Geographical Framework

The area of study considered in this work corresponds to a wide geographical space, approximately comprised between the Alcanadre River —in the Sierra de Guara, part of the Sierra Exteriores— and the Nogueras Rivers —in the Central Catalan Pyrenees. This area is defined as Southern Central Pyrenees, a large territory with an extension of more than 8.000 km². From an administrative point of view, from west to east, it includes the Hoya de Huesca, Somontano de Barbastro, Sobrarbe, Cinca Medio, la Litera the Ribargoza (all of them part of the Huesca province) and the Noguera, Pallars Jussá, Alta Ribagorça, Vall d’Aran, Pallars Subirá, Alto Urgell (all of them part of the Lleida province) and Andorra (Fig. 1.3, 1.5).

This large area is characterized by a strong topographical and altitudinal gradient, passing from the 200-300 m.a.s.l. of the Ebro Basin to the 3.000 m.a.s.l. of the Axial Pyrenees, which in the Central sector form a huge block of mountains that represent the highest range of the entire Pyrenees. All along this gradient one can see different environments: 1) the alpine landscape characterized by seasonal grasslands located over 2.300 m.a.s.l. until the top of the mountains (where there is enough soil and insolation) approximately around 2.800 m.a.s.l.; 2) a mixed-coniferous forests in the subalpine stage between 2.300-1600 m.a.s.l., dominated by mountains pines over 2.000 m.a.s.l. and eventually alternated with grazing areas; 3) a Mediterranean mixed deciduous forest in the mountain stage, between 1.600 and 800 m.a.s.l., quite degraded and with an extensive secondary vegetation dominated by shrubland formations; 4) a highly anthropized landscapes of floodplains, mainly dominated by cultivated fields or abandoned areas.

Actual climate in the Pyrenean region displays an oceanic influence in the west, becoming more Mediterranean toward the east. The Axial Pyrenees is mainly characterized by an Atlantic climate, with cold winters, mild summers and high rainfall throughout the year.

Annual precipitations overcome the 1000 mm/yr., while temperatures during winter are between -4 ºC and 2 ºC and during summer between 13 ºC and 18 ºC. The pre-Pyrenees are mainly characterized by a Mediterranean climate, with annual precipitations that oscillate between 500-800 mm/yr., while temperatures between 3 ºC-5 ºC during winter and 18 ºC-24 ºC during summer. An Atlantic influences is also present, but mainly toward the western valleys of the Aragon's pre-Pyrenees. Further south, the Ebro basin is mainly characterized by a Continental-Mediterranean climate, with low rainfall (300-350 mm/yr.), high insolation and high evapotranspiration (between 1000-1500 mm/yr.) and the prevalence of strong, dry north-westerly winds. Since it is enclosed by various mountain systems —the Pyrenean (to the north), Iberian Cordillera (to the southwest) and the Catalonian mountains (to the east)—

Ebro basin’s climate adopts continental traits: aridity, deficit of rain, drying winds, high insolation, elevated nocturnal radiation and strong annual and daily thermal oscillations. The resulting climate can be defined as semi-arid Mediterranean.

This environmental setting makes of the Southern Central Pyrenees one of the most interesting context for studying the human dynamics of mountain occupation; such altitudinal gradient implies not only a strong variation in climatic and vegetation patterns, but also in the type of resources and their availability.

From a geographical point of view, the main communication routes between the Ebro Basin and Pyrenees are represented by the Ebro tributaries that from the reliefs of the Axial Pyrenees and of pre-Pyrenees descend almost parallel toward the Ebro Basin. The main

Introduction

rivers of the area, from west to east, are: Alcanadre, Vero, Cinca, Ésera, Isábena, Noguera Ribagorçana, Noguera Pallaresa and Segre, among which the Cinca-Segre represent the main tributaries. Indeed, the Segre and, in lesser extent, the Cinca represent the widest and more accessible valleys, respectively connecting the eastern and the western sector of the Southern Central Pyrenees with the plain areas, while the Isábena, Ésera and the Nogueras are characterized by more narrow valleys, forming canyons and gorges when passing through the limestone formations of the pre-Pyrenean Sierras. Pre-Pyrenees traditionally represented the mayor obstacle to human mobility in the Spanish Pyrenees, especially on the west-east axis, between adjacent valleys. In this sense is remarkable that, historically, the main economic practices have been developed on a longitudinal (north-south) and not transversal (west-east) axis (Comas d’Argemir 1995; Benlloch 2002).

