Tuesday, September 2, 2014

Northern Rhone Rocks: Soils of the sub-region

The Rhone Valley vineyard architecture has been shaped by tectonic, hydrologic, and wind forces applied to the evolving landscape over eons, as described in a prior post. The result has been two distinctly different vineyard settings: (i) vineyards on the edge of faults (Northern Rhone) and (ii) vineyards set in sedimentary basins (Southern Rhone). We address the Northern Rhone vineyards in this post based on the work of Jacques Fanet (Great Wine Terroirs).

The aftershock associated with the Alpine folding (started in the Upper Cretaceous and reached a crescendo in the Miocene) caused an uplift in the eastern edge of the Massif Central with implications for Burgundy, Beaujolais and the Rhone Valley. In the Rhone Valley, the uplift was non-uniform. Rather, it occurred in fault-delimited tranches that front on the river's northern course. One such fault is responsible for the river's rightward jog at Vienne prior to returning to its "normal" southerly course in the area of Condrieu.

Bordering the river for its entire northern course, and varying in its width, is a band of alluvial soil which has been continuously brought downriver from the Alps. Most of the riverbank towns in the Northern Rhone are built on beds of this alluvia.

With the exception of the Croze-Hermitage and Hermitage AOCs, all of the Northern Rhone appellations are located on the right-hand-side of the river, clinging precariously to the steep metamorphic or granitic slopes of the Massif Central.

The Côte-Rotie is the first appellation encountered on the river's southward trek and its 60º, terraced slopes are dark brown, weathered as they are from Mica bedrock. The soils in the south of the appellation are thin, lighter in color, and weathered from Gneiss bedrock. The southwest jog of the Rhône between Vienne and Condrieu provides excellent southeastern exposure for vines. Syrah thrives here.

Beginning at Condrieu AOC, the right bank is granitic rock capped by loess. These granitic scarps give way to metamorphic rocks in the Saint-Joseph appellation opposite the confluence of the Dolon River. The north-south flow of the river in this zone provides an easterly exposure on the cliff face, undesirable for grape-growing in this region. Instead, a series of northeast-to-southwest valleys provide south, southeast, and east exposures and are exploited for vineyard plantings.

The Saint-Joseph appellation is granitic to the north, gives way to metamorphic rock just north of the St. Vallier cut, and then reverts to granite opposite Tain-l'Hermitage.

From Vienne to Saint-Vallier, the left bank of the river still reflects the multi-level terraces formed as the river recut its banks in the migration to its current course. In the places where best preserved, a total of four terrace levels are on display. Beginning at Saint-Vallier, the Rhone cut its way through the metamorphic rocks and basement granite, providing right-bank-style soils on the left bank between Saint-Vallier and Tain-l'Hermitage. The Hermitage appellation encompasees the granite of the Hill of Hermitage plus some of the upper-terrace strata. The Crozes-Hermitage appellation is inclusive of the Hermitage soils as well as the Miocene and lower-terrace soils. Terrace soils are primarily clay-limestone from the Tertiary period.

For the remainder of the river's Northern Valley course, the left bank displays lower terrace strata and one outcropping of upper-terrace strata overlaying Miocene deposits in the upper reaches of the Isere tributary. The river continues to hug the right bank along the Massif through the remainder of the Saint-Joseph appellation.

Cornas AOC is also a granitic appellation but is shelterted from the north winds by a Jurassic limestone outcrop called Les Arlettes. The river and the granitic scarp begin to diverge in the vicinity of Les Arlettes with the Massif trending southwest and the river to the southeast. The space thus created is occupied by lower-terrace strata.

Saint-Péray AOC is the southernmost of the Northern Rhone appellations. The Massif Central intrudes into the appellation in its northwest quadrant and combines with a similarly oriented Jurassic limestone outcrop -- Crussol -- to hem in a band of Miocene-Pliocene strata. The Saint-Péray soil is a complex mix of limestone, clay-limestone, and granite which owes its composition to a number of donors (rhone-wines.com): (i) granite from the Primary Period contributes a hint of silica; (ii) Jurassic limestones from the Secondary Era; (iii) marine deposits from the Tertiary Period are the source of today's clay-limestone soils; (iv) a veneer of loess from the Quaternary Period and Major Glaciations; and (v) alluvial deposits carried down from the Alps by the Rhône River. The vineyards themselves extend for 75 ha on the gentle lower terrace slopes at the foot of the Crussol Hill with south and southeast exposures. Marsanne and Roussanne are the allowed grape varieties in the appellation.

As the Massif Central rapidly exits stage right, space is created wherein Vocontian Trough and sedimentary basin geology dominate. This is the realm of the Southern Rhone. Its geology will be revealed in a subsequent post.

©Wine -- Mise en abyme

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