Grain Storage in Iron Age Britain

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by Preston J. Boylesa

"The method they employ of harvesting their grain crops is to cut off no more than the heads and store them away in roofed granges, and then each day they pick out the ripened heads and grind them, getting in this way their food."

Diodorus Siculus, The Library of History, V.21.5

"Some use underground caves as granaries, the so‑called sirus, such as occur in Cappadocia and Thrace; and still others use wells, as in the Carthaginian and Oscensian districts in Hither Spain. They cover the bottom of these with straw, and are careful not to let moisture or air touch them, except when the grain is removed for use; for the weevil does not breed where air does not reach. Wheat stored in this way keeps as long as fifty years, and millet more than a hundred. Some people, as in Hither Spain and in Apulia, build granaries in the field, above ground, so constructed that the wind can cool them not only from the sides, through windows, but also beneath from the ground."

Varro, on Agriculture, I.57

The two main types of grain storage features of Iron Age Britain are referred to by archaeologists as 'four-post structures' and 'storage pits'. Diodorus Siculus may have been alluding to the former when he wrote that the ancient Britons used "καταστέγους οἰκήσεις" (translated as 'roofed granges', V.21.5). Four-post structures are usually found within settlement sites, and are so-called because they survive as a set of four postholes (occasionally more) arranged to form the corners of a square or rectangle. These postholes are interpreted as having held the wooden posts that supported small, raised granary buildings, perhaps similar in appearance to the extant hórreos of Galicia; a reconstruction of an Iron Age settlement featuring such elevated granaries can be found on the Historic England website. Much like those used in parts of contemporary Hither Spain and Apulia (Varro, de Re Rustica I.57), these raised granaries would have allowed ventilation, keeping the grain inside dry whilst lifting it out of the range of pests. Diodorus wrote that the ancient Britons used their granaries for short-term storage, telling us that they kept whole ears of cereal in them, taking out and processing only as much as they needed each day: "each day they pick out the ripened heads and grind them, getting in this way their food." This was because the most common cereal crops in Iron Age Britain were spelt (Triticum spelta) and emmer wheat (Triticum diococcum), both of which require more intensive processing to remove the grains from their glumes than do free-threshing wheats (e.g., Triticum aestivum). Storing them still in the ear was therefore a common practice, which Varro also mentions (I.63).

However, for long-term storage of surplus grain, underground pits were used, similar to those Varro tells us existed in parts of Thrace, Cappadocia, and Spain. Where storage pits have been found in Britain, they are usually large and deep, being narrower at the top and broadening out lower down, so that they often have a beehive-shape when viewed in cross-section; Danebury Hillfort provides good examples of such pits.1 These pits would have been used to store processed grain, which had been removed from the ears of cereal through threshing and winnowing. As archaeologist Barry Cunliffe points out and experimental archaeology has proven (e.g., at Butser Ancient Farm),2 such storage pits are eminently practical and efficient if sealed correctly. They would be filled to the top with grain and then plugged and capped to make them airtight; hence why they are narrower at the top near the opening, to make this easier to do. Once sealed inside, the grain nearest the edges would be in contact with the small amount of air and moisture also trapped in the pit, and so begin to rot. The bacteria, fungi and moulds causing the rotting would remove most of the oxygen in the pit through aerobic respiration, replacing it with carbon dioxide. Eventually, so much oxygen would be removed and so much carbon dioxide will have accumulated, that it would kill off the very organisms causing the rotting; "the internal atmosphere becomes self-sterilising", as Cunliffe describes it. This will also suffocate whatever grain pests were living in there too; Varro observes of the Thracian and Cappadocian storage pits that "the weevil does not breed where air does not reach". It will also arrest the unwanted germination of the grain, because seeds need oxygen to germinate, requiring carbon dioxide once they grow leaves and start photosynthesising. The bulk of the grain would therefore be unspoilt, as long as the hermetic seal held and no more oxygen or moisture entered; as Varro says of Thracian and Cappadocian pits: "They cover the bottom of these with straw, and are careful not to let moisture or air touch them". When the pit was eventually opened and the contents extracted, the crust of spoilt material around the edges could be discarded whilst the rest of the grain would be in perfectly good condition. Pits like these were useful for long-term storage, but if they were periodically opened to remove grain (e.g., for daily consumption) or to inspect the contents, then the seal would be broken, fresh air and moisture would enter, and the rotting would start again. This could be why the raised four-post granaries were used in Iron Age Britain to hold cereal meant to be readily accessible for frequent, short-term use, as Diodorus records, whilst pits were for long-term storage of surplus processed grain.

