We have seen objects moving great distances within the Plains and coming onto the grasslands from much of the North American continent more and more over time, albeit in small numbers. However, objects move because people move them, and people rarely move the kinds of small, durable items that preserve in the archaeological record in isolation. Instead, they usually move them along with a variety of other things, including nondurable things like food. That things move with people also means that those people move, and that their knowledge – language, customs, technical information, ideology – moves, too. But objects, people, and ideas do not necessarily move together in equal proportion or with equal effect.

Dividing time into discrete chronological periods like those that organize the chapters here is a mixed blessing. On one hand, such divisions reflect real differences in the archaeological record, differences that tell us about important changes in the past. On the other, they often overemphasize those differences, as if massive changes occurred instantaneously. Looking for dividing lines teaches us to divide time into segments even though we know that it flowed continuously and it focuses our attention on archaeologically visible shifts, often blinding us to important patterns of continuity.