The north of Portugal consists almost exclusively of granite. This extremely hard stone with a thin layer of soil is practically of no use for agriculture. Interestingly, the Douro River also cuts through a massif of slate that happens to extend from Barca d'Alva almost to Régua. This slate frequently splits into vertical layers below the sur­face. This not only allows moisture to seep in, but also gives roots a place to grow (which is impossible in granite). Thus these soil conditions form the natural borders of viticulture in the Douro region: vines thrive as far as the slate reaches. The adjoining granite makes winegrowing impossible.

Slate is hard but brittle and breaks down into ubiquitous yellowish-brown dust in the Douro region and becomes a souvenir on the shoes and car of every visitor to the winegrowing region.

250,000 hectares consist of slate soil, of which 40,000 hectares are now used for viti­culture. Approximately half of these cultivation sites are located on slopes with in­clines of more than 30%. Those steep and stony slopes are difficult and costly to work.