PAST CRIMEShole, such as plant roots, fence posts or rubbish. Then the hole would be filled
in–either straight away in one action, or over time by natural silting episodes,
which might be visible in the section of the cut. By careful excavation, an
archaeologist can recognise each of these episodes and actions, and
reconstruct the whole sequence of events–the stratigraphy of your pit.
Stratigraphy helps us to understand the time sequences of a site. Soil forms
over the landscape through a series of natural processes over long periods of
time. But it can be affected by outside events–it can be eroded by wind or
water, and soil can build up due to silts deposited by floods or by material
washed down from hillsides. These episodes can often be recognised by
identifying layers of different coloured soils, and by the degree of compaction
of the layers. Generally, soil is uniformly compacted within a layer unless
there has been some sort of disturbance. This could be animal burrows,
agriculture, the digging of ditches or foundations, or the use of an area as a
roadway. Archaeologists can recognise (often by the feel of the soil under the
trowel) where these changes occur and spot marks left by tools or other
agencies such as animals, which mark the borders of each different area, and
can remove each layer very precisely, without disturbing older material below
or around it.
Soil that has been disturbed is often less compacted than the surrounding
layers, and (in Britain at least) frequently more damp, as there is more space
between the grains of soil for water to collect. Much depends on thetypeof
soil–sand, loam and clay are all very different–but in many instances,
changes can still be visible below the surface after thousands of years.
Each layer or change represents acontext. Each context is evidence of a
change in the activity in and around the soil at some time in the past. To build
up a picture of these activities, everything is carefully recorded in three
dimensions, measured, described, drawn and photographed. The process is
repeated for each layer that is exposed as the digging goes deeper into the site.
Every context is given a unique number. Measurements, drawings and
photographs are made of the vertical sides, or sections, of the excavation,
which can illustrate the stratigraphic sequences of events in that place.
On an archaeological site, many stratigraphic sequences may be recorded,
which can be combined and related to each other to give a picture of activities
across a wider area and a longer time period. The contexts can be laid out in a
table, called a Harris matrix, which can be used to establish the chronological
sequences across a site to help date them. Clearly, under normal
circumstances, the oldest events are also going to be the deepest, with more
recent activity appearing closer to the surface. (This is not always the case,