: in GU, horst-graben fracture structures to various brittle-ductile deformation stages and numerous weathering predominate and the valley profiles generally present a wide U- cycles. Thus, the tectonic structure (considered a relevant factor in these shaped morphology, whereas large ancient Variscan antiform-synform cirque development processes; e.g., Linton, 1963; Haynes, 1968; folding structures predominate in SO and the valleys are narrow with ir- Sugden, 1968; Vilborg, 1977; Olyphant, 1981a, 1981b; Evans, 1994) regular floors and profiles tending to a V-shaped morphology. and weathering profiles on substrata rocks (Centeno and Brell, 1987; As mentioned above, slope cirques are abundant in both GU and SO, Molina-Ballesteros et al., 1997), may significantly standardise the ero- forming floor cirques with steep slopes (between 23° and 24° on aver- sional behaviour of different lithologies. age; Table A1, Fig. 2D and E). This type of cirque is typical of glacial These weathering processes, have remained active until now in tongues located on steep slopes (slope glaciers) and was common in areas of fracture (Centeno and Brell, 1987), giving rise to bands of Iberian Central System former glaciers (Pedraza and Carrasco, 2006). weathered rock (grus) that replace the original lithologies. Due to Slope glaciers have also been described in areas with medium and their greater susceptibility to torrential and rill erosion, these weath- high mountain ranges (Vivian, 1975; Marinescu, 2007; Scherler et al., ered rocks formed substrata favourable to hosting the existing river net- 2011). In all cases, these are considered short glaciers with a low capac- works, and might also have facilitated the creation of cirques and ity for overdeepening that are highly susceptible to environmental determined their shape and size (Bullón, 1988, 2016; Pedraza, 1994; changes. Ballesteros-Cánovas et al., 2015). The influence of weathered crystalline Although the L/H and W/H ratios obtained for GU and SO show that rocks on glacial processes has also been observed in some glaciated vertical incision is very low in all cases (with ratios generally between 2 landscapes in Scotland (Krabbendam and Bradwell, 2014). and 3, Table 8), the behaviour of the cirques located on plateaus and val- A final factor to consider is the positive effect of lithological hetero- ley heads (classes 3 and 4) partly contradicts the hypothesis that slope geneities on cirque development. This process has also been detected cirques present a lower deepening capacity due to the steep slopes. in some areas of the Spanish Cantabrian Mountains, where cirques lo- Nevertheless, other data must be considered. For example, plateau cated in lithological associations are twice the size of those occurring cirques (minimum H value in GU) are to a large extent anomalous, be- in a single lithology (Ruiz-Fernández et al., 2009; Gómez-Villar et al., cause they formed in a subglacial environment due to irregularities in 2015). the plateau topography which supported the glacier and only became J. Pedraza et al. / Geomorphology 341 (2019) 153–168 163

Table 8 Analysis of variance. Mean values of the morphometric variables of cirques according to landform type, topographic features and pre-glacial torrential basins classes. See tables A6, A7 and A8 for the statistical significances (ρ) and others data.

Code (class)⁎ Guadarrama Somosierra

(1) (2) (3) (4) (5) (6) (1) (2) (3) (4) (5) (6)

