The fruits of glaciation
on terroir and the geology of wine bloomed during 22 years at Washington State
University in the midst of one of the worlds great wine regions
the Columbia Valley of Washington, nearby areas of Oregon and the Okanagan of
British Columbia. Recently I moved across the country to Smith College in New
England only to find that many of the same fundamental geologic factors, especially
glacial processes, are of pivotal importance to the local wine industry, such
as the emerging vineyards of the Finger Lakes and Long Island in New York (see
story). The glacial history of the vineyards of Washington
provides an excellent illustration of these general processes.
A barge travels across the Columbia River below the Wallula Vineyard in Washington.
Washington State is second only to California in terms of the volume of wine produced in the United States and, some would argue, second to none in terms of wine quality. Although there is considerable local variability, most Washington vineyards sit on soils formed from Quaternary sediments that overlie Miocene basaltic rocks of the Columbia River flood basalt province. The Columbia River Basalt Group erupted mostly between 17 and 11 million years ago (early Miocene) from north-south fissures roughly parallel to the present-day Washington-Idaho border. The estimated volumes of the groups individual flows are at least 3,000 cubic kilometers, making them the largest documented lava flows on Earth.
The basalt bedrock is overlain by unconsolidated sediments deposited by glacial outburst floods and eolian processes. About 18,000 years ago, a lobe of the Cordilleran Ice Sheet blocked the Clark Fork River near the Canadian border in northern Idaho and created glacial Lake Missoula, which covered 7,800 square kilometers of western Montana. The ice dam failed repeatedly, releasing the largest floods documented on the planet.
In south-central Washington State, the many paths of the onrushing floods converged on the Pasco Basin, where floodwaters were slowed by the hydrologic constriction of Wallula Gap before draining out through the Columbia River Gorge to the Pacific Ocean. This constriction caused backflooding of local river valleys and basins that resulted in deposition of relatively fine-grained slackwater sediments, characterized by rhythmically graded bedding; these graded rhythmites locally are called Touchet beds, and researchers have found multiple sets, indicative of multiple floods.
In the last stages of the Pleistocene and continuing through the Holocene, prevailing southwesterly winds eroded slackwater and other glacial sediments and redeposited them into the present thick blanket of loess that covers much of the Columbia Plateau. These windblown soils form the backbone of agriculture in most of eastern Washington: More than 90 percent of Washington State vineyards are planted in appellations affected by the glacial floods and loess. Geologists have made correlations between vineyard performance, wine quality and the underlying soil and rock units in Washington and in other world wine regions, such as France and Italy.
The terra in terroir
As interesting as the bedrock is to geologists, few grapevines can grow in solid
rock, and thus the soils developed on these materials are of immediate interest
to all viticulturists. Soil development is an ongoing process that is the culmination
of hundreds to millions of years of weathering.
Just as maps of bedrock geology are available for most of the United States, maps of soil types are available from the United States Department of Agriculture Soil Conservation Service. Usually organized by county, most maps include detailed aerial photographs or topography with overlays of the main soil series. Each soil series is named after a local town or geographic feature and has a characteristic soil profile. Soil series can be subdivided on the basis of slope, grain size and other characteristics that affect soil use by people.
Collectively, geologic, topographic and soil survey maps provide essential background data for any characterization of terroir. For example, the Ciel du Cheval vineyard in the Red Mountain AVA (American Viticultural Area) in Washington has a relatively low slope with homogeneous air drainage and regional climate. Three different soil types are exposed in the vineyard that cut across the north-south rows of vines. For nine years, Jim Holmes has owned and managed the Ciel du Cheval vineyard. Thus, the vineyard has a consistency of management style that might allow for examination of possible viticultural variations as a reflection of underlying geology. Initial results of grape analyses suggest that there are differences that might correlate with soil types. And an ongoing study of wine sensory analysis over a three-year period has been designed to test whether different soil types can be correlated with statistically significant wine flavor profiles and compositions. Previously, researchers have correlated such sensory differences with a variety of terroir variables.
Climate's starring role
Climate is one of the more important components of terroir. In some ways it
is the most difficult to evaluate because it varies in both space and time.