Incidentally, the oxygen-depleted, carbon dioxide-rich atmosphere within the pits could be dangerous to those who entered them to remove the grain. Varro’s advice was to let the pits air out first before anyone goes in, warning, "Those who keep their grain under ground in the pits which they call sirus should remove the grain some time after the pits are opened, as it is dangerous to enter them immediately, some people having been suffocated while doing so." (I.63). One of the very real dangers of working in a confined space is asphyxiation; carbon dioxide is not only undetectable to humans, being odourless and colourless, it is also denser than air and so a concentration will collect in the bottom of a hole like a storage pit. This would of course be dangerous to anyone breathing it in whilst labouring to shovel the grain out, particularly if they collapsed as a result of it, not to mention to would-be rescuers whose instinct would be to enter the same pit in order to retrieve the individual. Varro’s concern about the air in these pits makes sense in light of our modern understanding of what was occurring inside them when they were sealed.

It is interesting that the Roman period in Britain not only saw the arrival of new types of grain storage facilities (e.g., horrea warehouses), but with them came synanthropic insectivore grain pests, which show up in the British palaeoenvironmental record for the first time within a few years of the initial invasion (for example, for example, the research by Paul Buckland).3 Unlike the sealed storage pits and small four-post granaries of the Iron Age, the large horrea in the towns, military camps and villas of Roman Britain were an ideal environment for these invasive insects to flourish, being well-ventilated and containing large concentrations of processed grain. At these sites, archaeologists have found the notoriously destructive granary weevil (Sitophilus granarius); Columella speaks of the difficulties in preventing them from infesting granaries (de Re Rustica I.6.12). Weevils will act as a vanguard for other insects, including the rusty grain beetle (Cryptolestes ferrugineus), which generally prefers grain already damaged or spoilt, and the small-eyed flour beetle (Palorus ratzeburgii), which scavenges spoilt grain and predates on other insect pests. Evidence for all of these is attested in the palaeoenvironmental record for Roman Britain.

A Roman horreum, at Housesteads (Vercovicium) on Hadrian's Wall. We can see the pillars to hold the raised floor, and just about make out the remains of the buttresses on the left-hand external wall, suggesting it was not only a tall structure, but that it had to withstand the weight of the grain inside. There are holes located around the base of the wall to act as vents, allowing air to circulate underneath the floor and keep the grain dry — and let in the camp's cats and dogs to catch the rats. The large entrance would have allowed carts to back right up to it for unloading sacks of grain. The eaves of the roof no doubt projected well out from the walls, to stop rain ingress. A warm, dry, aerated environment containing enough food for about 800 soldiers: but perfect for weevils!

Not only were the large, well-ventilated horrea more conducive to such insect pests, as Varro (de Re Rustica I.57) and Vitruvius (de Architectura VI.6.4) well knew, although could not adequately explain, but storage practices may also have made a difference. Diodorus wrote that the ancient Britons stored the entire, unprocessed heads of harvested cereal in their granaries, taking out and processing only what they needed each day; since the granary weevil has difficulty in breaching husked kernels, this mode of storage may have prevented them from becoming too established in the Iron Age. By contrast, the horrea of the Roman period appear to have been used to store fully processed grain, which had already been separated from the protective husks and chaff through threshing and winnowing, making it more vulnerable to insect attack. The socio-economic conditions of the Roman period may also have facilitated the spread of these pests. Unlike the more localised and subsistence based agriculture of Iron Age Britain, grain was now being moved in bulk from the countryside to be concentrated in new urban centres through trade and as taxes paid-in-kind (e.g., tributum soli), whilst the army's agents were buying and requisitioning grain and moving it across the island to their permanent bases and temporary campaign camps; it is no surprise that the earliest examples of insect grain pests in Britain are from Roman military sites. Grain was also being moved in quantity between provinces, including Britannia (cf. Ammian. Res Gestae XVIII.2.3), and pests no doubt travelled with it. After the end of the Roman period, these insects largely disappear from the British archaeological record, returning after the Norman Conquest.

The Author's Notes:

1 See Fig. 3 in Marijke Van der Veen 2006, "A re‑analysis of agricultural production and consumption: Implications for understanding the British Iron Age" in Vegetation History and Archaeobotany.

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2 The results of the grain pit experiments at Butser Ancient Farm are discussed in R. A. Hill et al. 1983, "Storage of Barley Grain in Iron Age Type Underground Pits".

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3 Paul Buckland 1978, "Cereal production, storage, and population: a caveat" in CBA Research Report No. 21. Currently available as a PDF document on the Archaeology Data Service website.

Thayer's Note:

a The author has been working as a professional archaeologist since 2009, and at the time he wrote this article he was a Project Officer for Cotswold Archaeology; he has excavated a number of Iron Age storage pits and four-post structures. He is an alumnus of the University of York, where he studied archaeology; he explains his work — and gives some career path pointers to archaeology students — in this YouTube video interview.

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