Landform N° of cirques 39 24 10 6 ––6 1 11 0 –– Type L 489.59 570.63 740.45 627.62 ––525.00 544.00 461.18 ––– W 458.65 580.42 712.63 484.73 ––481.00 548.00 442.82 ––– H 217.15 223.21 251.80 77.33 ––231.33 223.00 212.18 ––– CR 1.08 1.12 1.14 1.13 ––1.09 1.12 1.13 ––– 2D area 23.40 31.45 48.75 32.96 ––24.73 25.25 18.33 ––– L/H 2.44 2.70 3.22 8.56 ––2.39 2.44 2.23 ––– L/W 1.13 1.07 1.09 1.44 ––1.09 0.99 1.10 ––– W/H 2.24 2.80 3.10 6.61 ––2.28 2.46 2.19 ––– Topographic features N° of cirques 48 14 17 –––8100––– L 597.72 500.44 484.81 –––533.00 450.30 – ––– W 516.27 447.06 401.79 –––487.25 440.70 – ––– H 320.00 139.64 215.59 –––222.63 216.40 – ––– CR 1.05 1.11 1.11 –––1.11 1.12 – ––– 2D area 24.91 18.57 18.49 –––24.52 17.91 – ––– L/H 1.87 5.03 2.26 –––2.56 2.08 – ––– L/W 1.16 1.19 1.29 –––1.12 1.06 – ––– W/H 1.61 4.33 1.87 –––2.41 2.09 – ––– Preglacial torrential basins N° of cirques 3 2 332 15242 0 0 0016 L 1241.24 1459.94 520.11 1747.99 509.69 509.11 710.00 ––––459.19 W 1243.71 1220.88 508.44 1989.09 478.45 456.48 623.00 ––––441.19 H 546.33 623.00 210.30 498.00 234.13 149.83 367.00 ––––200.69 CR 1.10 2.15 1.11 2.25 1.11 1.10 1.14 ––––1.12 2D area 125.91 81.50 25.36 177.98 23.01 22.22 46.32 ––––17.66 L/H 2.26 4.33 2.61 6.70 2.22 4.42 1.93 ––– 2.34 L/W 1.18 2.18 1.08 2.22 1.13 1.19 1.10 ––––1.09 W/H 2.26 4.17 2.58 7.39 2.03 3.81 1.80 ––––2.29

