GEOLOGY OF VINEYARDS IN THE WILLAMETTE VALLEY, OREGON
Corvallis lies near the center of the Willamette Valley, one of the world's premier grape growing and winemaking regions. The word Willamette (pronounced wil-LAM-et) means spilled water in the Kalapooya Indian language, and it originally referred to the Willamette Falls at Oregon City. The Willamette Valley Appellation is the largest of Oregon's six wine-growing regions. The others are the Umpqua, Rogue, and Applegate Valleys, all wholly within Oregon, and the Columbia Valley and Walla Walla, shared with the state of Washington.
The first grapevines in Oregon were planted in the 1850s, soon after the Oregon Trail was opened. The modern winemaking industry, however, dates from a much later time--from the period after World War II. By then, two factors led to the industry's rapid and efficient expansion. First, 2 centuries of grape growing in France had shown by trial and error which grape varieties in which microclimates make the best wine. And second, during the 1940s, Maynard Amerine of the University of California at Davis developed a climatic formula by which a potential wine-growing area could be compared with a matching area in France, so that its grape varieties could be adopted.
A free booklet Vintage Oregonhas been published by the Oregon Wine Advisory Board. It includes thumbnail roadmaps to the state's winerys and is available at chambers of commerce and information centers. The AAA county maps also spot the wineries and are excellent for winecountry touring.
CLIMATE AND GRAPES
The grape Vitisis a cool-climate genus that grows between latitudes 30 and 50 degrees. In Europe, a single species is native, Vitis viniferaLinné, but it has more than 700 varieties (4000, including crosses and clones). These constitute the world's wine grapes. Vitis viniferagenerally cannot tolerate temperatures below -15 degrees C (5 degrees F).
The genus Vitisalso has about 15 additional species in Asia and 20 in America, including the Concord grape, Vitis labruscaLinné. These other species make wine that is generally considered to be too grapey, but American grapes provide the grafted rootstocks for most commercial wine grapes worldwide.
Vitis viniferais believed to have originated in what is now the Republic of Georgia. Extensive cultivation of it occurred throughout the Roman Empire, including in France. But societal breakdown during the Middle (Dark) Ages partially disrupted the previously well-organized wine industry. It finally became established in its present form in France about 1720. During the intervening period, the grapes had continued living in the wild, where natural selection based on the various local environments across Europe produced the varieties that we know today.
During the period when the wild varieties of European grapes were evolving, birds were a principal agent of grape natural selection. Grapes are adapted to be sour and unpalatable while their seeds are not yet mature. During a brief period in the fall, just before the first local frost, the seeds mature, and the grapes abruptly turn sweet. The birds gorged on them and dispersed the seeds, producing the tight coupling between grape varieties and climatic zones. In the Willamette Valley today, propane cannons, nets, and synthetic bird distress calls repell the robins during their fall migration.
Genetic studies of the DNA of the grape varieties produced a surprise when researchers discovered that Cabernet Sauvignon, the premier variety in Bordeaux and Napa Valley, is a wild cross between Cabernet Franc and Souvignon Blanc. Pinot Noir and the other members of its heterozygous series, Pinot Gris and Pinot Blanc, is older than most other varietals. It originated in the Burgundy-Rhine region, probably before the Roman period. Wild crosses between it and Gouais Blanc, which the Romans brought from Croatia in AD 280, produced Chardonay, Gamay Noir, and several other varietals (Bowers and others, 1999).
The European grape varieties were planted around the world in suitable climatic zones during the 1800s, where at first they flourished. In 1858 and 1862, however, American Vitisspecies were sent as botanical specimens to France. They carried the yellow aphid-like root louse Phylloxera vastatrix.Soon the French grapevines were dying, and after that all French varietals around the world began to succumb. The American Vitisspecies are resistant to Phylloxera.But like the European diseases that came the other way across the Atlantic to decimate the Native Americans, Phylloxerawas devastating to the nonresistant Vitis vinifera.
