Why Are Archaeologists Excited About a Rubbish Heap? Everything People Threw Away

Most archaeological deposits are acidic, and acid dissolves bone, shell and anything else made of calcium carbonate, which is why so many sites yield stone tools and pottery and almost nothing organic. A mound made largely of shell does the opposite, since the shell itself slowly dissolves and buffers the surrounding soil towards alkaline conditions, which preserves bone, antler, fish remains, eggshell and worked shell in excellent condition. The result is that these deposits routinely contain the categories of evidence that are missing everywhere else, including the small delicate bones of fish and birds that record diet far better than the large mammal bones that usually survive.

The contents go well beyond the shells that make up the bulk:

• •Bones of fish, birds and mammals, recording the full range of the diet
• •Stone and bone tools, including fishing equipment
• •Hearths and burnt material, dating the layers directly
• •Human burials, which were frequently placed within the mounds
• •Ornaments made from shell and bone
• •Plant remains preserved by charring

The shells are evidence in their own right rather than merely the matrix holding everything else. The species present record which shores were being worked and how. The size distribution records harvesting pressure, since a population fished heavily shows a shift towards smaller individuals over time, and that shift is measurable across the layers of a single mound. Growth bands in individual shells, which form seasonally in the same way tree rings do, indicate the season of collection, so a mound can be shown to record year-round occupation or seasonal visits. Chemical composition of the shell records water temperature at the time of growth, which supplies a local climate record extending back thousands of years.

Interpreting these deposits as a record of what people ate turns out to be less straightforward than the contents suggest, and the arguments are instructive. Shellfish are conspicuous because shells survive and take up volume, while plant foods, which may have supplied far more calories, leave almost nothing, so the visible remains overstate the contribution of the visible foods. Calculating the meat weight represented by a quantity of shell is possible and gives figures that are frequently modest against the effort of collection. Several researchers argue that shellfish were a reliable fallback during shortages rather than a staple, which fits their availability year round and their low return per hour. Others point to sites where the volume is too great for that reading.

These deposits have been destroyed on an enormous scale and the reasons are mostly economic. Shell is an excellent source of lime and an excellent road surfacing material, and mounds across Europe, North America, Japan and Australia were quarried away in the nineteenth and twentieth centuries for exactly those purposes, with some of the largest removed entirely before anybody recorded them. Coastal erosion destroys others continuously, and because they sit at shorelines that are now retreating, the rate is increasing. Development along coasts removes more. Several countries have given the surviving examples protected status, and rescue excavation of eroding ones has become a recognised priority in coastal archaeology.

Dissolving shell buffers the surrounding soil towards alkaline conditions, so bone, antler, fish remains and worked shell survive where acidic soils would have destroyed them. The shells themselves record which shores were worked, whether harvesting pressure was rising, the season of collection and past water temperatures. Enormous numbers were quarried away for lime and road material, and coastal erosion is removing the rest.