View all newsletters
Receive our weekly newsletter - World Of Fine Wine Weekly
  1. News & Features
July 28, 2022updated 02 Nov 2022 11:49am

Limestone: Holy grail or charismatic illusion?

By Alex Maltman

With so many of the world’s finest wines grown on limestone and its relatives, it’s no wonder that many winemakers see it as the dream soil for viticulture. But is its exalted reputation justified? Alex Maltman reports.

Alex Maltman on flint

In classical times, the goddess Tethys and her consort Oceanus were believed to watch over all the waters of the Earth. (And Tethys also found time to raise more than 3,000 junior goddesses.) In modern times, she lives on in the name geologists give to a huge ocean that once stretched more than halfway around the Earth.

The western reaches of this vast Tethys Ocean were shallow, warm, and sunlit—as if today the Bahamas extended from what is now the British Isles to the Himalayas.

The waters offered just the conditions for a flourishing marine life, including all manner of shellfish, copious plankton, and widespread reefs of clams and corals. With their shells being made of calcium carbonate, this led to thick deposits of calcareous detritus accumulating on the sea floor.

What does all this have to do with wine? Well, with time the calcareous deposits hardened into rock—limestone—and Earth’s internal forces thrust the sea floor upward to make land, the very land where eventually wine-yielding grapevines were to originate and flourish.

In the Eastern Mediterranean/Caspian Sea region where Vitis vinifera began, there’s a huge amount of limestone, and as the vines spread westward they encountered yet more of it, together with its calcareous variations such as chalk and marl.

Content from our partners
Wine Pairings with gooseberry fool
Wine pairings with chicken bhuna 
Wine pairings with coffee and walnut cake 

About half of France is underlain by limestone, and nearly one third of Italy and Spain. In other words, thanks to the Tethys Ocean, limestone soils are prominent in the original milieu of the Eurasian grapevine.

And so, when in recent times certain areas were identified as producing superior wines, it was almost inevitable that these kinds of soils would be involved. And become famous. In the wine world, the marls of Chablis, the chalk of Champagne, the limestone of Meursault, and so on are legendary, so much so that limestone and its relatives are seen by some as the dream soil for viticulture.

Burgundians rejoice in their good fortune of having calcareous soils. In the New World, where limestone is relatively scarce, some growers passionately seek it out: “We pounced on this area because of its limestone, for wine the holy grail”; “like the holy grail, limestone has long been sought out by viticulturists and winemakers.”

Wine writers and promoters love limestone. The listings of one New York wine merchant mention the word more than 100 times.

Some wine descriptions use it to signify quality: “The more limestone is present, the finer the wines become”; “limestone is what gives this wine its soul.”

Some commentators actually taste the rock in wine: “The limestone rolled onto my tongue”; “you can smell and taste the limestone.” And all this enthusiasm is helped by our easy familiarity with the rock.

The friendly rock

As it happens, these same Tethys-born lands would also spawn Western civilization, which soon made full use of its ambient limestone. The pyramids of Giza and the great temples of Gozo are made of limestone, as is much of the Acropolis.

From more recent times, notable American buildings such as the Lincoln Memorial, the Empire State Building, and the Pentagon, together with well over half of the US state capitols, are built of limestone. So are a lot of celebrated European monuments, including many of the great castles and cathedrals. In London, Buckingham Palace, the Houses of Parliament, and the National Gallery, for instance, are made of limestone.

How many poems (as in WH Auden’s “In Praise of Limestone”) and novels have been written about a particular kind of rock? Fiona Farrell’s book Limestone opens with “I was born in limestone country. This has implications. It is a different thing to be born to clay, or schist, or basalt…”

Limestone Country by Fiona Sampson begins, “This book is about a love affair with limestone” and goes on to remark that “limestone is welcoming to humans, who have long found it adapts easily to their needs, as they to it.”

Limestone even has a part in Christian ritual. At the Feast of Epiphany, chalky limestone is blessed by a priest: “O, Lord God, bless this creature chalk to render it helpful to men.”

So, we are comfortable with limestone; unlike cryptic rocks like arkose and aplite, gabbro, or greywacke, it seems like an old friend. But does that make it a holy grail for vine growers?

Of course, superlative wines come from many kinds of soils, but all of the above illustrates that limestone is held in particular regard. It seems to have charisma. And certainly, if one rock were to be singled out as possibly being special for viticulture, then it would have to be limestone—because it is different.