Thomas Munson from Denison, Texas, an expert on American grapes, suggested a solution: Graft European vines onto American roots (Munson, 1909). That soon was done, although some grape varieties had been so thoroughly annihilated in France that restocking had to come from surviving scions in the United States. Today, several American species are used as rootstocks in France to provide both Phylloxeraresistance and adaptability to France's generally limy soils. In the Willamette Valley, with basalt and tuffaceous sandstone soils, about 20 percent of the rootstocks come from the American species Vitis riparia,native to the northeastern United States, and about 70 percent consist of several clones of hybrids between V. ripariaand V. rupestris,native to creek beds in Arkansas, Missouri, and Tennessee.
About 60 percent of vineyards in Oregon still grow V. vinifera varietals on their own roots, and they must scrupulously prevent contaminated material from coming in. Current plantings are generally of grafted stock. Once an infestation begins, about 4 years are available to bring grafted stock to production, at first commonly side-by-side with the old plants.
Maynard Amerine's climatic formula consists of a unit called degree days (Amerine and Winkler, 1944). Degree days are the number of hours annually between May 15 and October 15 when the temperature exceeds 10 degrees C (50 degrees F). Typical values average about 4,000 for Fresno, California, and 3,000 for both Bordeaux, France, and Napa Valley, California. Burgundy, France, and the Willamette Valley, Oregon, have a degree days average of 2,000, which explains why Willamette Valley winemakers mainly produce the principal grapes of Burgundy: Pinot Noir, Chardonay, and Pinot Gris.
The 45th Parallel passes through both France and Oregon. The winds in the North Atlantic and North Pacific, the Westerlies, drive oceanic currents toward France and Oregon. The narrowness of the North Atlantic, however, causes its current to produce warmer coastal temperatures in France than the wide North Pacific does in Oregon. Hence the degree days in coastal Bordeaux at latitude 45 degrees match those farther south in Napa Valley at latitude 38 degrees (Howell and Swinchatt, 2000). The Westerlies continue toward the east across both Europe and North America where they gain continental influence that eventually produces winter temperatures too cold for the tender Vitus vinifera.
An intermediate level of rainfall is an additional factor in grapevine growth. The intervening Coast Range limits rainfall in the Willamette Valley, which lies 50 km (30 miles) from the Pacific Ocean. The Massif Central controls the rainfall in Burgundy, 500 km (300 miles) from the Atlantic Ocean. The two places differ, however, in that Burgundy has more rainfall during the summer. Burgundy's greater distance inland adds a continental influence that compensates for the warmer starting temperature at the Atlantic Coast.
The Willamette Valley and Burgundy differ in that Burgundy has more rainfall during the summer. Both places are subject to the annual climatic variations that can lead to either great or not-so-great vintages. In the Willamette Valley, a cool summer can slow the ripening, and early rains can do the same, as well as diluting the fruit flavor and sometimes producing Botrytisbunch rot. These year-to-year climatic variations contribute to the diversity of the resulting wines.
The Earth's crust in western Oregon and the Willamette Valley is unusual. A borehole drilled there would pass directly from rocks of the Cenozoic Era (the age of mammals) into the Earth's mantle below the crust. It would not intersect any rocks of the Mesozoic Era (the age of reptiles) or the Paleozoic Era (the age of invertebrates). Western Oregon was created as "new ground" on the surface of the Earth soon after the dinosaurs had died out.
The area of the Willamette Valley was originally underlain by old rocks similar to those in the Klamath Mountains of southern Oregon. About 58 million years ago, however, northward drag from an offshore tectonic plate tore western Oregon and Washington away from the Klamath Mountains and transported them to southern Alaska (Moore, 1984).
The moving fragment resembled today's Baja California Peninsula. As it moved northward, it left behind new seafloor analogous to the floor of the present Gulf of California. The basaltic seafloor melted out of the underlying Earth's mantle as the seafloor stretched. That basalt, the Siletz River Volcanics, since then has been slowly covered by the various rock layers that are important to Oregon's vineyards (Yeats and others, 1996).
The hole in the ocean floor off Oregon initially had a depth of 2,500 meters (8,000 feet). Marine sandstone and shale eroded from the remaining land began to fill it in rapidly. The first deposits at the Willamette Valley, which belong to the Eocene Epoch (55 to 38 million years ago), started with the Tyee Formation and ended with the Spencer Formation. These were followed by sedimentary rocks of the Oligocene Epoch (38 to 24 million years ago), such as the Eugene Formation, and then of the Miocene Epoch (24 to 5 million years ago), such as the Scotts Mills Formation. These marine formations in places contain shells of fossil clams and snails.
