Eighteen thousand years is a conservative guess for some of the engravings on the rocks at Siega Verde, and the animals they show are still recognisable: horses, aurochs, deer, ibex and the occasional bison, scratched with flint into the flat grey-brown schist slabs that line the river Agueda. There are hundreds of them, and the open-air gallery runs for about a kilometre along the water. They were made by hunter-gatherers of the Upper Palaeolithic, and they are all outdoors, unprotected by caves, which makes the survival of so much of it remarkable. The site was found only in 1988 by a pair of archaeologists, and it was added to the World Heritage list in 2010 as an extension of the Coa Valley in Portugal, which has a related collection less than a hundred kilometres away.
Visits are by guided walk, and it is worth booking ahead. A guide shows you where to look, and angled light can bring out lines you would walk past. In the early morning or late afternoon, with the sun slanting across the stone, a horse's head or a pair of antlers suddenly appears out of the rock.
The setting is dehesa, the open woodland of holm oak and cork oak that covers much of western Spain, where black Iberian pigs feed on acorns and become the finest ham in the world. The nearest town is Ciudad Rodrigo, about fifteen kilometres away, a small fortified city on a hill above the river with a medieval cathedral, a castle that now houses a parador and a complete circuit of walls that was fought over in the Peninsular War. It has fuel, food and plenty of bars serving tapas. The village of Castillejo de Martin Viejo is closer but tiny.
Riding in this part of Castilla y Leon is relaxed. Roads are quiet and well surfaced, with long straights across the plateau, and the wooded ranges of the Sierra de Francia to the south offer twistier riding and villages of timbered houses, such as La Alberca. Summers are hot and winters are cold and wet, so spring and autumn are best.
Try the local hornazo, a pastry filled with ham and chorizo, and a glass of Rueda or Toro wine.