Limestone: A different kind of rock

Most rocks and soils are based on silicate minerals, in which atoms of silicon and oxygen are bonded together by various metal elements. In fact, though such rocks as schist, granite, gneiss, and shale have very different geological origins, they are chemically remarkably similar.

They all weather down to give somewhat acid soils, with a pH of less than 7, and with similar ranges of water-holding and drainage properties; they all potentially yield the same full range of mineral nutrients for the vine. Just as with, say, volcanic rocks and soils, none has any special ingredient. Neither does limestone, but in its chemical and physical properties, it’s different.

Pure limestone is made of calcium carbonate, based on carbon and oxygen. Chalk is one kind of limestone, rich in the fossilized remains of a specific plankton; marl is an intimate mixture of clay and limestone.

limestone with sea
Photography by Jennifer Sarabia / Shutterstock

All are referred to as being calcareous, and in contrast to the virtually insoluble silicate rocks, they dissolve fairly easily. Also, they weather to give soils that tend to be alkaline—that is, with a pH of 7 or more. These are two key properties, and I now look at them more closely, considering what they might mean for wine.

A rock that dissolves

First, the significance of limestone dissolving in water. Well, it led Auden to write, “If it forms the one landscape that we, the inconstant ones, are consistently homesick for, this is chiefly because it dissolves in water.”

Limestone can be a physically tough rock, commonly resisting erosion to give upland areas; this, coupled with its chemical solubility, leads to distinctive and striking landscape features. Examples are the strange tourist-attracting shapes of the Enchanted City near Cuenca in Spain, the magnificent fluted cliffs at Cassis, and the channeled escarpments that tower above vineyards in the Alps and the Jura.

Plateaus of limestone are well known in the wine regions of southern France, as is the heady mix of herbs that grow in their dissolved fissures, the garrigue. The word is a favorite in descriptions of wine aromas: “Wine with scents of the garrigue,” “expressing notes of garrigue,” and the like. (The equivalent word for aromatic shrubs growing not on limestone but on silicate rocks is maquis. When did you last see maquis in a wine tasting note?)

Most of the world’s caves have formed in limestone, where underground rivers have dissolved away some of the host rock. Some natural caves have been expanded over the centuries as underground quarries for limestone building blocks, only to become disused and now adapted for wine storage.

Putting aside claims that the walls of these cellars somehow improve the bottled wine—“You can taste these limestone walls in the glass” and the like—the great virtue of caves is their provision of ideal conditions for storing and maturing wine.

Famous are the limestone wine caves below Reims, St-Emilion, and parts of the Loire Valley in France; in Spain’s Penedès, Rioja, and Ribera del Duero; and the extensive ancient cellars below Chateau Ksara in Lebanon.

Less well known are those north of Moldova’s capital, Chișinău, reportedly the world’s largest. The tunnels at Mileștii Mici stretch for no less than 150 miles (240km) and contain the world’s largest collection of wine bottles (more than 1.5 million, according to the Guinness Book of Records). At nearby Cricova, you can drive your car along the 60 miles (100km) of limestone tunnels, bearing names such as Cabernet Street, Muscat Street, and Aligoté Street.

The solubility of limestone means that any fine particles in soil are soon dissolved away, so limestone soils tend to be coarse, stony, and very well drained, ideal for moist northern European areas such as England and Champagne.

At the same time, some limestones contain abundant little gaps between their constituent calcite grains, some enlarged by dissolution, and this allows storage of water. Such a balance between water drainage and storage, particularly well shown by some chalks, is ideal for viticulture. It’s why the chalk strata beneath London and Paris are such important aquifers for those cities.

The other side of the coin, so to speak, is water moving around underground and encountering new conditions such that any dissolved calcium carbonate has to precipitate out. This is how tufa, travertine, and stalagmites come about, together with hardpans in soil; it’s how the caliche of Chile and the so-called pedogenic limestone in the soils of Central Otago and Mendoza form.

The sweet earth

Now, the soil alkalinity. European farmers have known for millennia that vines grow well on calcareous soils: Pliny remarked that, for vines in Italy, “the chalky earth of Alba Pompeia is preferred.”