By Oligocene and Miocene time, the hole left by the northward rifting of the original western Oregon had been largely filled. Some of the marine sandstone and shale then interfingered with lava from the Cascade volcanoes on the east side of the Willamette Valley and with volcanic ash that made tuff which contains fossil leaves.
About 15 million years ago, during the Miocene Epoch, a new wave of basaltic lava began erupting from vents in far eastern Washington and Oregon. This highly fluid lava flowed as sheets across Washington and Oregon toward the sea, including across the northern half of the Willamette Valley. Solidified crusts on its surface retained the heat until the lava reached the sea. It flowed between the spaced-out Cascade volcanoes and down the valley of the ancestral Columbia River. The Coast Range had not yet been raised as a barrier to the flow of the lava, so it moved all the way to the shoreline and beyond. It left behind the Columbia River Basalt Group, the bedrock of the red hills that now bear vineyards in the northern Willamette Valley.
A great thickness of sedimentary and volcanic rocks by then overlay the basement of the Siletz River Volcanics. All the while, the floor of the Pacific Ocean was sliding toward the east down under western Oregon at the Cascadia Subduction Zone. The decending slab fed water to the roots of the Cascade volcanoes, which fluxed and melted the hot mantle material there. Some of the erupting melt fed back as volcanic ash to the sedimentary rocks of the valley.
But this well-oiled system had a stoppage about 5 million years ago, during the Pliocene Epoch. While the tectonic plate from the Pacific was moving down into the lower mantle underneath the Cascade volcanoes, North America had been moving toward it at about 20 km (12 miles) per million years. In the 50 million years since the Siletz River basement had been emplaced, North America had overridden the West Coast subduction zone by about 1000 km (600 miles).
Abrupt changes then occurred. The plate boundary moved inland at California to the San Andreas Fault, Baja California began to pull away from mainland Mexico, and the Cascade volcanoes jumped 50 km (30 miles) eastward from the old Western Cascades to the presently active High Cascades. The ultimate cause of the changes in western Oregon was that the subducting slab abandoned its old alignment and reinserted itself in a more stable position farther west.
The slab has a thickness of 100 km (60 miles). As its butt end reentered the Earth's mantle, it disrupted the edge of the continental crust. The formerly smooth sedimentary basin that extended seaward from the volcanic arc was pushed upward to create Oregon's Coast Range. Erosion of this new mountain range over the 5 million years since then has cut down to where even the Siletz River Volcanics now appear at the surface in the hills.
A final geologic event, and an unusual one, was next to affect Oregon's future wine industry. Although the Ice Age glaciers never reached the floor of the Willamette Valley, a distant glacier had a profound effect on it. A lobe of the Canadian Ice Sheet dammed a river in Idaho and created the large ancient Lake Missoula. After about 60 years, Lake Missoula filled to a level where it floated its ice dam. The dam broke, and a huge flood swept across the Channeled Scablands of eastern Washington and brim-full down the Columbia River Gorge. The water then swirled into and filled the Willamette Valley to a depth of about 100 meters (330 feet) (O'Connor and others, 2001).
The water drained out of the valley over about 1 week, and it left behind a layer of silt 10 centimeters (4 inches) thick. Meanwhile, the glacier readvanced and created a new ice dam and a new Lake Missoula. Again, after 60 years, the dam rebroke, the Willamette Valley was refilled with water, and another layer of silt was deposited. The cycle repeated itself about 40 times from 15,300 to 12,700 years ago. Today the Willamette Silt is about 10 meters (30 feet) thick. It covers the lower slopes of many vineyards to an altitude of 100 meters (330 feet) and creates the rich farmland of the valley floor.
