US Geological Survey


Date of this Version



American Antiquity 75(4), 2010, pp. 984-985


Copyright 2010 by the Society for American Archaeology


We appreciate and would like to respond to the comments made by Nolan and Cook. We make three points in our reply, which are directed at clarifying our position and responding to a few of Nolan and Cook's assumptions.

First, as a matter of clarification, the Palmer Drought Severity Index (PDSI) is not a measure of summer rainfall as Nolan and Cook (2010) imply. Instead it is a theoretical measure of soil-moisture, the value of which evolves over several months in response to fluxes of precipitation, evaporation, and runoff. In practice, PDSI is usually heavily weighted toward the precipitation side of the soil water balance.

Second, with regard to archaeological facts, the known developmental trajectory of the greater Cahokia region is not quite as Nolan and Cook describe it. In their discussion of the "timing of the precocious development of social complexity" in the American Bottom, Nolan and Cook refer to the American Bottom during both the Edelhardt (A.D. 1000-1050) and Lohmann phases (A.D. 1050-1100) as being relatively wet but not the wettest areas in the Upper Mississippi and Ohio Valleys. However, Cahokia's "big bang" dates to the end of the Edelhardt phase and continued through the Lohmann phase, so the overall climatic state of the Edelhardt phase is not at issue. Nolan and Cook's Figure 2 supports the concept that the Cahokia area was extremely wet during the Lohmann phase as opposed to the Edelhardt phase.