Chronological and geomorphological investigation of fossil debris- covered glaciers in relation to deglaciation processes: A case study in the Sierra de La Demanda, northern Spain Jos  e M. Fern  andez-Fern  andez a, * , David Palacios a , Jos  e M. García-Ruiz b , Nuria Andr  es a , Irene Schimmelpfennig c , Amelia G  omez-Villar d , Javier Santos-Gonz  alez d , Javier  Alvarez-Martínez e , Jos  e Arn  aez f , Jos  e Úbeda a , La € etitia L  eanni c , ASTER Team c a Research Group of High Mountain Physical Geography, Department of Geography, Universidad Complutense de Madrid, 28040 Madrid, Spain b Instituto Pirenaico de Ecología, Consejo Superior de Investigaciones Científicas (IPE-CSIC), Campus de Aula Dei, P.O. Box 13034, 50080, Zaragoza, Spain c Aix-Marseille Universit e, CNRS, IRD, Coll. France, UM 34 CEREGE, Technop^ ole de l’Environnement Arbois-M editerran ee, BP 80, 13545 Aix-en-Provence, France d Department of Geography and Geology, Universidad de Le on, Campus de Vegazana, 24071, Le on, Spain e Department of Agricultural and Forest Engineering, Universidad de Valladolid, Campus La Yutera, 34071, Palencia, Spain f Area of Physical Geography, Department of Human and Social Sciences, Universidad de La Rioja, 26004, Logro~ no, Spain article info Article history: Received 9 April 2017 Received in revised form 22 June 2017 Accepted 28 June 2017 Keywords: Debris-covered glacier Oldest dryas Cosmogenic exposure dating Sierra de la Demanda abstract In this study, fossil debris-covered glaciers are investigated and dated in the Sierra de la Demanda, northern Spain. They are located in glacial valleys of approximately 1 km in length, where several mo- raines represent distinct phases of the deglaciation period. Several boulders in the moraines and fossil debris-covered glaciers were selected for analysis of 10 Be surface exposure dating. A minimum age of 17.8 ± 2.2 ka was obtained for the outermost moraine in the San Lorenzo cirque, and was attributed to the global Last Glacial Maximum (LGM) or earlier glacial stages, based on deglaciation dates determined in other mountain areas of northern Spain. The youngest moraines were dated to approximately 16.7 ± 1.4 ka, and hence correspond to the GS-2a stadial (Oldest Dryas). Given that the debris-covered glaciers fossilize intermediate moraines, it was deduced that they developed between the LGM and the Oldest Dryas, coinciding with a period of extensive deglaciation. During this deglaciation phase, the cirque headwalls likely discharged large quantities of boulders and blocks that covered the residual ice masses. The resulting debris-covered glaciers evolved slowly because the debris mantle preserved the ice core from rapid ablation, and consequently they remained active until the end of the Late Glacial or the beginning of the Holocene (for the San Lorenzo cirque) and the Holocene Thermal Maximum (for the Mencilla cirque). The north-facing part of the Mencilla cirque ensured longer preservation of the ice core. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction A debris-free glacier becomes a debris-covered glacier when supraglacial debris, delivered by a combination of mechanisms such as avalanching and rock fall, cover 50% or more of the ablation zone (Kirkbride, 2000, 2011; Brenning, 2005; Az ocar and Brenning, 2010), and the thickness of supraglacial debris layer, which nor- mally increases towards the glacier snout, is more than 0.5 m (Whalley, 2004; Hambrey et al., 2008; Monnier and Kinnard, 2015). Despite this definition, the separation between debris-free and debris-covered glaciers is typically not clear, and the genetic pro- cesses that trigger the transition from debris-free to debris-covered glaciers is still debated (Berthling, 2011; Janke et al., 2015). Differ- entiating debris-covered and rock glaciers is also difficult because there is no agreed definition of a rock glacier (Hamilton and Whalley, 1995a,b; Berthling, 2011; Janke et al., 2015). Rock gla- ciers are considered to have a thicker surface rock layer compared Abbreviations: LGM, Last Glacial Maximum; OD, Oldest Dryas; YD, Younger Dryas; CED, Cosmogenic Exposure Dating; ELA, Equilibrium Line Altitude; HTM, Holocene Thermal Maximum; AAR, Accumulation Area Ratio; AABR, Area Altitude Balance Ratio; AWMA, Area-Weighted Mean Altitude. * Corresponding author. E-mail address: josemariafernandez@ucm.es (J.M. Fern andez-Fern andez). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev http://dx.doi.org/10.1016/j.quascirev.2017.06.034 0277-3791/© 2017 Elsevier Ltd. All rights reserved. Quaternary Science Reviews 170 (2017) 232e249