Cirques in the Sierra De Guadarrama And7 Somosierra Mountains (Iberian Central System): Shape, Size and Controlling Factors
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Geomorphology 341 (2019) 153–168 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph 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