A wine-growing region rests upon a combination of physical and biological environments and depends on the soil, subsoil, relief, and climate. The subsoil of Champagne is a superposition of layers alternating chalk, sand, and limestone marls. It is favorable to soil drainage and gives a particular minerality to certain Champagne wines. Its chalk is rich in marine microorganisms and mollusks from the Mesozoic Era (252 to 66 million years ago) that make it particularly porous. Rainwater accumulates and constitutes a natural reservoir consisting of 65 to 87 gal per m3 (300 to 400 liters). It allows the vines to benefit from a water supply even during the driest summers.