With an area of 1,534 km2, almost 25,000 inhabitants and an average elevation of 783 meters above sea level, the region contains 37 villages and the city of Dueñas.
Its capitals throughout history have been Castroverde de Cerrato, Palenzuela and, from the late fifteenth century, Baltanás, the largest municipality.
Some sources say that the name comes from the Latin cirratus, which gave the zerrato romance, with the meaning of "land dominated by rolling hills or cerrales".
Along the river channels there is the presence of herons (nycticorax nycticorax), otters (lutra lutra), the Pyrenean desman (galemys pyrenaicus), while in the moors are found the Great Bustard (otis tarda), Little Bustard (tetrax tetrax) and montagu's harrier (circus pygargus), a species listed in the Catálogo Nacional de Especies Amenazadas or "National Catalogue of Endangered Species" (Royal Decree 439/1990, of 30 March).
As for vegetation, the indigenous forest is Mediterranean in nature, made up of oak and holm, adapted to the climate and soil.