Romans of the time tasted water that had percolated through baskets of soil in order to gauge whether the soil was “sweet” or “sour”; they also knew that the latter could be “sweetened” by spreading lime on them. Pliny reported that the tribes living in (granitic) western Burgundy added ground limestone to their vineyards and that farmers in Britain had mastered the process of spreading marl to sweeten and improve their lands.

Marl spreading was important in England for a long time. A millennium after the era of Pliny, Arthur Young of Bradfield, Suffolk, lost his wholly unloved wife, and the only tribute he could think of to have engraved on her church memorial was that her great-grandfather had used marl on his fields.

Numerous texts on liming were published over the years, giving precise recipes and instructions, though floundering as to how it worked. John Evelyn (1675) felt that the “fecundity of the earth” was improved by lime because it contained the “alchemical spirit” of nitre (the basis of gunpowder).

By 1764, the chemist Johan Wallerius had established that the sourness of soils was due to their acidity and that the sweetening effect came about because limy materials were alkaline, but he then faltered by suggesting that somehow this involved dissolving the “fattiness” of the soil. He did, though, correctly point out that sweetening the soil is temporary, thus explaining a proverb of the time: “Liming soil makes fathers rich but sons poor.”

It was around 1840 at Rothamsted Manor in Hertfordshire, UK, that scientific experiments were first carried out to find exactly how this lime effect worked. There, the celebrated Broadbalk Field yielded the first understanding of how soil acidity, which depends on the lime content, affects nutrient availability to plants. (One visiting American later remarked, “We have learned more about agriculture from this field than from any other patch in the world.”)

Today, a farmer can simply tap the Rothline app on his cellphone to find instantly how much lime to add to improve his soil’s nutrition.

Many agriculturists, however, still rely on the standard chart that cleverly depicts how nutrient availability comes and goes with differing soil pH values. It was invented by Nicholas Pettinger of the Virginia Agricultural Experiment Station, who died tragically young, with his contribution now rather unacknowledged.

Early in 1935, besides being busy with such things as university lectures, talks to the local farmers’ institutes, and chairing committees, Pettinger was busy preparing his “Useful Chart for Teaching the Relation of Soil Reaction to the Availability of Plant Nutrients to Crops.” This was the origin of today’s much-used table. Sadly, however, while developing the chart he contracted a serious blood disease and died a mere six months later, aged just 35.

Albeit unsung, his efforts live on: Look in any popular account of soil fertility today, and you will likely see some version of Pettinger’s pH chart.

Most bedrock tends to yield rather acid soils, typically with a pH of 5 or 6 at the most. A glance at Pettinger’s chart shows that at such values, the vine’s access to essential nutrients such as magnesium, phosphorus, and molybdenum will be curtailed to the extent that root growth and plant development will suffer.

Richly purple and deep emerald vine leaves may look strikingly beautiful but are probably a sign that they have too little magnesium. However, soils with a greater lime content will approach an ideal pH of around 6.5—that of many calcareous soils. Here, access to essential nutrients is at its optimum.

Further virtues of calcareous soils come from their calcium interacting with any clay to weaken the bonding of soil particles. This makes the soil open up, to give spaces that not only promote warmth, good drainage, and root penetration but also enhance the diversity and activity of microorganisms.

While we still don’t know the extent to which the soil microbiology affects wine flavor, there’s no doubt that a healthy, living soil has huge ecological benefits and can indirectly benefit the finished wine.

So, in ways such as these, a “sweet” soil is desirable, and that usually means one with a good lime content. Does that mean, therefore, that a limestone soil is indeed the holy grail? Hmmm. Unfortunately, too much lime can be problematic, and a pure limestone soil consists of little else but lime.

So, what’s the catch?

Two major challenges can face growers working with limestone soils: the possibility of nutrient deficiencies and problems arising from soil pathogens, especially the threat of phylloxera.

First, nutrition. Purer limestone soils contain less clay, quartz, and other silicate minerals; calcite increasingly dominates, and pH levels increase as a result. As the chart invented by Dr Pettinger shows, this can introduce nutritional problems.

limestone nutrient chart
Chart redrawn from one by the Potash Development Association, “redrawn from E Truog (1946),” who based his version on Pettinger’s original

Take boron, for example, with an availability starting to decline at pH values of 7. It’s needed by grapevines in only tiny amounts, but even so, boron deficiency is a widespread micronutrient issue. It ruined one of Japan’s major wine producing areas, Yamanashi Prefecture, until it was established that one of the causes was over-liming of the soils.