⁎ See Table 3. true cirques (direct accumulation basins) in the final stages of deglacia- Nevertheless, the influence of topography on cirque shape is minimal tion (Carrasco et al., 2016b). In SO, slope cirques obtained the highest H at these locations. The values for CR and L/W (ratios 1.1–1.2) at GU values, which may also be conditioned by the differential behaviour of and SO alike are very similar for all topographic features. the geological materials due to lithological alternation and ancient GU cirques which reach a main divide present a degree of quilibrium folding. between lengthening-widening-deepening, whereas those which reach Lastly, as mentioned in the analysis of the aspect, the general land- a secondary divide present a clear imbalance between widening- form type of mountains where the main ridge runs NNE-SSW, NE-SW lengthening and deepening; and those which do not reach any divide and ENE-WSW has exerted significant impact on glacier location, con- present an intermediate position. Regardless of their topographic loca- tributing to their development on slopes prone to accumulating snow tion, all SO cirques present an imbalance between lengthening, widen- drifts. ing and deepening. This greater efficiency in cirque development when they are connected to orographic divides confirms the contribu- 4.3.2.2. Topographic features. Given the narrow altitude range within tion of topographic features to snow drift, as already noted when which GU and SO cirques could develop, the general trend was to oc- analysing this process in the context of aspect (Section 4.2.2). cupy the highest zones of the relief (see Section 4.2.1). Thus, the Emax Therefore, topographic features, landform type and aspect exert the is located on a main or secondary divide for 78.5% and 100% of GU and greatest impact on cirque location and the configuration of some of their SO cirques, respectively (Fig. A2). Although the valley heads of some parameters. The combined action of these three factors could be respon- cirques on opposite slopes were connected, generally by valley head sible for the gradient in glacial erosion from the lower to the highest fracture corridors, this phenomenon was very local, and it has not summits observed in GU, which was analysed in the section on altitude. been possible to identify any truncation processes in the relief due to This is because there is less flat surface area (plateau) to facilitate snow overriding between cirques (White, 1970), or a hierarchy or accumulation on the highest and intermediate summits (L1 and L2) compartmentalised fractal sequence (Evans and McClean, 1995; than on the lower summit (L3) (Figs. A2, 2A and 2C). Evans, 2003, 2010) similar to that described in other areas (cirques in The influence of topographic plateaus on snow redistribution due to cirques in cirques; Delmas et al., 2015). In general, GU and SO cirques drifting and avalanches has been described in England and Wales can be classified as simple cirques (Gordon, 1977; Benn and Evans, (Western Pennines and Cadair Idris) as an important factor capable of 2010) and present a unitary morphology. Consequently, the erosive ca- generating smaller glaciers in areas on the margins of those typically as- pacity of these glaciers as regulators of altitude (see ‘buzzsaw’ hypothe- sociated with regional glaciation patterns (Mitchell, 1996; Plummer and sis by Mitchell and Montgomery, 2006) was insignificant. As also Phillips, 2003; Radbourne, 2015). This type of process could be respon- observed in other mountain ranges (Mîndrescu and Evans, 2014; Crest sible for the location of some of the smaller, isolated cirques (Fig. A1) in et al., 2017), pre-glacial erosion surfaces on GU and SO summits are con- GU and SO that are connected with the topographic summit. served, slightly modified by nival and pluvio-nival action during the Another process associated with topography is the transfer of ice be- Quaternary (Fig. A2). tween glacier feeding basins connected by passes or saddles, which is Low or very low r values (0.44–0.02) were obtained for correlations usually associated with fracture corridors. This process is equivalent to between morphometry/topographic features (Table A2). Nevertheless, the transfluence widely described in many glacial areas, particularly in in both GU and SO, cirques with headwalls reaching a main divide pres- the Scottish Highlands, the Alps and some Spanish mountains (Linton, ent the largest dimensions (Table 8). This is consistent with the data ob- 1949, 1963; Florineth and Schlüchter, 1998; García Ruiz et al., 2001; tained from an analysis of cirque size/altitude ratios (see Table 6). Carrasco et al., 2015; Seguinot et al., 2018). However, in GU and SO, 164 J. Pedraza et al. / Geomorphology 341 (2019) 153–168 this was limited to overfeeding of some glaciers, contributing to greater the glacial and post-glacial stages, but in some areas, old periglacial de- development of the cirques (Carrasco et al., 2016a, 2016b). At the same bris slopes have been discovered, corresponding to cold periods prior to time, these processes could be contributing to elongation caused by the Würm glaciation (Vaudour, 1979; Bullón, 1988, 2016; Sanz-Herráiz, retro-erosion in the headwall of the cirques in favour of fractures (Fig. 1988). This suggests that many of these torrential basins were ideal for 2B, D and E). cirque formation, but bulldozing processes limited the capacity of the glacier to deepen and widen the cirque. 