The physical setting

Much of the traditional literature on deserts focuses on categorizing and classifying them on the basis of such physical characteristics as morphology, process, and climate. Classification schemes abound, two of the most common being the largely topographic/structural distinction between shield-and-platform deserts and basin-and-range deserts and the distinction between aggradational and degradational deserts. The latter is based on the dominance of erosion or deposition but largely corresponds to the morphological classes as well. The shield-and-platform deserts (Fig. 2.1) are broad plains of low relief, covered with stone surfaces, sand seas, or finer materials. They tend to be areas of warm continental climates, like the Sahara or the Australian deserts. The basin-and-range deserts (Fig. 2.1), such as those in the cordillera of the Americas and in Asia, are areas of high relief superimposed on low plains and are often tectonically active. The associated climates are generally those of the mid-latitudes, with cold winters.

These distinctions are convenient, but somewhat inappropriate. They stemmed from a belief both in uniformity of process and character in the desert environment and in a clear-cut distinction between the processes and characteristics of desert and humid environments. This paradigm has been replaced by one that recognizes the tremendous diversity of desert environments and the commonalities that exist between dry and wet environments. The classical categorizations are further misleading because the desert environment is a continuum of surface types, processes, and climatic conditions, with considerable overlap between niches. Moreover, the environment is dynamic, continually changing both to attain an equilibrium with the forces shaping it and in response to changes in these forces, e.g., climatic change. This state of dynamic equilibrium reflects the integration of a multitude of forces and factors acting on a spectrum of time and space scales, including the long-term geologic past. The deserts can only be understood within this context.