Drewett_1999_Field_Archaeology

Figure 5.12 Open-area excavation.

settlement sites, and many other types of site, open-area excavation is the best approach. For some types of site, or for preliminary evaluation, other approaches may be more appropriate but must be used carefully, selectively, and flexibly.At the end of the day a well excavated, well recorded, fully published 1 × 1 m unit does considerably less harm than a huge, badly controlled, unpublished area excavation. Time, resources and ability remain crucial elements in deciding excavation strategy.

The principle behind open-area excavation is ideally to clear the whole of a site (Fig. 5.12) revealing, recording and removing each archaeological deposit in broadly the reverse order of their deposition. There are either no, or only temporary, soil sections on site. The sequence or stratigraphy of the site is carefully recorded through continuous recording of surfaces of deposits (Chapter Seven). The size and shape of the area excavated is in no way predetermined by the field archaeologist. It will depend entirely on the size and shape of the site.As a public relations

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exercise, however, archaeologists usually edge the area with straight, vertically cut sides. To the wider public this will make the excavation look serious and not like a building site.

Generally, because of the scale of open-area excavations, deposits above in situ archaeology are removed by machine. Sometimes this is a highly efficient way to start an excavation, saving huge labour costs. On shallow, arable sites there is, however, the problem that by machinestripping all the plough soil off the site you may only be left with sub-surface features. A surprising amount of site detail can survive in plough soil and be recovered through magnetic susceptibility, phosphate analysis and careful artefact recovery. People in the past, as today, lived and undertook their daily tasks on surfaces, not down pits and postholes (Clarke 1990, 106 7). We shall return to the details of excavation in Chapter Six.

Levels of recovery

Having decided upon a broad approach to an excavation and the size and shape of holes, or the hole, to be dug, then levels of recovery should be considered. In fact this has to be considered as every single discrete archaeological entity (or context) is located whether it is plough soil, an in situ floor, or the contents of a pit. Even in total area excavation you will be sampling only aspects of the site. You may attempt to recover all walls on a site, or all potsherds (at least those visible to the human eye) but is there any point in collecting every pollen grain, even if it were possible?

Levels of recovery range from very coarse to very fine. The coarsest form of recovery is to use heavy earth-moving machinery. This can be used to locate features like walls, but virtually the entire archaeological content of the soil machined out is lost. Picking up odd bones or potsherds from machined-out soil is almost pointless as the sample will be so biased. There will, however, always be contexts which can be safely machined off, like concrete floors in an urban context, wind-blown sand on a desert site, or perhaps plough soil moved down a slope to bury an archaeological site below. In a rescue or salvage situation it may be the only way to get enough dug in the time available to make sense of the site. For archaeological work the best types of machine, some would say the only types, are those which pull soil off the site rather than pushing it off. Machines with a back actor (a hydraulic arm with bucket) are frequently used (Fig. 5.13) but any machine with a blade that can be pulled away from the archaeology could be used. Bulldozers which push the soil forward and then drive over the cleared archaeology are not to be recommended.

The next level of recovery up from machinery comprises hand-held heavy digging tools like picks, mattocks, hoes and shovels. Used skilfully these can be considered precision tools, but generally they are used for disturbed contexts rather than, for example, in situ floors. A good excavator using a pick and shovel can recover a surprisingly high percentage of the artefacts and ecofacts in a deposit. The problem is that the sample recovered will be biased in favour of the big and against the small, and in favour of colours contrasting with the background soil matrix

Figure 5.13 Coarse excavation: digging with a JCB 3c.

and against those of similar colour. From a mid-grey soil, therefore, the sample recovered may include all cattle bones but few small fish bones, and all red pottery but only big grey sherds.

A finer level of recovery will involve small hand tools like the mason s pointing trowel. Usually the excavator is crouching or kneeling when trowelling, so is much closer to the area being dug than when standing up using a pick and shovel. By being closer and moving smaller quantities of soil, artefact and ecofact recovery will increase but it will still have a size and colour bias. Depending on the nature of the context and the type and period of site, this may not be important. If you are excavating a medieval pottery kiln site, for example, and already have a million potsherds, how useful are those few extra flecks of pottery missed? However, if excavating a native Amerindian site with perhaps few surviving inorganic artefacts, greater recovery may be considered advisable.