4.3.2.3. Pre-glacial torrential morphology. The existence of pre-glacial tor- Despite limitations due to the presence of only two classes, SO pre- rential basins was determined from indicators such as the piedmont sents the same trends as GU in the relative size and shape of the cirques: fans associated with these basins and pre-glacial age deposits a marked increase in cique size associated with torrential valley heads (Table 3). Class 3 accounted for 41.77% of GU cirques, even more than but no influence of this morphological factor on cirque shape. However, in class 6, which does not present any restrictions (Table 8). This may a slight trend can be detected in SO as regards development of the be due to the concurrence of several factors: aspect, altitude and land- deepest cirques when these are associated with torrential valley head form type facilitated the formation of a considerable number of cirques areas. and torrential basins on one of the slopes of the Lozoya Valley (Karampaglidis et al., 2014, 2016)(Figs.A1andA2). 4.3.3. Fracture network Due to the tilted block mountain landform type of the SO, basins as- sociated with old piedmont deposits solely appear on the steep north- 4.3.3.1. Fracture direction. The main orientation of the Spanish Central ern side where only two cirques have been found (11.1%). The System is governed by the combined effect of inherited tectonic struc- remaining cirques (88.9%) are located on the gentler slopes on the tures and variations in the lateral strength of the lithosphere (Vegas southern side, where the geomorphic indicators of pre-glacial torrential et al., 1990; De Vicente et al., 2007; Fernández-Lozano et al., 2012). activities were not found. Mountain massifs are pop-up-like structures affected by a strong, This factor exerts a clear influence on the dimensions of GU cirques superimposed structural pattern that exerts a major influence on cirque (Table 8). Except for class 4, which accounts for most of the larger location (Table 9). cirques, the remaining classes present a direct cirque size/fan size For instance, given the fracture density observed throughout the ICS ratio. These data suggest that the concurrence of cirque/torrential valley (Pedraza, 1994; de Vicente et al., 2007), the data on the limited number head area is a factor contributing to cirque development. However, as of cirques not located on fractures are reasonably congruent (3.8% and the r values (all negative, around 0.4 and 0.6 in GU and SO, respectively; 0% of cirques in GU and SO, respectively). In general, the largest number Table A2) indicate, alluvial cone size is a random indicator. This may be of cirques is found in the least restrictive classes, i.e. those formed by the because alluvial cones correspond to different accretion sequences; thus presence in a single cirque of various basic classes (4, 5, 6 and 7). Thus, the largest alluvial cones do not necessarily imply the largest torrential whereas orthogonal fractures clearly predominate in GU in the SO there valley heads or those most favourable to the formation of the largest is a slight predominance of orthogonal fractures (Table 9). cirques. An analysis of correlations (Table A2) and variance (Table 9), The scattered values for cirque shape indicators show that the effect suggests that contrary to correlation estimates, since the r values of this factor is negligible, in agreement with the data obtained for cor- are very low, (lower than 0.1 in all cases), this factor exerts an influ- relations (absolute values of r lower than 0.1; Table A2). In relation to ence on GU cirques. However, fracture location in relation to the L vertical incision indicators, all values for L/H and W/H ratios are very axis of the cirques exerts little control over their shape. Both the high (2.03–7.39), indicating shallow cirques with a marked predomi- CR and L/W ratio values present negligible variance in the different nance of lengthening-widening. classes (CR 1.13–1.08 and L/W 1–1.35). If we compare with present-day dynamics, The slopes of most of the In none of the classes are the L/H and W/H ratios conducive to heads of torrential basins have superficial deposits forming debris deepening processes, with the least favourable ratios corresponding slopes and cones caused by flash floods and debris flows (Palacios to class 8 where there are no fractures, suggesting that fractures et al., 2003; Ballesteros-Cánovas et al., 2015; Rodríguez-Morata et al., exert a negligible influence on this process (Table 9). For example, 2016). These deposits mainly correspond to paraglacial activity during class 2 (fractures parallel to the L axis) is the least conducive to

