Driving north from the city of Bordeaux, the first area you reach is the southern section of the Médoc. It is called the Haut-Médoc, or the "Upper Médoc," because the elevation here is slightly higher than elsewhere in the Médoc, which is barely above sea level.
Since cold air drains down off higher elevations, the Haut-Médoc's ever-so-slight elevation means that its vineyards are warmer. The area's gravel soils also warm the vineyards by retaining heat and providing good water drainage.
This combination of elevation and gravel soils helps Cabernet Sauvignon ripen.
Common wisdom maintains that the best vineyards are within sight of the river. Their wines tend to have more finesse.