The soils on the Right Bank of Bordeaux are dominated by clay and limestone. The terroir and soil varies from château to château, the best vineyards have soils with limestone, clay, gravel, sand and iron deposits with some elevations, slopes and good drainage. Clay soils consists of very small particles from degraded limestone which have the ability to retain water and to allow the vines easily penetrate the soil, searching for nutrients. Merlot is the most important grape in the region, due to the clay rich terroir. Petrus is worldwide famous for its blue clay soils.