Tutorial:Midden

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Middens
Middens are dumped accumulations of the remains of human occupation including ash, charcoal, stone tools, ceramic, slag, construction materials, seeds, shells and animal bone that take the form of a mound or a layer in a stratigraphic sequence. Middens can be classified by their most common ingredient such as a shell midden, which is often found in coastal areas, or an ash midden, comprised of hearth layers.

Midden stratigraphy
Middens can be stratigraphically complex with abundant layering of greater or lesser amounts of different components such as shell or bone in a varying darker and lighter matrix.

Ash middens
Fragments of hearth layers are commonly mixed with bone, coprolites and local sediments to form ash middens. Within them the ash is variously preserved in patches or is vestigal, according to local soil conditions. In the latter, especially in the case of grass ash middens, relic ash material has a delicate, yellowish, phosphatic fabric, characterised by many phytoliths.

Shell middens
Shell middens are usually located adjacent to aquatic environments. Sites, which contain shell, have: Thus the stratification of a shell midden will result both from initial deposition and post-deposition effects.
 * 1) Increased porosity, permeability and alkalinity.
 * 2) Low densities of historically diagnostic artefacts.
 * 3) High probability of being saturated by the adjacent body of water. Water table rises cause saturation and chemical alteration of the midden.

Midden analysis
Some of the elements in a midden can be readily identifiable in the field and can be studied macroscopically or with a binocular microscope under lower magnification. In thin section, mollusc shells are easily identifiable by their external shape, crystalline internal fabric, and the high birefringence (calcite or aragonite). This association of elements, the overall angularity of the fragments, as well as the loose, porous nature of the deposit, set middens apart from natural death assemblages that may have been thrown up on a beach or left by sea birds.

Post-depositional processes
Post-depositional effects are poorly discernible in the field. In thin section, the activity of earthworms and the translocation of charcoal are evident. In secondary midden deposits, the admixture of natural soils and specific pedofeatures may indicate manuring for cultivation. In the case of grass ash middens, the midden material becomes diluted by soils and inclusions such as ash are dissolved.

Precipitation or streams can erode a shell midden while groundwater can produce chemical weathering. Stein (1992: 137-8) working at British Camp Shell Midden on San Juan Island in Washington recorded organic matter and clay hydration where the shell midden intersects the water table causing the colour of the organic matter to darken, and the consistency of the organically draped clays to feel greasy. The hydration provides the soil solution in which weathering occurs, and the vehicle through which dissolved ions are leached.

Organic acids released during the decomposition of organic matter dissolve the carbonates of the shell in the midden. The dissolved carbonates are flushed from the shell midden as the water table flows along the hydraulic gradient. The archaeological record changes from an alkaline shell-bearing site to an acidic non-shell bearing site.