L is for Limpet

For well over 100 years, archaeologists have been working in the windswept environment of the Isle of Oronsay on the west coast of Scotland to discover more about the people who lived on this tiny patch of land as long as 6,000 years ago – and how they exploited the natural environment around them.

Oronsay is remarkable for its stark beauty and its role as a habitat for wildlife. The island is also known for its five shell middens. Heaps of ‘kitchen waste’, they were left by people living at a period known as the Mesolithic.  Also found are evidence of structures and hearths used for boiling and cooking food gathered from sea and shore. These too date from the Mesolithic.

Archaeologists have identified bones and shells from at least 30 species of marine life in the debris of these ancient rubbish heaps which, by virtue of their remoteness, lay undisturbed for so long under a thick covering of sand. Easily the most abundant of the molluscs to be found in the debris of the middens is the humble limpet.

Limpets can be found clinging to rocks all around the coast of Britain. Resembling the stereotype of a Chinese hat, the shell of the mollusc is conical but with ridges running from its outside edge to the peak. Inside, the soft body of the limpet is as vulnerable as a Dalek without its armour.

In the tough history of the Scottish islands, limpets were eaten mainly at periods when other foods were scarce. Today they are used by fishermen as bait and feature on the menus of only the most adventurous free food enthusiasts.

Archaeologist Sir Paul Mellars, emeritus professor of Prehistory and Human Evolution at Cambridge, first visited Oronsay in the mid-1970s. Over the course of five field seasons, often working in driving rain and at the mercy of Scottish midges, Mellars and colleagues excavated samples of shell material from all five middens on the island.

Using Radiocarbon dating, a technique that transformed the chronologies of human prehistory, archaeologists were able to show that the middens belong to the final stage of the Mesolithic period. This finding pointed to an intensive and relatively short-lived exploitation of the island by Mesolithic communities around the fourth millennium BC. 

Modern visitors to Oronsay might well ask themselves why people would have chosen to live in such a far-flung environment, at least eight miles from the mainland. “As fishermen will tell you even today, the waters around Oronsay and the bigger island of Colonsay are rich in seafood. And the calcite-rich shell sand that covers much of the islands makes the land fertile enough to grow crops,” says Mellars.

“What’s so fascinating about the Oronsay middens is that they date from a time when people were on the point of moving from being hunter-gatherers, or hunter-fisher-gatherers, to farmers. These heaps of debris give us a glimpse of lives at a time of transition.”

Mellars’ early work focused on the range and relative importance of the different food resources as revealed by the contents of the middens.  Debris from both fish and shellfish was evident. Analysis of these deposits showed that one species – saithe or coalfish – accounted for more than 90% of fish bones. Among molluscs, limpets greatly predominate over other molluscs such as whelks and periwinkles.

“It’s not hard to come up with plausible explanations for the preference for limpets, which are plentiful. They taste like bits of car tyre – but they are meatier, and more nutritious, than whelks and winkles. Furthermore, limpets are a lot easier to extract from their shells. These factors combine to make limpets a more energy efficient resource,” says Mellars.

The shapes of limpet shells vary according to where they are found. Limpets inhabiting the lower parts of the tidal range are generally much flatter than those occupying the higher parts of the shore. Measurements of the height of limpet shells found in the middens revealed that they had been collected almost exclusively from the lower part of the tidal range – and chiefly from the very low tide situations exposed only during spring tides.

Mellars proposed two possible explanations. Firstly, limpets from low-tide zones are tenderer than others. Secondly, continuous harvesting of limpets may have led to an over-exploitation of more easily-accessible areas of the shore.  With the disappearance of limpets from the upper levels, the human population sought foodstuffs close to the low tide line.

Advances in technology are now enabling Mellars to come up with answers to other key questions – especially the question of seasonal patterns of saithe fishing and harvesting.  Although abundant around Oronsay and other islands in the summer, the fish migrate into deeper water late in the autumn.  This means that communities relying on marine life would need to look elsewhere for sustenance during winter and spring.

In a collaboration between the Departments of Archaeology and Earth Sciences, limpet shells from the middens were embedded in resin and then cut in half to expose the calcite interiors. The oldest material deposited by the shell is at the pointed top while the shell deposited just before the limpets were harvested lies on the outermost edge.  

Rebecca Vignols, an Earth Sciences student, used a computer-controlled microdrill to obtain tiny samples of calcite from points less than half a millimetre across in a closely-packed series at the edge of the shells. To get the full picture of seasonal change at the location the limpets lived, some limpets were drilled in a detailed series right to the top of the calcite in the shell. The ratio of light and heavy isotopes in water is affected by water temperatures, and the limpets build these changes into their shells as they grow.

The oxygen isotope ratios in the shell samples were measured in the Department’s mass spectrometers. In this way, the water temperatures at the time the shells were harvested could be compared with the full seasonal temperature range at the site where they lived. This data reveals that limpets were harvested throughout the winter months. Limpets may have been consumed in combination with other seasonal foodstuffs such as hazel nuts. 

The five middens on Oronsay are located around I km apart from each other. All are close to the beach and face east, away from the worst of the Atlantic gales. “The spacing of the middens seems to suggest that they were made by communities who moved along the shoreline. As they exhausted the supply of limpets in one half kilometre stretch of beach, they moved on to the next, perhaps making two runs per year in order to let the molluscs regenerate. You could almost describe it as a kind of farming,” says Mellars.

No pottery has been found in the middens but hundreds of ‘limpet scoops’ were retrieved. Some are simply finger-shaped pebbles from nearby beaches; others are made from deer horn. “Horn from at least two types of red deer was used. There have never been deer on Oronsay or Colonsay, which means that the horn may have come from Islay, Rum or Skye, and would have come by boat, suggesting a flourishing trade in this raw material for tool making,” says Mellars.

Oronsay’s middens all date to a similar period late in the Mesolithic and are composed of layers of material assembled over some 300 years. The apparent ceasing of limpet harvesting, which had been a way of life for generations, marks a significant change in lifestyle. Narratives about the spread of farming have shifted radically over the years.

“There’s been huge debate about the arrival of farming in the British Isles. “When I was a young archaeologist, it was thought that farming spread up the Danube and came north as a result of colonisation. In the 1980s, it was trendy to argue that local people made the transition to farming independently or with the influence of occasional incomers,” says Mellars.

“The consensus today, and the theory I support, is that the communities who lived on Oronsay and other islands interbred with incomers who didn’t wipe them out but introduced them to other ways of ensuring a supply of food during the lean winter months. Something a lot more palatable than limpets – such as mutton and beef!”

Next in the Cambridge Animal Alphabet, M is for a small creature that can cause a big nuisance but also tell us a lot about pollution in water.

Inset images: Oronsay (Guy Beauchamp); Limpets (Kim Freeman); Drawings of shell sampling (Rebecca Vignols); Oronsay landscape with cows (Carron Brown).