The soil name for the Willamette Silt is the Woodburn Series, named after a town in the valley. It is rich in nutrients, hence grows wine grapes very well. Their flavor, however, is less complex than that of grapes grown on the soils of the bedrock slopes. Vinyards blessed with both types of soil can often achieve the best of both worlds by blending their wines. Typical wineries whose estate vineyards incorporate some grapes grown on the Woodburn Series are Duckpond Cellars, Houer of the Dauen, and Tyee Vineyards.
The unique origin of the Willamette Silt produced an almost instant soil. But the 13,000 years since the silt was deposited has made a true soil. Its A horizons at the top, where some clay has been leached downward, are 40 cm (17 inches) thick. The B horizons, enriched in clay, extend from 40 to 140 cm (17 to 54 inches). Below that, the C horizon consists of the parent silt. These soils of the Woodburn Series are very dark brown (wet), brown (dry), and have a moderately acidic surface pH of 5.9.
Soils are defined first by climate, second by slope angle, third by soil thickness, and forth by parent material. All the Willamette Valley soils have the same climate, which consists of about 1000 millimeters (40 inches) of precipitation, an average annual air temperature of 12 degrees C (53 degrees F), and a frost-free season of about 180 days. Altitude of most vineyards ranges from 90 to 200 meters (300 to 700 feet), because a thermal inversion warms the air within that altitude range. The Woodburn Series has a slope of 0 to 1 degrees, and during some years its lower reaches below the thermal inversion can be subject to vine-killing temperatures from the topographic drainage of cold air.
Most vineyards on hard bedrock slope from 1 to 7 degrees, values that result in good drainage but are gentle enough to provide resistance to soil erosion. For reasons partly of proximity to the state's population center at Portland, many vineyards lie on soils of the Columbia River Basalt Group, because it makes nearby hills of the requisite slopes. Their soils belong to the Jory Series, named after a basaltic prominence in the Salem Hills. Soils of the Jory Series are dark reddish brown (wet), reddish brown (dry), 150 cm (60 inches) to bedrock, well drained, and have a surface pH of 5.6. Their thickness indicates that they are at least 1/2 million years old.
Oregon's analog of Burgundy's Cote d'Or (Slope of Gold) is the southeast-facing slope of the Dundee Hills. Several vineyards that produce Pinot Noir and other varietals on the southeast front of the hills are Archery Summit, Cameron Winery, and Sokol Blosser Winery.
Uplift of the Coast Range 5 million years ago folded the rocks of the Columbia River Basalt Group. In the Dundee Hills, three individual lava flows dip toward the southeast. Erosion has etched the outcropping edges of these flows into ridges somewhat like those on weathered wood. The erosion produced excellent higher level vineyard sites back from the front of the hills, such as the Prince Hill Vineyard of Erath Vineyards.
Vineyard soils on tuffaceous sandstone and shale in the Willamette Valley commonly consist of the Willakenzie Series. Its name comes from the confluence of the Willamette and McKenzie Rivers north of Eugene, and typical parent materials consist of the Eugene and Spencer Formations. Soils of the Willakenzie Series are dark brown (wet), brown (dry), 80 cm (30 inches) to bedrock, well drained, and have a surface pH of 5.9. Typical wineries with vineyards on the soils of the Willakenzie Series are Airlie Winery, Elk Cove Vineyards, and Willakenzie Estate.
The Nestucca Formation of Eocene age consists of tuffaceous sandstone containing beds of basalt formed before the Western Cascades took their final alignment. Coleman Vineyards, on the Nestucca Formation near McMinnvile, has soils that range from those of sandstone to basalt, and a bed of pillow lava crops out on the road leading to the winery.
Several other soil series grade into the principal soils that support vineyards on bedrock slopes in the Willamette Valley. On basalt, the Yamhill Series is thinner to bedrock (100 cm; 38 inches) than the Jory, and its B horizons contain less clay introduced from above. The Nekia Series is still thinner (75 cm; 30 inches) and contains more fragments of basaltic colluvium. On tuffaceous sandstone, the Bellpine Series overlaps with the Willakenzie Series at higher altitudes and extends up to 1000 meters (3000 feet). Slopes carrying it range up to 35 degrees, the depth to bedrock averages 60 cm (33 inches), and sandstone colluvium fragments make up a higher precentage of the soil than with the silty Willakenzie. The Amity Series represents poorly drained parts of the Woodburn Series (on the Willamette Silt). It has a conspicuous light-colored leached layer (the E horizon) between the A and B horizons.
