- Chapter Nine -
and political change, the environmental consequences of which are examined.
These changes relate first to the geographical expansion of the Celtic world in the
first millennium Be, then to the northward expansion of the Roman Empire over
most of the area and then in the fifth century the empire's collapse, the southward
migrations of Germanic people and the resurgence in the wreckage of the Roman
Empire of pre-Roman Celtic traditions which were strongest in the peripheral
areas of Ireland, Wales and Brittany. These are issues which have traditionally been
examined through historical sources, art history, place-names, material culture,
etc. Environmental archaeology also has a distinctive contribution to make in
evaluating the degree of continuity and change over this period. Its contribution is
sometimes independent of the vicissitudes in the lives of individual settlements which
dominate many aspects of the archaeological record. It may tell us whether there was
continuity of land-use in the wider surrounding environment.
CLIMATIC CHANGE
The extent of human environmental impact by this date makes it difficult to
disentangle the role of natural factors such as climate. Even so, the first half of the
first millennium Be experienced one of the most widely accepted climatic changes of
the Postglacial, with evidence for higher precipitation and lower temperatures during
the climatic episode known as the Sub-Atlantic. This can be seen as the culmination
of a long-term climatic deterioration which began around 3000 Be. In the Greenland
ice caps changing ratios of the oxygen isotopes^160 and^180 represent a palaeo-
climatic record which shows that the deterioration was progressively more marked
between 1000 and 500 Be (Dansgaard et al. 1982). A pronounced change around 500
Be is also indicated by the oxygen isotope content of lake carbonates in Gotland,
southern Sweden, where mean summer temperatures may have been as much as
3.7°C lower (Marner 1980). Alpine glaciers re-established themselves and advanced
between 900 and 300 Be (Grove 1988). The most widespread evidence of climatic
change comes from peat bogs. The preceding Sub-Boreal climatic phase was
relatively dry, with slow peat growth and humification. Many bogs then show a
renewed and rapid peat growth recurrence horizon, known as the Grenzhorizont.
Radiocarbon dates show the onset of climatic deterioration, particularly in the more
maritime western areas, from about 1250 Be, with most of the dates from the British
Isles clustering between the eighth and fifth centuries Be (Turner 198 I). Boulton Fell
Moss in Cumbria became much wetter in the ninth century Be and a detailed study
by Barber (198 I) gives good reason to infer that changes in the growth of this bog
provide a faithful record of climatic change. Danish bogs indicate wetter conditions
from around 500 Be (Aaby 1976). In the Netherlands the Bourtanger Bog grew
rapidly as a result of increased precipitation then burst catastrophically around 530
Be, destroying a recently built trackway (Casparie 1986).
Glacier advances indicate that the temperature reduction may have been in the
order of I-2°C (Grove 1988). That change, and indeed the glacier movements them-
selves, is comparable to that which occurred in the Little Ice Age between AD 1550
and 1850, which was a partial cause of the abandonment of farms in uplands. The146