Some of the soils at Montalcino in Italy reach pH values approaching 8, and as a result, most of the soils are lacking or severely lacking in accessible boron. It’s a similar story with zinc: More than half of the Montalcino soils lack available zinc.

One of the results of boron and zinc deficiencies is shriveling of the vine leaves. In South Africa’s Cape Province, even though zinc is mined in the northwest of the region, in the wine-producing southwest it’s sparse in the soils: Consequently, the growers there talk about kleinblaar—little-leaf disease.

Another symptom is uneven fruit-set, known in France as millerandage and in English, picturesquely, as “hens and chickens” or “pumpkins and peas.” Not good news for the harvest.

But the big problem is with iron. The difficulty here is not only that limestone soils contain less iron than those derived from, say, basalt or schist but that the element is usually locked into insoluble compounds that roots cannot absorb.

The problem worsens with higher values of soil pH, and a one-unit increase in pH decreases the solubility of iron a thousandfold. The ensuing lack of iron makes it difficult for the leaves to produce chlorophyll, and the leaves turn yellow as a result. It’s called iron chlorosis; if you’re a gardener, you may have seen it on your roses or tomato plants.

Or you may have seen yellowing, chlorotic leaves in vineyards, since it’s not uncommon in areas with limestone soils. In the Corbières district of France’s Languedoc, two or three rows of vines can look distinctly yellow among the rich green foliage of neighboring rows. That’s because the intricate geology of the district can give very thin strips of high pH limestone soils, even within a single vineyard.

Iron deficiency has been a long-standing problem in the chalky soils of the Champagne region. For centuries, the ground there has been supplemented with coaly materials containing iron: Coal waste was brought from the mines around Lille, not far away; the low-quality lignite that occurs around Reims was dug out of numerous small pits for this purpose. Several still operate.

Right up until the end of the last century, in a further attempt to “improve” the soils, municipal waste—the infamous boues de ville, containing things like broken glass, batteries, and plastic bags—was spread on the ground.

The soils of Champagne might attract the holy grail sobriquet, but writing as recently as 1984, one French authority remarked on their “foul odor” and the “uninviting aspect to the vineyards, disconcerting for the visitor who too often sees the spaces strewn with plastic debris.”

These nutritional problems might seem surprising, given that vinifera vines had largely originated and, for millennia, flourished on calcareous soils.

In fact, as well as growing perfectly well on most other kinds of soils, for limestones the vinifera species (alone) has a cunning mechanism to combat chlorosis: If needed, the roots are able to pump out hydrogen and organic acids into the immediately adjacent soil in order to increase its acidity and thus help the iron become soluble and available for uptake.

What, then, has gone wrong? Why are there now nutritional problems with limestone? The answer lies with the dreaded phylloxera.

Continents apart

The watery empire of the goddess Tethys included the great and mysterious sea that stretched out westward from the Mediterranean: the Sea of Atlas—or, as we know it, the Atlantic Ocean.

Geologically, the North Atlantic began to form about 130 million years ago, when the continent that until then had contained Eurasia and North America began to split apart. As a result, the existing terrestrial life forms now found themselves on diverging land masses, on opposing sides of the infant ocean. They proceeded to evolve separately, and this included the ancestors of the plants we now know as the genus Vitis, the grapevine.

As we have already seen, in Eurasia Vitis vinifera evolved largely on calcareous deposits originally on the Tethyan sea floor.

But on the American side, different species arose, such as Vitis labrusca, and it developed on the distinctly acidic gneiss and granite soils of what is now New England. And alongside it, the phylloxera aphid was also evolving, specifically equipping itself to suck sap from the roots of grapevines.

Limestone phylloxera
Different representations of the phylloxera louse; Natural History Board of 1910. Fototeca Gilardi / Bridgeman Images

Some of the American vine species responded to the pest by developing biochemical ways of repelling it and its associated pathogens—and of growing a corky membrane to heal any nibbled roots. But over on the other side of the ocean, in the absence of phylloxera, Vitis vinifera developed no such tricks.

The 18th century, however, saw the inadvertent importation into Europe of the American phylloxera louse: The ensuing devastation of the defenseless European vines is well documented.

