The processes by which a community produces its basic food supply are conditioned by climate, soil and the technological capability of society at the time. All three are variable. Climate, as we have seen (pp. 57-8) can change even over comparatively short periods, soil can degenerate and lose its nutrients through over-use, while better tools or new varieties of cereal can be introduced at any time. There is a dynamic relationship between these three constraining variables. Together they control the food producing strategy, but since the variables are constantly, if often only imperceptibly, changing, so the food producing regime is never entirely static. Another factor adds to the variety. A country such as Britain, with an immensely varied geomorphology and microclimate, is a palimpsest ofdifferent bioclimatic regions. Each provides a framework within which society has to adapt its systems. Thus, in attempting to study the way in which Iron Age communities raised their food we are dealing not with a single system but with many, each with its own trajectory of change.