Among the various geographical features in the region, one can distinguish some elements especially relevant in relation to the studied area and the selected sites (Fig. 1.5):

 Sierra the Guara: it is part of the Sierras Exteriores of Aragon, which represent the frontal part of the Pyrenean fold and thrust belt. Sierras Exteriores run for about 125 km in west-east direction, parallel to the Axial Pyrenees. Those ranges are mainly characterized by sediments of Triassic, Upper Cretaceous, Tertiary and Eocene period.

The Sierra de Guara is located in the eastern sector of the Sierras Interiores, 25 km a)

b)

Fig.1.3. Geographical framework. In red is indicated the area of the study.

Introduction

north from the city of Huesca; Sierras de Guara extends between the Guatizalema and Alcanadre Rivers. It reaches altitudes of 2.077 m.a.s.l. with the Tozal o Pico di Guara.

 Sierra Ferrera: it is part of the Sierras Interiores system; is a WNW-trending mountain range that bounds the Axial Pyrenees to the south. It runs between the Cinca and the Ésera river, and it is part of the Cotiella massif. It is mainly composed of limestone (in the upper sedimentary sequence), and marls (in the lower formations). The highest peak is called Peña Montañesa (2.295 m.a.s.l.).

 El Turbón: it is a massif located in the Huesca’s Pre-Pyrenees. It is about 6 km long and it is aligned N-S. It is mainly composed by limestone and marl formations.

The Isábena River flows on the eastern side, separating El Turbón massif from the Sierra de Sis. Highest altitude is of 2.492 m.a.s.l.

 Sierra de Sis: is a 28 km long mountain range of the Pre-Pyrenees that runs in N-S direction. Is located between the valleys of Isábena and Noguera Ribagorçana rivers. It is mainly formed by Eocene-Oligocene materials. The highest summits of the range are the Pico de l’Amorriador (1.791 m.a.s.l.) and Puialto (1.782 m.a.s.l.).

 Serra de San Gervàs: it represents the main range among the reliefs of the Sierras Interiores that run between the Noguera Ribagorçana and Pallaresa rivers. It is mainly constituted of Upper Cretaceous formations of limestone and marls. The main peak is the Pala del Teller (1.890 m.a.s.l.).

 Sierra del Montsec: it is constituted by a series of calcareous mountains that run east to west for about 40 km. Is located in the provinces of Huesca and Lleida. It is divided into three main massifs by three of the most important rivers of the Catalonian Pyrenees: the river Segre in the eastern sector, the Noguera Pallaresa in the middle and, to the west, the Noguera Ribagorçana. It is composed of a thick layer of Upper Cretaceous limestone, followed by Lower and Middle Eocene conglomerates, sandstones and shale. The highest mountain, the Santalís peak, is located in the central massif at 1.678 m.a.s.l.

 Sierra del Castillo de Laguarres: part of the Sierras Exteriores, it runs between the Embalse de Barasona and the Noguera Ribagorçana, in the province of Huesca. It is composed of Paleogene formations, mainly conglomerates and sandstone. The highest peak is the Pico de Laguarres (1.100 m.a.s.l.).

 Besiberri: it is a mountain massif located in the central area of the Axial Pyrenees, located at the western limit of the Aigüestortes i Estany de Sant Maurici National Park.

The main summits of the massif are the Comaloforno (3.029 m.a.s.l.), Besiberri Sud (3.024 m.a.s.l.), Besiberri Nord (3.008 m.a.s.l.) and Besiberri del Mig (2.995 m.a.s.l.).

 Aneto: it is the highest mountain of the Pyrenees, 3.404 m.a.s.l. It is located the Posets-Maladeta Natural Park, near the town of Benasque, Huesca province. It forms the southernmost part of the Maladeta massif. It consists of Palaeozoic formations of a granitic nature and Mesozoic materials. Its northern slope holds the largest glacier of the Pyrenees.

Introduction

 Ainsa basin: it is a foreland sedimentary basin part of the South Pyrenean foreland basin along with the Jaca and Tremp-Graus basins. The separation of the three basins took place during Eocene, due to the development of the Mediano and Boltaña Anticlines. It presents reduced dimensions of about 25 x 40 km. Basin-fill materials are mainly constituted by marls and limestone of Upper Cretaceous and Palaeocene periods.

 Tremp-Graus basin: it is a E-W foreland sedimentary basin located between the Sierras Interiores and the Montsec range. It is made of a syncline of Cretaceous and Palaeocene units, with basin-fill materials of Eocene age. The basin is about 20 km wide and almost 100 km long. Lithologies vary, from north to south, from marine (marls and turbidites) to continental materials (conglomerates and sandstone). In the Noguera Ribagorçana and Isábena basins marls predominates, while in the western sector, in the Ésera basin conglomerate prevails.