Table 9 Analysis of variance. Mean values of the morphometric variables of cirques according to fractures location and density classes. See tables A9 and A10 for the statistical significances (ρ)and others data.

Code (class)a Guadarrama Somosierra

(1) (2) (3) (4) (5) (6) (7) (8) (1) (2) (3) (4) (5) (6) (7) (8)

Fractures@location N° of cirques 9491062216323403330 L 397.49 351.34 469.26 984.46 547.20 534.23 552.29 345.31 575.00 526.33 484.50 – 628.33 376.67 361.67 – W 364.96 367.14 456.74 1017.74 424.16 473.20 559.61 300.46 408.50 382.67 443.50 – 675.00 376.00 471.00 – H 182.11 102.75 211.56 324.30 167.83 196.45 234.00 179.33 233.00 208.00 198.00 – 335.33 182.67 169.67 – CR 1.08 1.08 1.08 1.13 1.12 1.13 1.09 1.08 1.07 1.15 1.11 – 1.15 1.10 1.13 – 2D area 13.13 10.53 18.36 92.16 19.57 23.39 28.00 8.46 18.30 17.10 18.26 – 40.34 15.13 15.96 – L/H 2.25 4.00 2.42 3.25 4.56 3.64 2.44 2.11 2.51 2.67 2.50 – 1.85 2.06 2.19 – L/W 1.16 1.00 1.05 1.01 1.35 1.22 1.04 1.21 1.41 1.41 1.12 – 0.91 1.02 0.76 – W/H 2.07 4.79 2.65 3.43 3.08 3.02 2.42 1.91 1.79 1.85 2.42 – 2.17 2.02 2.94 – Fractures@density N° of cirques 57 14 6 2 ––––15300–––– L 626.99 417.99 316.36 235.55 ––––523.47 305.00 –––––– W 603.91 365.24 333.41 157.50 ––––483.60 350.33 –––––– H 234.46 170.93 138.33 110.50 ––––233.67 146.67 –––––– CR 1.11 1.07 1.11 1.19 ––––1.12 1.11 –––––– 2D area 37.00 12.63 9.87 3.69 ––––23.19 9.12 –––––– L/H 3.13 3.31 2.34 2.26 ––––2.33 2.09 –––––– L/W 1.12 1.18 1.00 1.55 ––––1.12 0.92 –––––– W/H 2.95 2.76 2.57 1.45 ––––2.20 2.41 ––––––

