form, especially after the retreat of glaciers during the Holocene. Soil fossils can also exist where a younger soil has been eroded (for instance by wind), as in the Badlands of South Dakota. (One must exclude areas where present-day soils are relics of former wetter climates, as with Australia and Southern Africa. The soils of these regions are proper paleosols.) Soil fossils, whether buried or exposed, suffer from alteration. This occurs largely because almost all past soils have lost their former vegetative covering and the organic matter they once supported has been used up by plants since the soil was buried. However, if remains of plants can be found, the nature of the soil fossil can be made a great deal clearer than if no flora can be found because roots can nowadays be identified with respect to the plant group from which they come. Patterns of root traces including their shape and size, is good evidence for the vegetation type the former soil supported. Bluish colours in the soil tend to indicate the plants have mobilized nutrients within the soil. The horizons of fossil soils typically are sharply defined only in the top layers, unless some of the parent material has not been obliterated by soil formation. The kinds of horizons in fossil soils are, though, generally the same as those found in present-day soils, allowing easy classification in modern taxonomy of all but the oldest soils. Analysis Chemical analysis of soil fossils generally focuses on their lime content, which determines both their pH and how reactive they will be to dilute acids. Chemical analysis is also useful, usually through solvent extraction to determine key minerals. This analysis can be of some use in determining the structure of a soil fossil, but today X-ray diffraction is preferred because it permits the exact crystal structure of the former soil to be determined. With the aid of X-ray diffraction, paleosols can now be classified into one of the 12 orders of Soil Taxonomy (Oxisols, Ultisols, Alfisols, Mollisols, Spodosols, Aridisols, Entisols, Inceptisols, Gelisols, Histosols, Vertisols and Andisols). Many Precambrian soils, however, when examined do not fit the characteristics for any of these soil orders and have been placed in a new order called green clays. The green colour is due to the presence of certain unoxidised minerals found in the primitive earth because O2 was not present. There are also some forest