An additional parent material that resembles the Willamette Silt, but which is older and has a more mature soil, is wind-blown silt in the northern part of the valley. This loess, older than the young loess of the Portland Hills Silt, was produced from dust blown onto the hills from glacial outwash during the early part of the Pleistocene Epoch (Scott F. Burns, personal communication, 2002). It contains 1-centimeter (1/2-inch) pisolites (spherical cemented bodies), which attest to the oldness of the unit. Tualatin Estate Vineyards contain about 3/4 meter (2 feet) of this material over an old Jory soil, and Shafer Vineyard Cellars has several meters of it over the Pittsburg Bluff Formation (Willakenzie Series). The vines likely tap the bedrock, but the nutrient level is slightly elevated in the old loess, and the plants require some leaf pulling and grape-cluster pruning.
Tuffaceous siltstone of the Spencer Formation underlies Bellfountain Cellars, southwest of Corvallis. The vineyard soil, mapped as the Bellpine (Willakenzie) Series, in places is redder than usual. Basaltic intrusions crop out near the vineyard, and thin sills of basalt cut the Spencer Formation there. Because the vineyard is planted in a south-facing basin, it can ripen Cabernet Sauvignon as well as the Burgundian varieties.
Reed Glasmann of Oregon State University has analyzed the clay mineralogy of several Willamette Valley vineyards. For the present study, he analyzed Jory soil from Prince Hill Vineyard (Erath Vineyards) and Willakenzie soil from Willakenzie Estates. The main difference between soils on basalt and on tuffaceous sandstone is that the clay-sized fraction over the sandstone contains quartz and that over the basalt does not. On both rock types, the clay minerals consist of halloysite and gibbsite, and the red to yellow iron oxide coloring agents consist of hematite and goethite.
Native vegetation on both the basalt and tuffaceous sandstone of the Willamette Valley consists of Douglas fir, Oregon white oak, wild rose, poison oak, snowberry, and bracken fern. Native vegetation on the Willamette Silt consists of grass and widely spaced Oregon white oak.
Two alternative methods are used in the Willamette vineyards to prevent erosion and to retain moisture between the rows of grapes. In the first method, the previous winter's cover crop is lightly tilled in the spring to produce a water-retaining mulch, then red clover and oats are replanted near grape harvest time to prevent erosion from the winter rains. In the second method, mixed natural vegetation consisting of up to 11 species is allowed to grow continuously between the vines, where it is mowed and resembles a wildflower-studded lawn.
During grapevine pruning in most vineyards, two strong canes produced during the preceding year are tied to supports and cut back to about ten buds on each. The other annual growth is removed. The buds begin to leaf out by early April, and tiny flowers, which are self pollenating, appear in June. Leaves are pulled during the growing season, chiefly to provide mildew-preventing air circulation but also to let the sun strike the grapes. This mild "suntanning' can alter their flavor, while the main leaf canopy cools them.
The Scott Henry trellis system, developed in Oregon, is now increasingly used around the world (Henry, 1992). Two levels of two canes each provide two lines of grape clusters, one above the other. A gap between the lines gives excellent air circulation and protection from powderly mildew.
Vineyards commonly grow several clones of each grape variety. Also, different fermentation yeasts may be used to vary the effects from the different grape clones, from the various soils within the vineyard, and from the different ripening conditions in diffferent years. The resulting wines are barrelled separately, and they then are blended before bottling in proportions that give the best flavor.
White grapes are generally crushed as whole clusters, and the juice is removed from the skins before fermenting to reduce tannins, maintain higher acidity, and retain delicate aromas. Skins remain on red grapes during fermentation. The skin pomace floats to the top of the usually stainless-steel fermentation tanks. It is pushed down regularly by the winemakers to maintain contact with the developing wine.
Some wines are stopped after the yeast fermentation is complete, whereas others are taken through a subsequent bacterial fermentation. This malolactic fermentation utilizes the already present or introduced bacteria Leuconostoc oenos to convert the grapes' malic acid into the milder lactic acid. The process increases the wine's pH from about 3.4 to 3.9, and depending on the strain used, it can impart a buttery flavor.
