Stages Of Soil Formation Research Articles

THE ALTERATION OF PLAGIOCLASES AND AUGITES UNDER DIFFERING PEDO‐ENVIRONMENTAL CONDITIONS

SummaryThe alteration of plagioclases and augites in lithorelicts were studied in thin sections of five selected profiles which differed in their mineralo‐chemical composition and stage of soil formation. The scanning electron microscope was employed to monitor the products of alteration of the pseudomorphs.In the base‐rich environment of the Eutropept and Tropudalf, both primary minerals alter to montmorillonite with small amounts of amorphous alumino‐silicates present on the plagioclases, and amorphous materials of undetermined composition on the augite. Local desiliciflcation of the feldspars results in gibbsite.In more acid environments, as in the Dystropept, the plagioclases alter to metahalloysite and some amorphous aluminosilicates, and the augite to amorphous iron hydrous‐oxide and goethite. In a freely drained environment as in the Tropudult, the alteration of plagioclases leads to kaolinite and locally to gibbsite; all the augite is converted to goethite. No rock relicts are present in the Oxisols.The differential alteration is a function of the pedo‐environment which is determined by the rate and amount of water percolating down the profile.

Middle Pleistocene stratigraphy in south-east Suffolk

A conspicuous horizon of rubification, clay and iron enrichment, involutions and ice-wedge casts is attributed to two stages of soil formation. It comprises a rubified sol lessivé formed in warm temperate conditions, which was altered to an arctic structure soil by periglacial processes. These palaeosols indicate that the sand and gravel deposits beneath the Lowestoft Till comprise two units, and provide the basis of a revised Middle Pleistocene stratigraphy. The Kesgrave Sands and Gravels were formed during the Beestonian in a periglacial environment by a major river which drained towards the north-east. Subsequently the Valley Farm Rubified Sol Lessivé was formed during the Cromerian Interglacial, and the Barham Arctic Structure Soil and the associated Barham Loess were formed during the early Anglian Glacial. Both soils and the wind-blown sediments were formed on low relief terrace topography and are the remnants of a formerly extensive land surface. This feature was subsequently trimmed by meltwater rivers, then overlain by the Barham (outwash) Sands and Gravels and the Lowestoft Till which were deposited during the Anglian Glaciation.

Post-Glacial changes in soil profiles

In assessing the importance of soil genesis in the development of habitat conditions through the post-Glacial, we need to know first of all the sequence of stages which a soil goes through in maturing, and secondly the time required for this sequence to be completed. Estimates of the first come from studies of the processes which are involved and comparisons of soil sequences seen in the field today. Inevitably, perhaps, we know more about the early stages of soil formation on new parent material and about the mature profile than we do about the long developmental stages in between. The time scale, too, has been estimated by extrapolation from known circumstances, such as the rate of soil formation after the draining of Lake Ragunda in 1796 (Tamm 1920), but this type of estimation involves assumptions about the constancy of the processes involved; allowances for climatic, hydrologic, or biotic environmental change are difficult to make with any precision. Nevertheless, on the rare occasions when direct estimate has been possible, as for instance the series of sand bars investigated by Burges & Drover (1953) in Australia, the results indicate that our indirect estimates are at least of the right order. It appears that in temperate regions two to four thousand years are necessary for a primary soil profile to mature. This may be an underestimate for soils derived from calcareous parent material, but in what follows, reference will be mainly to non-calcareous conditions, so it is unlikely that serious error will be introduced by taking this figure. It should be noted, however, that secondary soil development can take place at a very much greater speed. The ten thousand or so years of the post-Glacial have clearly provided ample time for the primary soils to reach maturity; in fact, if the estimated time scale is correct, and making generous allowance for possibly less favourable climatic conditions in the early stages of the post-Glacial, it seems that soils in Britain could have been mature (under normal free-draining conditions) by the end of the Boreal period. By then the poorest parent materials would have developed mature podsols if they were going to, and the more base-rich ones some form of brown earth. This conclusion can only be checked by studying soils of this age which have been preserved in some way. Buried soils appear to retain their visible profile charac­teristics relatively unchanged. Soil profiles may be buried artificially or by some natural process involving the mass movement of large quantities of material; or by the formation of peat. However, the formation of peat in Boreal or earlier times implies special hydrological conditions. Nevertheless, Havinga (1963), in Holland, has recently provided indirect evidence of ‘ homogeneous forest profiles ’ under a variety of forest types in pre-Boreal and Boreal times. In some cases bleached soils had succeeded these homogeneous profiles, usually due to a change in hydrologic conditions, and he points out that a homogeneous profile is never found directly under peat, the soils under peat always being more or less podsolized.