History records how the most effective solution turned out to be grafting vinifera, with its superior tasting fruit, onto the phylloxera-resistant rootstocks of North American vines. Less recounted, though, is the role that limestone soils played in the story. Here’s an outline.

The root of the matter

Early French attempts at grafting used Vitis riparia because, being the most widespread North American wild vine, it clearly had good phylloxera resistance.

But as its name indicates—riparia meaning to do with rivers—it flourishes on moist, fertile riverbanks, and this presented a problem in the dry calcareous soils of Europe. So, rootstocks of Vitis rupestris were tried, and—rupestris meaning rock-living—these fared better in stony soils but, again, not if they were calcareous.

Might there be an American wild vine living happily in alkaline, calcareous soils? As the beleaguered viticulturists became increasingly desperate, they urged government action.

So it was that in 1887 that a young botanist, Pierre Viala, was sent to the United States to search for a grapevine that was tolerant to limestone soils. He visited a host of vineyards in the eastern states but with minimal success, largely because there was little limestone exposed at the land surface.

And wherever he did find some, any local vines were invariably struggling. “Not one of the varieties of the north and the east has value for calcareous and marly soils,” he concluded.

Viala was sent extra funding in order to continue farther westward, across “Indian territory” and eventually all the way to the West Coast. But there he found imported European vines already decimated by phylloxera—and no limestone.

It must have been demoralizing for the young man, knowing that so much rested on his shoulders and yet with things looking so hopeless. His diary one day records, “Despair and woe! Now my little suitcase with my most precious possessions: all my collected notes since I’ve been here” has vanished. “I cried like a child.” (It was later recovered.)

Then came a change in fortune. After a tip-off, Viala traveled to the little Texas town of Denison, north of Dallas, right up by the Oklahoma state line, to meet Thomas Volney Munson, an indefatigable cataloger of American vines. (The two immediately hit it off, and Munson later named one of his daughters Viala.)

Munson not only understood vines, but he knew their habitats and, crucially, the soils in which they grew. And yes, he knew exactly where vines were thriving on rocky limestone.

Thus, Viala rode down to Texas Hill Country, to a place just west of Belton called Dog Ridge. It was “horribly dry land, with Indians on it,” but it was limestone, with soils remarkably similar to those in France and with vines growing contentedly in them.

Viala swiftly identified the particular species that Munson had recommended—Vitis berlandieri—and soon 15 wagon-loads of cuttings were being taken away and loaded onto three ships bound for France. Salvation for the viticulturists was on its way…

Viala’s return, however, wasn’t wholly triumphant. Among other things, back in France his cuttings proved reluctant to take root, such that scientists had to resort to hoping that selective crossings might produce some viable offspring.

And eventually they did. A cross between Vitis berlandieri and Chasselas (Vitis vinifera) yielded the famous rootstock 41B. It rescued the particularly badly stricken Charente vineyards and is still used for more than 80 percent of the vines in the Champagne region. But the chlorosis challenges remain. The North American varieties generally needed in order to improve rooting tendencies still dislike high-pH limestone soils.

The shortcomings work in reverse; early vineyards in the US clustered along Lake Erie and the southern Finger Lakes where the indigenous vines were well suited to the acid gravels, sands, and shales. But they produced mediocre wine. Farther north, up toward the Niagara limestone escarpment, vinifera made better wine and was quite at home nutritionally—but on its own roots it was completely defenseless against the rampant phylloxera.

Different, but how special?

So, is limestone special for wine? Although the rock has never had a monopoly on superlative wines, arguably there was some justification for the claim in pre-phylloxera days when vinifera grew happily in limestone soils on its own roots; there are plenty of modern commentators who believe that those pre-phylloxera vines produced wines unmatched today.

Nowadays, however, it’s a risky proposition to use ungrafted vines, even in regions that so far have avoided phylloxera, because of the ever-present and perhaps increasing chance of infestation by a whole array of soil pathogens.

As a result, most modern growers resort to grafting, which involves the careful selection, after weighing various pros and cons, of an appropriate rootstock—and consequently, having to accept some degree of compromise.

It’s worth pointing out that a disadvantage of limestone (and clayey) soils is that they seem particularly attractive to phylloxera, as the French experience illustrates. In volcanic and sandy soils, however, the insect struggles: The few surviving pockets of pre-phylloxera vines in France are almost all on distinctly sandy soils.

