Soil genesis in south-west Dyfed, Wales

Abstract

A study of soil genesis in a small area of south-west Wales is presented. The soil-forming factors are considered first in the context of time. The evolution of the factors and the importance of past environments and conditions in affecting these factors, and hence less directly the present-day soils, is emphasised. The soil-forming factors are also examined in the spatial dimension. Two of these factors - "topo-drainage" and "parent material" - are particularly important as determinants of soil type and are plotted on the Pedogenetic Map. The Pedogenetic Map is a map of the soil-forming environment and not of soils as such. Its relationship to the soils of the study area is discussed, however. Soil types are also described according to the new classification (Avery, 1973) of the Soil Survey of England and Wales, and selected properties are examined. The second part of the thesis concerns itself principally with the soil sand fraction. The lithological composition of the sand fraction is investigated, and it is shown that the coarsest sand fraction {2,000pm - 1,200pm) can be used as an indicator of the provenance of the soil "parent material". Contamination of finer sand fractions is also encountered and this is pursued in the analysis of detailed size distributions of the soil sand fractions. These studies also suggest a mechanism of weathering by surface granular disaggregation of sand-sized red silt- stone fragments in the soils. Additional evidence is given for this mechanism from scanning electron microscopy, followed by a brief discussion of the theory of grain size distributions. Finally, the different sources of information are drawn together in an attempt to effect a reconstruction of the environment that has influenced soil genesis in the area.; [brace not closed]