Three types of oak are used for barrel aging: French Quercus robur,(eastern) American Quercus alba,and Oregon Quercus garryana.The mild French oak is generally favored for the Willamette Valley's Pinot Noir, whereas the stronger American oak can be used for the more robustly flavored Cabernet Sauvegnon. Oregon white oak is intermediate between the other two.
The oak has two functions: to impart its own flavor to the wine, and to provide an active surface for the winemaking process. For delicately flavored wines, batches from new oak barrels are usually blended with batches from old barrels that don't provide flavor but do supply the reaction surface. The old barrels last indefinitely.
The wood of most barrels, as in the case of Oregon oak barrels, is split rather than sawn into boards, so that the grain is parallel with the surface. Then they are planed to the proper thickness and shape for the staves and heads. Finally, the staves are heated to bend them, they are drawn together by an encircling steel cable, and the steel hoops are put in place.
The barrels are gently toasted to carmalize the sugars remaining in the oak pores. This process differs from the charring that is used for whiskey barrels, which is intended to impart a smoky flavor. Wine barrels are toasted before the heads are installed. The open-ended barrels are placed over small fires burning the same wood as the barrels. Production is maintained by moving them along a series of fires. Then the staves are tightened down on the heads.
To illustrate the importance of blending to winemaking, Scott Shull of Raptor Ridge Winery, Scholls, Oregon, published the tests that went into blending his 1995 reserve Pinot Noir (Shull, 1996). The test used six colleagues and himself and three Pinot Noirs of several clones grown on two bedrock types. The initial wines and then the three tested blends were presented to the panel from numbered beakers.
Sample 1 was mostly a Pommard clone grown in Willakenzie soil on tuffaceous sandstone, colored pale ruby, with tones of violet, almond, plum, and a woodsy nose. Sample 2 was mostly a Dijon clone grown in Willakenzie soil, colored medium garnet, with tones of black cherry, flowers, earthy wood, and coffee. Sample 3 was a Wadenswil clone grown in Jory soil on basalt, colored almost black as ink, with tones of chocolate, menthol, vanilla oak, blackberry, dust, and heather.
An equal 33 percent blend of the three wines, as initially had been planned by the winemaker, had good color, but its taste was flat. Next came a blend of 45 percent Pommard-Willakenzie, 45 percent Dijon-Willakenzie, and 10 percent Wadenswil-Jory. This was good. The cherry fruit flavor came through well, but the desired earthiness was masked. The final blend was 50 percent Pommard-Willakenzie, 40 percent Dijon-Willakenzie, and 10 percent Wadenswil-Jory. The small percent of Wadenswil gave good color to the blend. The cherry tone came through well, and hints of oak, blackberry, and heather remained. This was released as the Winemaker's Reserve, and another release consisted mostly of the Wadenswil.
Medical demographers have known since the 1970s that populations which drink red wine regularly, such as in France, are less subject than others to cholesterol clogging of their arteries. Laboratory studies have recently offered an explanation. The cell product endothelin is known to cause arterosclerosis. Polyphenols in red wine (but not in white wine or in unfermented red grape juice) inhibit endothelin production and hence seem to be the cause of this beneficial effect (Corder and others, 2001).
Winemakers blend their wines to increase the complexity, an especially useful practice within a single estate, or amongst neighbors. Shipping grapes for long distances to blend them may produce commercial wines for national distribution, but it suppresses the wines' terroirs--their unique vine-bedrock-soil-microclimate linkage (Wilson, 1998). King Estate in Lorrane, Oregon, for example, which has a large acreage of vinyards at the winery on tuffaceous sandstone of the Spencer Formation, imports and blends from the whole state, even from different appellations. The winemaker does make the wines separately, but the blending is cosmopolitan from across the state. A different practice is that used by other wineries, including St. Innocent Winery, Salem, where the vineyard names are printed on the labels, even though they may lie some distance away from the winery.
As Oregon's wine industry continues to develop, the individual terroirs may gain further name recognition, so that preserving and specifying them can become even more marketable.
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Updated September 16, 2002