The natural area of Ayllón extends towards the south following the valleys of the main rivers (Lozoya, Jarama, Sorbe, Bornova, Cristóbal), in whose canyons habitats and communities of great ecological value are located.

The last glaciation partly affected the Macizo de Ayllón forming some glacial cirques such as those found at the source of the Berbellido river, next to the Pico del Lobo, and at the Peña Cebollera Vieja.

Likewise, the slate areas can be classified according to the type of slate that gives shape: There are three relief units that form the Macizo de Ayllón: the peneplain, areas of undulating surface located at the ends of the massif, mainly to the north and south; the valleys, short in the north and very long and narrow in the south, leaving little room for meadows, and the mountain range itself.

The thermal amplitude in autumn and spring periods can be very wide, with pleasant days and extremely cold nights in some cases, becoming more severe in the valley bottoms.

From the Tejera Negra beech forest, to the west, flows the Lillas river, which rises under Pico de la Buitrera and which in turn receives water from another tributary with year-round flow, the Zarza river or Arroyo de la Hoz, which also crosses Tejera Negra after rising under Alto del Parrejón.

Further north, the Sonsaz river, which gives its name to the extensive hunting reserve that encompasses a large part of the valleys and mountains of the Sierra de Ayllón.

It rises under the Atalaya and follows a winding path between mountains until it flows into the Sorbe river between Peña del Pinarejo and the Loma de las Piquerinas.

[6] Today the National Hunting Reserve of Sonsaz, which extends its 68,106ha over a large part of the mountain chain, and the beech forests de Tejera Negra and Montejo are the perfect refuge for the wild fauna of the area.

Depending on the location in the massif, different animal species can be found: Among the insects it is worth mentioning the great diversity that exists in the Macizo de Ayllón.

The humid forests of the mountain chain also provide habitat for nemoral species such as the woodcock (Scolopax rusticola), a scarce nester, wintering and regular migrant, the barbastelle bat (Barbastella barbastellus), the also nesting tree pipit (Anthus trivialis), the song thrush (Turdus philomelos)and the red-backed shrike (Lanius collurio), with breeding areas further north.

Birds such as the kingfisher (Alcedo atthis) and mammals such as the desman (Galemys pyrenaicus), which maintains here isolated populations of the main core distribution of the species in the Sistema Central, very vulnerable to external disturbances and probably threatened by the proliferation of the American mink.

The importance of the natural area of Ayllón for some rare or endemic invertebrate species, such is the case of Lucanus cervus, Parnassius apollo sp.

The Sierra of Ayllón is included in the Carpetano-Ibérico-Leonesa[8][9] chorological province, in the Guadarrámic sector, forming the Ayllonan subsector with its own chorological identity, the most mature arboreal vegetation corresponds in most of the supramediterranean territory to Pyrenean oak groves, reserving the holm oak groves to the basal areas, sloping sills and the steep slopes of the fluvial gorges.

There are many types of vegetation that can be found in the Sierra de Ayllón, although the forests are not very extensive due to the incessant hand of man who for centuries has been obtaining firewood and charcoal and has been opening pastures for livestock, mostly transhumant.

The peaks of the Sierra de Ayllón are located in the bioclimatic levels gold[10] and cryo-Mediterranean,[11] with communities characteristic of these high altitudes, very rich in endemic species.

[12] Thus, there are psicroxerophilous grasslands at the highest altitudes (Hieracio myriadeni-Festucetum indigestae), cytisus (Senecioni carpetani-Cytisetum oromediterranei), chionophilous cervunales (Campanula herminii-Festucetum ibericae), hygrophilous oromediterranean cervunales (Luzulo carpetanae-Pedicularietum sylvaticae, Campanulo herminii-Festucetum rivularis), rupicolous communities (Saxifragetum willkommianae) and oromediterranean glericolous (Digitali carpetanae-Senecionetum carpetani, Rumicetum suffruticosi, Criptogrammo crispae-Dryopteridetum oreadis).

Of special interest are the alder groves (Galio broteriani-Alnetum glutinoseae), of great extension and an excellent degree of conservation in the rivers Jarama, Sorbe and Bornoba, this type of habitat is in increasing expansion, due to the abandonment of traditional activities.

In the domain of humid beech and Pyrenean oak forests there are heaths (Erico arboreae-Arctostaphylletum crassifoliae, Halimio ocymoidis-Ericetum aragonensis, Junipero nanae-Vaccinietum myrtilli), of great importance because they are very scarce in the rest of the Sistema Central.

Two types of oaks are found in the area: The Pyrenean oak groves of the Sierra de Ayllón, (Festuco heterophyllae-Quercetum pyrenaicae and Luzulo forsteri-Quercetum pyrenaicae), are the most extensive and representative of Castilla-La Mancha, presenting as characteristic cortege Arenaria montana, Luzula forsteri, Viola riviniana, Poa nemoralia, Primula veris, Viola sylvatica, Holcus molli, Conopodium bourgaei, etc..

The abandonment of charcoal production and the great reduction in the use of firewood outlines an optimistic future for the natural recovery of the Pyrenean oak grove in this mountain chain.

These forests, together with the humid Pyrenean oak groves (Festuco heterophyllae-Quercetum pyrenaicae), constitute important refuges of Eurosiberian plant species absent in other areas of the Meseta.

[15] The extension of pine in the area corresponds mainly to reforestation policies in the second half of the 20th century due to its rapid growth, the high value of its wood and its great geological amplitude.

Also noteworthy is the isolated presence of a juniper in Tamajón, which occupies both Cretaceous dolomites and silurian slates, a very singular fact given the ecology of Juniperus thurifera.

With the conquest in 1085 by Alfonso VI of what from then on would be called Extremadura Castellana and the consequent transfer of the fighting frontier to the banks of the Tagus river, the valleys of the Macizo de Ayllón would be repopulated from then until the end of the 13th century with Castilians, Leonese and Galicians, with people who were once again cattle breeders.

Monastic repopulation was granted by Alfonso VIII in 1164 and was carried out by the Knights Templar of the houses of Albendiego and Bonaval, who founded small monasteries such as Royal Site of Santuy.

The peace of these lands was only punctuated by the war against the French Napoleonic Empire between 1808 and 1810, when the Macizo de Ayllón was the scene of battles for the control of the mountain passes.

During the Confiscation Law promoted by Juan Álvarez Mendizábal in 1836, the communal properties, which were many in the area, were put up for sale, and thus began the first economic and demographic decline of the Sierra of Ayllón in the Late modern period.

The National Plan of Economic Stabilization of 1959 and its consequent industrial development makes that during the decade of the 60s the region loses almost half of its population by the emigration of this one to the great metropolitan areas in growth, mainly those of Madrid, Barcelona and Bilbao.

Due to the living conditions in a high mountain climate, the population in the area has always been scarce, although nowadays, and for decades, it also suffers from the problem of depopulation.

On the northern slope is more common the "red architecture", so called due to the reddish tone of its constructions based on mixing clay and slate.