ABSTRACT

The term Paleosol is vague and generic. Actually two principal kinds must be distinguished. Polyphasic soils or Vetusols. The term Vetusols was recently proposed (Busacca and Cremaschi 1998). They have the ancient features that do not correspond to present-day environmental conditions. Vegetation, climate and hydrologic regime have changed over time. But these soils continued to evolve till now and, therefore, also have features in equilibrium with the present conditions. Fossil soils are generally buried and their evolution has been blocked abruptly. This could be related to various phenomena: • aeolian (deposition of loess or sands on an existing soil); fossil

soils are found in the centre of sand dunes; • fluvial (covering of alluvial soils by new additions of sediments);

it is a very classic case, especially in the mountains because the buried organic materials are not mineralized, which enables later identification of the ancient surface of the soil(s); there will be nearly 1200 superposed Paleosols in the Eocene formations of Wyoming!