Cycles of glacial activity shape the contours and composition of the Médoc through continued erosion and deposition. Massive glaciers sculpt the land as clay sediments are compressed and moved around beneath the immense weight of the ice. Mixed rock fragments and other debris are dragged in the basal ice and deposited as the ice melts.

Pre-Ice Age tectonic upheavals create fissures in the Médoc that cause the Garonne river to change direction - flowing from south to north, instead of east to west. In the interglacial periods that follow, melting ice gives rise to new rivers with strong currents carrying assorted gravel and sand, which is deposited when sea levels drop again. Riverbeds are left as deep clay ravines with gravel deposits on the sides, as their paths gradually shift due to repeated subsidence.

The sea levels fluctuate, periodically rising and falling to the glacial rhythm. The gravel deposits, essentially different kinds of pebbles and sand, are moved around until dropped in their final position. Early periods of intense warming causes fine clay particles to be released from the more fragile pebbles, binding the gravel, thus achieving a more compact clay subsoil that is particularly prevalent in the Grand Enclos.

Proximity to the estuary, and unique proportions of soils and interlocking layers, define the enigmatic character of the Grand Enclos. A compact, clay-rich base, blanketed with white stone galets and thin gravels, creates significant hydric constraint. High quantities of ripe, elegant tannins elevate the wines with rare balance and structure. There is mystery too: the pH of the soil is lower than would be expected for the terroir, bringing freshness that is part of the secret of age-worthiness.