III. STRATIGRAPHYIt is not usually

III. STRATIGRAPHY
It is not usually possible to excavate vertical cross sections under water, except when working in thick mud; therefore, stratigraphy can be difficult to record under water. In sand, silt, or gravel areas it is impossible to excavate
in vertical sections, so excavation strategy will need to be carefully thought out if stratigraphy is to be recorded. The merits of working over a large area and excavating systematically downward have to be considered
in relation to the difficulty of doing this evenly over the whole area. The problem with working in small grid squares (about 2m2), as discussed previously, is that the excavation ends up with a conical hole or pit, simply because the sand or silt will not hold any appreciable wall.Working along a front enables systematic recording and some degree of stratigraphy can be observed, although inevitably the working face will have slippage. The methods used will depend on the circumstances and the correct choice will only come with experience. Alternatively, careful excavation of layers is a possibility. With the judicious use of excavation tools, the excavator can remove layers over quite large areas, so for a start, the sterile overburden can be removed in one stage.
In many cases there is no stratigraphy, but rather a sterile overburden, followed by an archaeological layer, followed by a sterile layer. This is not always the case, and excavators must be cautious not to miss the subtle changes. Particularly, inside ship structures, it is possible to observe different stratigraphical layers trapped in compartments or on decks representing different phases of the wreck disintegration process. Additionally, when changes are observed, these are often difficult to record because of problems in establishing vertical datum points. This can be an extremely difficult problem and bubble tubes or depth-measuring devices will have to be used to make these measurements. These problems are discussed in Chapter 4.
It is additionally worth noting that under water, archaeological chronology can have a different significance than that for an archaeological site on land. In the excavation of a shipwreck, stratigraphy usually relates to a single event in time. Consequently, the stratigraphy may have little or no temporal significance, but it may have a particular spatial significance.Thus a shipwreck lying upright on the seabed will disintegrate in time. Any thing lying on top of another is determined by a spatial relationship rather than a temporal one. If the ship settled upright on the bottom, material would generally collapse downward and outward. If a ship sank heeled over on its port side, the guns (for example) on the starboard side would lie on top of the port guns after the wreck collapsed. By interpreting the events subsequent to the wreck, the excavator can thus determine more information about the ship. The unusual circumstance of a wreck, with the immediacy of the event, makes the spatial aspect of the site of much greater significance than the temporal aspect.This does not mean that one should ignore stratigraphy. The point is simply that the vertical component may be of no more significance than the horizontal component.As noted above, localized stratigraphy inside the structure of a shipwreck can have great significance.
Stratigraphy has played an essential part in the excavation of a number of shipwreck sites. In the IJsselmeer polder, sites can be dated using stratigraphical evidence. Because the vessels sank at a particular point in time
archaeologists can identify the stratigraphy of the IJsselmeer and thus date the event (Reinders, 1982; Reinders et al., 1978, 1984). Similar approaches have been made on the Mary Rose (Marsden, 2003) and the HMS Pandora
(Gesner, 2000). Likewise, inundated land sites have an essential stratigraphical component. In the past, stratigraphy on underwater archaeological sites has often been ignored or not properly examined. It is essential in
planning modern underwater archaeological excavation that the question of stratigraphy is taken into consideration. It is advisable to thoroughly understand the implications of stratigraphy on a wreck site as it will have quite a different significance to that of a land archaeological site.
Many other new and interesting underwater excavation techniques have been pioneered in the last few years. Some of these have been standard on land excavations for many years, but as the practice of maritime archaeology
improves, so the technology moves with the times. On the Amsterdam project (Gawronski, 1986, 1987) the excavation work has developed into a multifaceted scientific study taking into account a wide variety of excavation
strategies. Likewise, the examination of the mud in a late Saxon logboat found at Clapton shows the extent of the information that can be recovered using suitable excavation strategies (Marsden, 1989).