Cylindrical structures up to 70 cm or more in diameter and 100 cm or greater in depth are found in soils on upland landscapes in south Louisiana. The cylinders are in two stratigraphic units differing in age and lithology, the late Pliocene or early Pleistocene Citronelle Formation and the overlying Sicily Island Loess (thermoluminescence-dated at 75,000-95,000 B.P.). By morphology and stratigraphy we infer that the cylinders are fossil tree-root casts preserved by pedogenesis and diagenesis. The cylinders can be divided into circumferential zones including a reduced core and an indurated rim that are morphologically, physically, and chemically distinct from each other and from the surrounding soil. In the Citronelle Formation, the rim can be subdivided into an inner rim, a black hand, and an outer rim; a transition zone between the core and the rim also may be present. Differences in time of development and lithology have resulted in differences in the morphological, physical, and chemical properties of the cylinders, but their effects could not be separated. Our model for these tree-root casts explains how they were formed and preserved.