Many of the physical attributes of limestone can be emulated by other kinds of rock. Plenty of soils are free-draining and, usually through clay being present, can offer water storage. I mentioned that most limestone wine cellars aren’t natural caves but have been fashioned from disused underground quarry workings; so, similarly, cellars can be excavated in many rock types.

They range from soft materials such as loess, as in Kamptal, Austria, to tough granite, as in South Africa’s Western Cape. In Hungary, the huge Rákóczi cellar in Sárospatak and the four-story, 9-mile- (15km-) long Ungvári wine cellar in Sátoraljaújhely—both UNESCO World Heritage sites—are carved from volcanic tuff.

A limestone finesse?

One apparent attraction of limestone soils is the distinctive quality they’re supposed to give to wine character (seemingly irrespective of the vine rootstock), though there is much inconsistency in the claims. Most commonly, the trait is expressed with words like “liveliness,” “edge,” “nervousness,” and “finesse,” perhaps in line with the notion that the alkaline, high-pH soils of limestone produce low-pH wines.

This thinking possibly stems from the revered and influential wines of northern France, especially Burgundy and Champagne, from their calcareous soils—though even in the Côte d’Or, for some writers the limestone soils lead to “big, strapping wines” and “unmatched opulence.”

Many wines from northerly countries such as Germany, Austria, and Hungary attract similar descriptions of “finesse” but are from acidic and not calcareous soils. Conversely, wines from limestone soils in warmer climates tend not to be noticeably acidic—Chardonnays from Lebanon’s Bekaa Valley, for instance—as with many red wines.

Presumably, the greater acid levels from the northerly areas mentioned above have a lot to do with their cool climate, but designing a scientific experiment to isolate and assess the contribution of the soil is difficult. In a recent Dutch experiment, however, it turned out that the obstacles had been circumvented.

Grapes from three different cultivars were grown similarly but each in several different soils. They were then uniformly vinified. When independent trained tasters assessed the wines, they were unable to detect any significant differences among the wines of each varietal, irrespective of the soils.

In an extension of the experiment, the tasters did detect variations when the wines were made using different yeasts—but the soils, which included limestone, appeared to be playing no role in the taste.

It’s instructive to look at areas where limestone occurs in close association with other rock types.

The Leithaberg DAC in Austria is dominated by two soils, limestone and slate, but the official regional specification makes no differentiation between them.

It’s similar at St-Chinian in southern France, where the appellation covers both limestone soils in the south of the region and the schist in the hillier north. Interestingly, and in contrast with northern France, commentators find fuller, fatter, more powerful wines coming from the limestone soils, whereas it’s the schist soils that yield wines with more acidity, tension, elegance, and perfume.

limestone st-chinian
St-Chinian. Photography by iStock / Getty Images Plus

The schistose areas are higher in altitude, so a cooler climate may be involved, but then in Bourgeuil, on the Loire, light, fruity, “thirst-quenching” wines come from the gravel down by the river and “richer, well-built, ageworthy wines” from the higher limestone bluffs.

It would seem, then, that limestone doesn’t confer a special character on wine taste, at least in a consistent way. And as we have seen, for the grower, limestone soils can present challenges.

So arguably, all of this is hard to square with the idea of limestone being a holy grail for vineyards—if that implies something of a consummate ideality.

Near where I am writing this in mid-Wales is the old manor house of Nanteos, which until recently kept a wooden object believed by some Christians to be the holy grail.

Supposedly, following the crucifixion, this fragment of the cross had been brought by Joseph of Arimathea to Glastonbury Abbey in England, then at the time of the dissolution of the monasteries was spirited to Nanteos for safekeeping.

There, over the ensuing centuries the object was held sacred, bringing miracles to all who beheld it, as befitted a fragment of the True Cross. Until, that is, radiocarbon dating spoiled the whole story. And so it seems that the search for the holy grail goes on…

Topics in this article : ,
Select and enter your email address For award-winning content from the world’s most respected and intellectually satisfying wine magazine, sign up to our newsletter here
Visit our privacy Policy for more information about our services, how Progressive Media Investments may use, process and share your personal data, including information on your rights in respect of your personal data and how you can unsubscribe from future marketing communications.
Thank you

Websites in our network