The officially demarcated area where the grapes of Port are grown starts 100 kms inland from the coastal city of Porto (Oporto) and stretches another 100 Kms along the course of the Douro Valley to the Spanish border. The vines are planted on terraces wrought from the precipitous slopes flanking the beautiful course of the Douro river and its many tributaries. The wine country begins just west of Régua and here it is known as the lower (or Baixo) Douro where grape yields are high and the vineyards produce good fresh relatively early maturing wines. The better quality grapes are found further up in the upper (or Alto) Douro district where there is much lower rainfall and consequently lower yields. These vineyards produce rich dark and very concentrated wines. The best vineyards or "Quintas" are situated principally around Pinhão and the tributary valleys of the Torto, Tua, Pinhão e Távora rivers. These are also smaller pockets of first class vineyards in the remoter upper reaches of the Douro Valley. The Douro climate is harsh with sub-zero winter temperatures contrasting with the unrelenting heat of the summer. The rainfall is only about half that of Porto as the Marão mountain range prevents much of the Atlantic humidity from penetrating further inland. The foothills of the Marão form the western limit of the demarcated area. The combination of the great heat and the low rainfall produces grapes with intense colour, flavour and a high sugar content, all crucial for the prodution of good quality Port. The stony nature of the schistous soil allows the heat of the sun to be retained during the warm nights of the growing season so that the grapes continue to develop even after sunset. The grapes thus achieve exceptionally balanced maturity ensuring Ports of high quality. It may seem an improbable region for the growing of wine, but it is the intrinsic qualities of the soil and climate which give the Douro its unique characteristics for the production of Port.