Horcajo de la Sierra is located in the southern foothills of the Sierra de Ayllón, in the Sierra Norte Region of Madrid. It perches on a cornice overlooking the Del Valle Creek, although the main water course of the area is the Madarquillos River, where trout (Salmo trutta), barbel (Barbus sp.), carp (Cyprinus carpio) and several frog species can be found. The municipality occupies a 20,600 km2 area comprised by two urban settlements, Horcajo de la Sierra and Aoslos. As is the case with many of the villages of the Middle Lozoya River Valley, their history was determined by the Señorío de Buitrago history.
Although the exact date of the first settlement is unknown, it is probable that the decision to settle the area was for defensive -as it is perched on a hilltop overlooking the valley and what was then the only roadway communicating the region with the northern part of the Iberian peninsula- and ranching purposes, given the abundant rich pastures of the region. In fact, during the XIII century, the communally held pastures were under the rule of the Villa de Buitrago. The territory is crisscrossed by eight royally endowed sheep drover's roads -the most representative being the Cañada Real Segoviana or "Segovian Merino Sheep Road", the Cañada de la Risca and the Cañada del Cerro- and by an extensive network of rural ways that are now used as paths by hikers, bicyclist and horse riders. These paths traverse areas of great natural beauty and incalculable ecological value. The rugged orography characteristic of the terrain creates marked differences in altitude in a relatively small area that provide for an abundant and significant diversity of flora and fauna. The predominant vegetation is Quercus pirenaica, the characteristic "robles melojos" oaks clustered in extensive woodlands, and the native walnut tree (Juglans regia). There are some centenary specimens, such as the "Nogal del Cruce" (the Crossroads Walnut Tree), declared Singular Tree by the Madrid Regional Government. The landscape is a mosaic of forest, agricultural and pasture lands delimited by hedges made of the remnants of the original holm oak and gum cistus association (Quercus ilex and Cistus ladanifer).
Sheep ranching, however, was the force that shaped both the landscape and popular architecture, determining the urban development trends of the settlements in this area, characterized by short and windy streets flanked by one story houses, corrals, hay barns and cattle pens made of the typical drywall masonry made of granitic gneiss -the local stone- roofed with clay tiles. The very characteristic shoeing frames still standing also attest to the cattle ranching past of the area.
Horcajo de la Sierra developed around the San Pedro in Cathedra Church located at the southern end of the original settlement. The church is also made of drywall and brick trimmed by ashlar stone corners and was built in the XV century. It has a linteled entrance, a nave with elevated choir on the narthex end and a polygonal apse abutting the belfry tower. As the village continued expanding southwards, developing an urban center along the Calle Mayor, the old town was left on the north side, something that was made more evident when the Calle de la Carretera -the current access to the A-1 highway- was opened.
Three kilometers southeast, on a plain surrounded by meadows dotted by ash trees (Fraxinus angustifolia), rustic melojo oaks and orchards lies Aoslos, the other village. The first written historical reference to Aoslos dates back to 1752, when Horcajo residents settled the site. It grew along the Calle Real, which continues to be its vertebrating axis and where the San Isidro church was built in 1936 with irregularly cut limestone. The church has the traditional rustic aspect of the area of the local architecture and it is topped by a belfry.
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