Valderejo forms the western closure of the Lahoz-Sobrón anticlinal fold. Erosion has left the core of the anticline uncovered, and it currently constitutes a wide valley surrounded by limestone rock faces. The highest are:.
- Recuenco (1.239 mts.)
- Vallegrull (1.226 mts)
- Santa Ana (1.042 mts.) .
These correspond to the flanks of the anticline which, centuries ago, formed part of the same structure.
The area enjoys a climate in transition between the Atlantic and Mediterranean environments, with hot, dry summers and winters in which there are heavy snowfalls. These conditions result in a wide biodiversity, a great wealth of both flora and fauna due to the existence of species that are typical of both these environments.
The main river in the Reserve is the Purón, whose source lies to the north of Lahoz. It runs north-south before flowing into the river Ebro after crossing parts of Burgos province. The Ampo and Polledo are its chief inflows.
Woodland constitutes almost 60% of the total green area. Most of this is pinewood, followed by holm (evergreen) oak, beech and a small area where gall-oak predominates.
The diversity of ecosystems and scant human presence has encouraged a rich and varied fauna.
Particularly worthy of mention are large mammals such as roe deer and wild boar, as well as an interesting community of medium-sized carnivores.
The sheer rock faces are nested in by a sizeable colony of Griffon vultures and other rock breeding birds, while woodland birds in general are also plentiful.