Saint-Estèphe is best known as the northernmost, and one of the four major wine-growing appellations of the Médoc. The area is separated from Château Lafite-Rothschild in Pauillac to the south by the Jalle de Breuil stream and its vineyards cover around 12 km2 (4.6 sq mi) on the banks of the Gironde estuary. The grounds of St.-Estèphe are varied. To the south-east corner of the appellation area, they are similar to those of Pauillac, with deep gravels lying on a hard sandstone base. Progressively to the west and north, they get more clayey, less gravelly, and become heavier. In parts, veins of alios (a reduced form of iron) can be found. The layers of gravel on top of clay lead to comparatively poor drainage, and the harvest is one of the latest of the whole region, although this water retention can be of use in vintages of low rainfall.