Three different scales delineate the many weather variables: macroclimate, mesoclimate
On a continental to regional scale, macroclimate controls the length of the growing season and other long-term trends and extremes. On a regional to vineyard scale, mesoclimate encompasses topography, elevation, slope, aspect and proximity to bodies of water or other moderating influences. Microclimate ranges from the scale of a vineyard down to individual vines, grape clusters and even smaller domains if measurement permits. While macroclimate can vary on a geologic time scale (millions of years), both mesoclimate and microclimate can vary seasonally, daily or even hourly. Human activities, such as urban development, wind machines, irrigation and canopy management, affect mesoclimate and especially microclimate.
Although many climatic variables can be measured, four of the important factors are temperature, humidity, wind and sunlight (solar radiation). These data and others are collected systematically by a variety of meteorological services and are often automated and publicly available. For several regions of Washington, such climate data show that Red Mountain has on average 200 more growing-degree days (a measure of effective photosynthesis) than nearby Yakima Valley, and Walla Walla has more than twice the precipitation of Red Mountain.
Gravel and ice
regional comparisons are possible using other types of terroir data. For example,
more than 90 percent of Washington vineyards are located in areas affected by
glacial outburst floods thousands of years ago. In the Red Mountain area, these
flood sediments were mostly deposited from the swirling back-eddies behind Red
Mountain and include numerous lenses of relatively coarse gravel. In the Walla
Walla area, the flood sediments are generally finer grained due to deposition
from ponded floodwaters, although there are some zones of coarse gravels in
modern river channels. Many other wine-producing areas of the world also have
links to glaciation. This is primarily due to two factors: worldwide lowering
of sea level during maximum glaciation and the locally abundant sediments produced
Winemaker Gary Figgins of Leonetti Cellars holds a granite glacial erratic found in the Seven Hills Vineyard, Walla Walla, Washington. Glaciation is important to winemaking, largely in providing sediments conducive to grape growing.
A prime example of these processes is the Graves-Medoc region of Bordeaux, France. Outwash gravels from glaciation in the Pyrenees Mountains along the French-Spanish border and the Massif Central in central France overloaded the Garonne and Dordogne rivers leading to the Gironde Estuary, which itself had been enlarged and deepened by the lowering of sea level. Each period of glaciation produced its own series of gravel outwash floodplains along the rivers and the best (so-called First Growth) vineyards are all on the same type of gravel. Names such as Chateau Lafite Rothschild, Haut-Brion, Latour and Mouton-Rothschild are well-known to wine lovers throughout the world, and each of these estate vineyards is located on these gravel mounds, particularly the stratigraphic unit of the Günz gravel.
Less well-known are the gravel outwash plains of the South Island of New Zealand, which were fed by the extensive alpine glaciation of the Southern Alps mountain range that transects southern New Zealand. These gravels form the substrate for many of the vineyards in the Marlborough area of New Zealand, and some of the wineries of this region focus on the coarse gravels for their best vineyards. Another area in New Zealand, Gimblett Gravels, is perhaps the first viticultural region in the world to specifically define itself on the basis of the gravel. Legally, wines from this appellation have to consist of at least 95 percent grapes grown on the Gimblett Gravels.
Gimbletts wines owe their unique flavor not only to their gravelly existence, but also, like all wines whether in Washington, France or elsewhere to large differences in climate, culture and viticultural practice. The study of terroir does not aim to minimize the different factors or the importance of human ingenuity in making great wine from great vineyards, but simply to illustrate the importance of understanding the physical environment as one essential element of terroir a topic that should be of interest to all geologists.
the beaten trail
For 30 years, petroleum geologist Bill Gipson has followed his curiosity
about Earth to an unlikely spot: winemaking. He owns the Pheasant Ridge
Winery, just 10 miles north of Lubbock, Texas. Wine in Texas? You bet.
In fact, people have been growing wine grapes in Texas since the late
1800s, when the Val Verde Winery (still in operation today) started up
in Del Rio, west of San Antonio on the Rio Grande. Now more than 50 wineries
strong, Texan wines place in national competitions and are grown in several
areas of the state.
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