Evolución geomorfológica en tiempos históricos recientes de cárcavas del borde del piedemonte norte del Guadarrama (Segovia, España)estudio a partir de fuentes documentales

Resum

Near the edge of the Northern piedmont of the Guadarrama Mountains (at the surroundings of the Pedraza historic town, at the Centre-South of the province of Segovia), a series of slope gullies cut on silica sand, shale and gravel sediments of the Upper Cretaceous period are widespread. They occur on the slopes of a set of mesas and cuestas capped by more resistant rocks, limestones and dolostones. The objectives of this paper are to know whether these erosive forms have expanded during recent times or if, on the contrary, they are being healed and stabilized by vegetation colonization. For that, different documentary sources of information have been used. They revealed that there is a significant scarcity of data and of historic references about these erosive forms. The best results have been obtained from the multi-temporal aerial photographs and the field photographies. Based on the former source of information, a characterization of the geomorphological evolution of these gullies in recent historical times has been made. The comparative study from this source cover the last sixty years, and it has shown that there is a tendency towards the backwasting of the gullies� headcuts and headwalls, and therefore there is an extension of the catchment areas. According to this method, a maximum rate of backward movement of 0.78 m/year has been obtained. Additional field work shows how the headwalls are backwasting mainly due to mass movements and hydric erosion. Paradoxically, these active processes coexist with another tendency towards a stabilization and vegetation colonization of the inner areas of the catchments. Thus, spontaneous tree and shrub vegetation is spreading within the gullies, with rates of increasing vegetation cover as larger as of more than 50% in nearly 50 years. This is due to the fact that it is the factor of �colluvium presence� the one which allows a spontaneous recovery of the soil and the vegetation. Regarding the uncertain origin of these gullies, a direct relationship with historical human activities has been established for some of the studied gullies, but not for all of them. Historical human activities such as mining, quarrying and overgrazing widespread over this region from the Middle Ages, and they were very intense since the eighteenth century. According to a direct spatial association between very old limestone quarries and the occurrence of gullies, it is hypothesized that the former could have triggered the latter, but an unequivocal relationship could not been established.