[en] 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.

[es] Cerca del borde del piedemonte norte de la Sierra de Guadarrama (comarca de Pedraza, provincia de Segovia) existe un conjunto de formas erosivas en cárcavas bien definidas, desarrolladas sobre arenas silíceas del Cretácico Superior. En este trabajo se analiza la evolución geomorfológica en tiempos históricos recientes de esas cárcavas, realizada a través del análisis de diferentes fuentes documentales. Dicho trabajo ha revelado la escasez de datos y referencias históricas a estas formas del terreno. Los mejores resultados se han conseguido a partir de secuencias fotográficas de campo y de fotografías aéreas verticales de distintas fechas. La evolución geomorfológica experimentada por estas cárcavas a lo largo de los últimos sesenta años, establecida mediante el estudio comparativo de fotografías aéreas verticales de distintos años, ha puesto de manifiesto un retroceso de los bordes de las cárcavas, y por tanto un aumento de la superficie ocupada por sus cabeceras. En concreto, se han obtenido tasas máximas de retroceso de hasta 0,78 m/año. Ese retroceso ocurre sobre todo por procesos gravitacionales (caídas, deslizamientos y flujos), y en menor medida por erosión hídrica en surcos y regueros. Estos procesos coexisten con la colonización de la vegetación en el interior de las cárcavas, sobre todo arbórea y arbustiva. El aumento espontáneo de la cubierta vegetal que se ha registrado supera, en ocasiones, el 50% de su superficie; dicha colonización contribuye a la estabilidad geomorfológica de algunos sectores del interior de las cárcavas, al disminuir la actividad erosiva sobre sus paredes. Sobre el origen incierto de estas formas erosivas, se confirma, su relación con fenómenos inducidos por el hombre; sin embargo, no se ha podido constatar para todas ellas como causa única y común.