The Chablis region of Burgundy is classified according to four tiers of Appellation d'origine contrôlée (AOC) designation. The top two are the crus of Chablis and include the 7 Grand cru vineyards followed by the lower Premier crus. Wines made entirely from fruit from these vineyards are entitled to list their wines as cru classe Chablis on the wine label. Below these tiers are the lower designations of basic Chablis AOC and Petit Chablis. Soil and slope plays a major role in delineating the quality differences. Many of the Premier Crus, and all the Grand Crus vineyards, are planted along valley of the Serein river as it flows into the Yonne with the best sites located on a southwest facing slope that receives the maximum amount of sun exposure. All of Chablis' Grand Cru vineyards and many of their better Premier Cru vineyards are planted on primarily Kimmeridgean soil (a composition of limestone, clay and tiny fossilized oyster shells) which is believed to impart more finesse and structure to the wines. Other areas, particularly the vast majority of Petit Chablis vineyards, are planted on slightly younger Portlandian soil.