Sauternes soil: the contribution of geology

Sauternes was predestined to be a wine-producing area. This can partly be explained by the geological composition of the two, very different areas which make up the region.
On the right bank of the Ciron river, a tributary of the Garonne, the villages of Preignac, Fargues, Sauternes and Bommes, which constitute the Sauternes appellation, lie on an east-facing plateau. The substratum of this plateau is shell limestone, marl or argillaceous sand, all dating from the tertiary era. During the quatenary ice ages,  this substratum was covered with enormous layers of gravel carried by the Garonne river or rather the Garonnes, since over the years the river has successively occupied several beds, moving from west to east. Recent scientific findings have indicated that the Garonne used to be a network of meandering channels, hence the creation of the vast fluvial terraces on the rectilinear banks and the deposits on the concave sides of the river or along the river bed. As the climate warmed, the glaciers melted and the sea level rose, slowing down the current of the river and allowing alluviae to settle. When the next ice age arrived, the sea level fell, the river flowed more rapidly and fresh new beds were uncovered. This explains the layering of the terraces and their geological complexity. The highest and oldest terraces lie to the west, the lowest to the east.

The Garonne and its tributaries transported moraine from the Pyrenees and Massif Central to build up the layers, several metres thick, of Sauternes gravel. The biggest rocks that can be found, admittedly quite rare, are up to one metre long. Geologists suggest that they were carried along frozen within enormous slabs of ice. In general, this gravel is formed of ovoid pebbles of varying sizes – from one to several centimeters - mixed with a limestone or argillaceous matrix, or more often sand. White and pink quartz, black Lydiens and green sandstone from the Pyrenean mountain range, conglomerate from the Albigeois and even basalt and volcanic rock from the Montagne Noire can all be found. The (rock) solid proof of the variety of soil in the region
can be seen in the surprising mineralogical collection at Château de Rayne-Vigneau.
Erosion has gradually transformed the gravel into gently sloping hillsides, 15 to 60 metres high, which are very characteristic of the Sauternes landscape. Well-drained by the Ciron and local streams, the white, pebbly soil absorbs sunshine.