a See Table 4. J. Pedraza et al. / Geomorphology 341 (2019) 153–168 165 deepening processes and the most prone to lengthening/widening. ratios clearely display an inverse trend in GU, the opposite is the This implies that fracture sets parallel or almost parallel to the L case in SO. axis are effective in this cirque development process, being thus Thus, it can only be established that a higher fracture density was considered allometric. Moreover, there is a clear relationship be- not a relevant factor in cirque development here. Nevertheless, tween fracture clusters/cirque size. Thus, the largest cirques (2D these initial results require further validation (almost cirque by area = 92.16 ha) appear in class 4, which groups fractures corre- cirque), since: (i) in other locations and situations, the opposite ef- sponding to the three individual classes. fect has been demonstrated (greater fracture density facilitates In contrast to GU, SO cirques in classes formed by fracture clus- cirque development; Olyphant, 1981a, 1981b), and (ii) a detailed ters (5, 6, and 7) present a clear widening trend (L/W values lower evaluation of fracture effectiveness should take into account both than or equal to 1.02), while cirques belonging to the unitary classes the dip (vertical fractures are effective for wall but not bed erosion; (1, 2 and 3) present a tendency to lengthening (Table 9). This is in Haynes, 1968) and the inward- or outward-dipping position agreement with the r values (r = −0.60) for the L/W ratio obtained (Vilborg, 1977), with respect to the topographic surface. in the correlations (Table A2). On the other hand, the L/H and W/H Recent studies suggest that fracture density beneath glacial ice ratios are near to or higher than 2 (1.79–2.94) in all classes, indicat- may induce erosion by strain localisation, which in turn contributes ing that the different fracture types present homogeneous behav- to enhance glacial quarrying (Dühnforth et al., 2010; Becker et al., iour in the vertical incision process. This result shows a slight 2014; Leith et al., 2014). For instance, in hard crystalline rocks discrepancy with the r values for the correlation (absolute value of with densely spaced joints the intersection between vertical frac- r = 0.40). tures and stress-release joints can induce rapid deepening as a re- An interesting point to note is that class 2 is the second least con- sult of ice plucking. However, in soft rocks as well as regolith ducive to vertical incision deepening, but at the same time, this class subglacial abrasion is particularly efficient when the spacing be- is the most prone to cirque elongation. This implies that, as with GU, tween joints is greater (Krabbendam and Glasser, 2011). fracture sets parallel or almost parallel to the L axis in SO are the most effective for cirque development, which may be considered al- 5. Conclusions lometric. In addition, and consistent with GU cirque size, the largest cirques (2D area = 40.34 ha) belong to class 5 and are formed by Most of the cirques analysed here are simple cirques, with a cir- fracture sets. cular morphology (concave cirques) formed on steep slopes. The As shown above, the fault orientation analysis suggests that the largest cirques occur in: medium altitude classes, topographic loca- length and geometry of the glacier basin are strongly influenced by tions connecting them to the main divides, former torrential valley fault network distribution. Although the influence of fractures on heads and on uniform slopes. A comparison with data on other cirque development has been widely accepted (Guerra-Zaballos European mountains indicates that GU and SO cirques can be classi- and Sanz-Donaire, 1987; Alonso, 1994; García-Ruiz et al., 2000; fied as medium-small cirques with an almost circular shape and lit- Dühnforth et al., 2010), no morphological analysis has yet been per- tle deepening. formed. Nevertheless, the fracture pattern and orientation shed The overall distribution of these cirques is the result of decisive light on cirque evolution in GU and SO. The main fault orientations topoclimatic control: altitude, orographic structure and Atlantic trending N-S, E-W and NE-SW (Fig. A1) have a profound effect on storm tracks. In both mountain ranges, the cirques are only occur metamorphic lithologies, where faults follow the Variscan structure in reliefs with summits reaching or exceeding 1900 m asl. The pre- and erosion strongly influences valley configuration. The cirque dominant aspects are east-facing, with configurations linked to widening effect of parallel structures is most marked in crystalline snow drift from western (windward) to eastern (leeward) slopes. rocks, as homogeneous lithologies seems to favour the formation This process implies a Circulation Weather Type (CWT) model for of blocks that are easily removed during cirque development. Simi- the Iberian Peninsula during the Last Glacial Cycle, similar to the lar observations have reported for other glaciated regions with crys- present day. However, no determining factor has been established talline rocks, where fractures led to the evolution of steep slopes for the specific position of each cirque, although pre-glacial fluvial and widely developed ramps (Mîndrescu and Evans, 2014). morphology and tectonic structure showed a major correlation with this process. 4.3.3.2. Fracture density. The location of the cirques in relation to this pa- In general, SO and GU cirques may be classified as tending to rameter (number of fractures/2D area ratio; Table 9)showsaclearpre- isometry. However, in the cirques located on former torrential val- dominance of classes with a lower fracture density: class 1 (minimum leys heads, exceptionally high values for the ratio L/W were re- density), with 72.15% in GU and 83.33% in SO, and class 2 (low density), corded, implying significant elongation. Other factors modifying with 17.7% in GU and 16.67% in SO. The other two remaining classes this trend to isometry, although to a lesser extent, are crystalline (regular and medium density), with 10.12% in GU, do not exist in SO. rocks (prone to widening), low transformation metamorphic rocks In general, the morphometry/fracture density correlations and (prone to lengthening) and parallel fractures (prone to widening). variance (Table 9, Table A2) suggest that CR variations are minimal All the cirques analysed are relatively superficial. Three factors (1.07–1.19) in GU and SO alike, consistent with the r correlation may explain the low overdeepening of these cirques: (1) the crystal- value (absolute values of r around 0.15). Meanwhile, variations in line lithologies were not deepening-prone; (2) the previous mor- the L/W (0.92–1.55), L/H (2.09–3.31) and W/H (1.45–2.95) ratios phology of the terrain, which conditioned the location of cirques are higher but fairly random, as the firsttwodonotpresentany on steep slopes, was not conducive to stabilisation-expansion- trend influenced by the density ratios. This was also the case for deepening processes of the ice mass; and (3) the essential factor of the correlations (absolute values of r: 0.06–0.12 in GU; 0.21–0.33 the time or duration of the glacial activity, since both the GU and in SO). SO cirques only developed during the Last Glacial Period (around As regards size, a clear inverse hierarchy was detected whereby 26 ka BP; MIS 2) with an early phase of post-glacial activity (around the largest cirques correspond to classes withthelowestfracture 12 ka BP). density. This coincides with values obtained in the correlations (r varies between −0.37 and −0.68). The variations in the vertical in- Acknowledgements cision indicator ratios do not display a definite trend. The highest L/ H ratio in GU corresponds to class 2, although classes 1, 3 and 4 dis- This work was supported by the Spanish National Parks Agency play a definite inverse trend, as in SO. Nevertheless, while W/H (Spanish initials: OAPN; project 1092/2014) and the Spanish 166 J. Pedraza et al. / Geomorphology 341 (2019) 153–168

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