This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
The wine country proper is along the Lower Douro - a stream called the Corgo separating the Lower from the Upper Douro. Along each bank, for the distance of nearly thirty miles, and varying from six to eight miles wide, are the vineyards. Limestone rock, clayey slate, and red clayey soils, compose the formation -of the hills or mountains, sometimes so precipitous that it is no very easy task to climb them, as their elevation is many hundred feet above the river, with the vines planted frequently to their very summits. Every hill is cut into innumerable terraces, the walls forming their sides being about five to six feet high, and composed of large stones - for any slight structure would quickly be washed away.
The situations regarded as best calculated to produce the richest grapes are those on the sides of the hills facing the south, and which enjoy the greatest quantity of sun; the lowest and most sheltered spots being in greatest esteem. The grapes growing near the summits of the mountains are more watery, and form a lighter and thinner wine. The labor and expense incurred in the formation of these vineyards may be imagined.