54 Port anD the Douro
virtually unworkable. Vines, which need to root deeply in order to flourish, are impeded
once they reach the impregnable granite. On the planaltos or high plateaux to the north and
south of the River Douro, growers can do little more than scratch a living from the land.
The geology of the upper part of the Douro valley stands out from the remainder
of northern Portugal. The greater part of the region belongs to a geological formation
known as the pre-Ordovician Schist-Greywacke Complex or merely ‘schist’ for short.
Unlike the surrounding granite, this foliated, slate-like rock can be worked into a coarse
soil. Lumps of schist on the surface shine almost like polished steel in strong sunlight,
reflecting and absorbing heat according to the angle of the sun’s rays and reducing erosion
during heavy winter downpours. Over time the schist weathers into a fine silt-like dust,
clouds of which billow up from trails and tracks, covering cars during the dry summer
months. The Douro’s soils tend to be fairly acid (pH 4.6–5.5), high in potassium but low
in calcium and magnesium. This causes a condition known locally as moromba (curling
and drying of the leaves during the growing season). Originally thought to be a virus,
this is now successfully treated by applying borax to the soil. The proportion of organic
material in the Douro’s schistous soils is low (typically less than 1.5 per cent) and regular
application of both organic and inorganic fertilisers is necessary to correct the imbalance
of nutrients.
Below the topsoil the schist tends to fracture vertically, facilitating both the infiltration
of water and the penetration of roots to depths far beyond those possible with granite.
It is not surprising that to those farming the Douro schist is almost a religion, and it is
not uncommon to come across an outlying chapel dedicated to São Xisto (‘Saint Schist’).
Here and there huge outcrops of rugged granite pierce the schist, notably from the town
of Carrazeda de Ansiães to the Douro at Cachão de Valeira, which marks the natural
boundary of two of the Douro’s three sub-regions (see below). There is not much you can
cultivate in these intractable soils.
The boundary of the demarcated Port region has been modified a number of times
since it was first established in the mid-eighteenth century and now it mostly follows the
outline of the schist. Travelling up river from the west, the region starts at Barqueiros, 75
kilometres inland from Oporto, and stretches eastwards to Barca d’Alva on the frontier
with Spain, 160 kilometres as the crow flies from the coast. It encompasses a total area
of 250,000 hectares, of which just over 45,000 hectares – representing 18 per cent of the
total – are planted with vines. This makes the Douro region the largest area of mountain
vineyard in the world.
Forming a deep cleft in the rock, the River Douro runs from east to west and forms
the natural axis for the region. The river itself, progressively dammed to form a series of
placid finger lakes, flows at an altitude of between 60 and 140 metres above sea level. The
terrain either side of the Douro is irregular in the extreme, with deeply incised tributaries
draining the mountains to the north and south. These rise in places to over 1,000 metres
on the margins of the region. With half the region’s vineyards planted on slopes with a
gradient in excess of 30 per cent, there are few wine regions that are so arduous and costly
to cultivate as the Douro.