Increased recovery can be introduced by using sieves (screens or meshes) (Fig. 5.14). These are used on a regular basis in the USA but much more selectively in Britain. This partly relates to the relative volume of material on sites in the two countries, but also goes back to Richard Atkinson s statement in his text book, Field Archaeology: A fairly coarse-meshed sieve may sometimes be needed to search for coins, beads, and other small objects, but its use should not be encouraged, as the exact original position of finds made in this way cannot be determined (Atkinson 1953). If soil is simply shovelled through a sieve, this point is worth considering, but generally sieving takes place on soil already carefully trowelled, so the objects, although moved from their exact location, would be lost without sieving. By sieving soil from discrete contexts separately, objects can be tied clearly to their context of origin.

Figure 5.16 Recovering carbonized material by water flotation.

Sieving for tiny, abraded potsherds on sites that have hundred-weights of sherds is probably pointless, but small beads and coins can be missed in trowelling. Coins, however, are often crucial for dating on some sites, so their exact location may be important. These can often be located in situ by using a metal detector across a context about to be trowelled (Fig. 5.15). The location of metal readings, which may be coins or other datable metalwork, can be carefully marked so the troweller can look even more carefully in these areas. Sieving is, however, essential in reducing bias in bone assemblages, especially in the case of fish and small mammal bones. For these bones a 1 mm mesh size should be used. This may require wet sieving to get the fine soil particles to go through the sieve.

The next level of recovery will involve wet sieving or water flotation (Fig. 5.16). Wet sieving can be used to recover any small material, but water flotation is usually used to recover carbonized

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Figure 5.17 Recovering carbonized material with a froth flotation unit.

seeds and charcoal. By sieving in water you will reduce the abrasive effect of dry sieving. Lithics will not be re-touched and organics will float to the surface of the water where unabraded charcoal, for example, can be collected.

The process of recovering organic material can be greatly accelerated by using a water flotation unit. Evolved by Gordon Hillman to recover plant remains in the Near East (Williams 1973), it can be run either by using mains water or by recycling the water using a pump. The flotation unit consists of a tank of water with soil held in a 1 mm mesh in the top of the tank. Water is pumped through the soil, breaking it up and releasing organic materials like seeds and charcoal. This light fraction flows over the lip of the tank to be collected in a nest of sieves. The water is then either passed through resettling tanks for recycling, or discarded if mains water is used. A modification of water flotation using chemicals to break up soil and create a froth was developed, but is now less used than simple water flotation (Fig. 5.17).

The next level of recovery involves attempting to recover very small materials or those not visible to the human eye. This can be done only by taking bits of the site away (as soil samples) and undertaking extraction under laboratory conditions. This is how land mollusca and pollen are recovered, both being identified and counted under a microscope. Clearly, at this level sampling is crucial, as not all soil can be removed from a site. Generally these samples should only be taken from dated, uncontaminated, sealed deposits. Buried land surfaces are particularly suitable for such sampling (Fig. 5.18).

Figure 5.18 Sampling for pollen.

The final level of recovery is locating the chemical traces of artefacts or ecofacts which simply do not exist any longer.Aburied body, for example, may survive only as a concentration of chemicals, including phosphorus, in the soil. At every stage of an excavation the director or delegated site supervisors must make informed decisions about the level of recovery appropriate to a particular context. Remember, however, that whatever you do, you will be recovering only a sample.

Figure 6.1 The layout of an excavation. Note particularly the position of soil dumps (spoil heaps).

multi-season excavation to resolve the soil dumping problem. When excavating the top of the motte at Lewes Castle, Sussex, for example, not only did the soil have to be kept on the top of the motte, but also half the top had to be kept accessible for tourists paying to visit the castle. The area excavation therefore had to take place over four seasons. Each season one quadrant was excavated, while another quadrant was used for soil dumping. The remaining two quadrants remained open to the public. Each year the excavation and spoil heap moved around one quadrant (Fig. 6.2). The location of site offices and facilities also has to be considered in relation to both the excavation and spoil heap, and also access into and out of the site for site huts, and so on. Usually these temporary buildings are best situated as near as possible to the road access.

The physical layout of the excavation will depend very much on how the site is to be dug. Do not expect machine drivers to be able to see thin string. If using machinery, it is often best either to lay out the trench roughly, and then cut back to its final shape by hand, or to dig a clear spade-width trench inside the edge of the trench as a marker. Rough shapes for machining can be marked with sand, spray paint or ranging poles. If hand-digging a trench, then the edge should be clearly marked with string held down with 6-inch (15 cm) nails larger in sand. Remember that if you put a nail in the corner of a trench and then dig up to it, it will fall out. This problem can be avoided by placing the nails outside the trench and making sure the strings cross at the trench corner (Fig. 6.3). The edge of the trench can then be started with a spade to mark the vertical cut. Finally the excavation is under way.