A midden, also known as a kitchen midden, or a shell heap, is a dump for domestic waste. The word is of Scandinavian via Middle English derivation, but is used by archaeologists worldwide to describe any kind of feature containing waste products relating to day-to-day human life. They may be convenient, single-use pits created by nomadic groups or long-term, designated dumps used by sedentary communities that accumulate over several generations. In the latter case, a midden's stratigraphy can become apparent.
Generally, a midden is laid down in deposits as the debris of daily life are tossed on the pile. Each individual toss will contribute a different mix of materials depending upon the activity associated with that particular toss. During the course of deposition, sedimentary material is desposited as well, by any number of mechanism, from wind and water to animal digs. This creates a matrix which can also be analyzed to provide seasonal and climatic information. In some middens individuals dumps of material can be discerned and analysed.
A shell midden or shell mound is midden comprised mainly of mollusk shells. Like all middens, shell middens also contain the debris of human activity and remains of their meals. Some shell middens are processing remains: areas where aquatic resources were processed directly after harvest and prior to use or storage in a distant location. Some shell middens are directly associated with villages, as a designated village dump site. In other middens the material is directly associated with houses in the village; each house would dump their garbage directly outside the house. In all cases, shell middens are extremely complex and very difficult to excavate fully and exactly. However, the fact that they contain a detailed record of what food was eaten or processed and many fragments of stone tools and household goods makes them invaluable objects of archaeological study.
Shell have a high calcium carbonate content, which tends to make the middens alkaline. This slows the normal rate of decay caused by soil acidity, leaving a relatively high proportion of organic evidence (food remnants, organic tools) available for archaeologists to find.
The archaeological study of shell middens began in Denmark in the latter half of the 19th century. The Danish word for shell mound or midden mound køkkenmødding or koekken-moedding is now used internationally.
Shell middens are found in coastal zones all over the world. Consisting mostly of mollusc shells, they are interpreted as being the waste products of meals eaten by nomadic groups or hunting parties. Some are small examples relating to meals had by a handful of individuals, others are many metres in length and width and represent centuries of shell deposition. In Brazil they are known as sambaquis, having been created over a long period between the 6th millennium BC and the beginning of European colonisation.
On Canada's west coast there are shell middens that run for more than a kilometer along the coast and are several meters deep. The midden in Namu, British Columbia is over 9 meters deep and spans over 10,000 years of continuous occupation.
Shell middens created in coastal regions of Australia by indigenous Australians hold particularly significance in Australia today. Aboriginals were a hunter-gatherer nomadic people who left no permanent structures, and middens provide evidence of prior occupation in native title claims and indigenous cultural assessment of mining and other developments. Some caution should be exercised in interpreting whether these middens were man-made or developed by natural wave action. One would expect hunter-gatherers to efficiently harvest only edible shellfish and transport them to a secluded campsite to cook and eat. Yet many of these middens contain a high proportion of small and non-edible shells, and the sites are often on exposed promontories devoid of wood fuel. There are good examples on the Freycinet Peninsula in Tasmania where wave action currently is combining charcoal from forest fire debris with a mix of shells into masses that storms deposit above high water mark. Shell mounds near Weipa in far north Queensland that are up to 13 meters high and several hundred meters long were originally considered to be middens, but are now attributed to natural causes.