Geology

Following the formation of the massif central and at the same time as the Alps were forming to the east (during the Tertiary era), the Rhone Valley was formed as fault lines running north to south collapsed.

At the end of the Tertiary era, and the beginning of the Quaternary, the erosion of the Alps led to an accumulation of mainly quartzite pebbles, which when cemented and consolidated would form the thick Bonnevaux-L'Amballan formation, ribbons of which can be found on the highest Côte-Rôtie hillsides.

The most recent deposits date from the Quaternary era. These belong to autogenic formations (loess and silt) and other fluvial formations on the hillside terraces, dozens of metres above the current level of the Rhone.

• Nature: metamorphic rock

• Colour: shades of brown.

• Texture: fine grains.

• Structure: Marked cleavage running NE-SW in a subvertical dip to the east (N50 70E). The cleavage contains folds whose decimetric effect can be seen on rocky outcrops, giving an appearance of wavy sheet metal. Sometimes very large, they may also be eroded and break into sheets. Significant veins of quartz centimetres thick often run parallel to the cleavage.

• Patina: dark brown

• Weathering: ferrous rust and manganiferous ore very common.

• The mica are adulterated with chlorites.

• Mineral composition: Quartz, very dark brown to black biotite flecks, very light beige to greyish muscovite.

• Associated soils: within this geological formation, the soil is composed of a very fine matrix of clay and small jagged stones, which make up between 30% (not very stony) and 80% (very stony). Between these two extremes the soil is of an average stoniness (50%). The thickness of the soil varies depending upon its position in relation to the slope: at the base of the hill, small fallen particles form colluvia and produce soil with few pebbles (30%) that is about one metre thick. On the slope, most of the small particles have fallen, and the soil is very stony (80%) and about 40cm thick. High on the hillside, the gentler slopes retain the small particles and the soil is of an average stoniness (50%) and about one metre thick.
• Hydrology: the water in soil lying upon mica schist quickly drains away due to the abundance of stones. The vines planted in this kind of soil may, however, easily find water in the rock itself, due to the many cracks and sub-vertical cleavage.
  

• Formation: metamorphic rock
• Texture: fine grains.
• Colour: very light and white in colour (rust when deteriorated).
• Weathering: the rock degrades into sand. However, most outcrops in Côte-Rôtie are solid.
• Mineral composition: alternating beds of quartz and
• feldspar and beds of white mica (muscovite). The latter are well crystallized and forms uneven leaves, and flecks of biotite and garnet crystals are sometimes found.
• Structure: quite pronounced bedding.
• Soil: this soil is comparable to that which lies atop granite. It is made up of a mixture of rough to fine sand (depending on granularity of bedrock) with the same composition as the bedrock and rounded stones.
• Acidity of soil: acid pH of between 4.5 and 5.5.

• Nature: rock formed by the partial fusion of metamorphic rocks.
• Texture: mixed with pockets of schist or gneiss
• Colour: dark due to the high biotite content.
• Patina: brown-grey.
• Weathering: breaks up into subangular cobbles, up to tens of centimetres in size. The high content of biotite, a highly degradable mineral, facilitates the rock's breakdown into sand. This formation is often found in loose, crumbly outcrops.
• Mineral composition: rich in biotite, quartz,
• feldspar; often with nodules of cordierite.
• Structure: mixed. The orientation of the minerals can be seen, as well as biotite aggregate.
• soil: light brown sand with small centimetric rocks. Generally around 50cm thick.
• Acidity of soil: acid pH of between 4.5 and 5.5.