The Origin and Development of Brown Earths on Clay‐with‐flints and Coombe Deposits

Abstract

Field investigation of the origin and development of three brown-earth (sol Zessiwk) profiles has been supplemented by mechanical and mineralogical analyses and micromorphological studies. Two profiles, representative of the Batcombe and Winchester series, were on Clay-with-flints and the third, of the Charity series, on a Coombe deposit.Particle-size distribution and petrographic data strongly suggest that all three soils are derived in part from loess. The Charity is interpreted as an autochthonous profile formed in a Head deposit composed of chalky and flinty detritus mixed with loess by soliflwion, whereas the Batcombe and Winchester are twostage profiles, formed in composite (non-uniform) parent materials resulting from the superficial incorporation of loess with the truncated or redeposited remains of previously weathered horizons with rotlehm and/or braunlehm fabrics. These Clay-with-flint substrata contain materials derived from Chalk and Eocene beds in varying proportions, weathered in Tertiary or interglacial periods and rearranged by periglacial agencies. Clay-with-flints sensu stricto, as represented in the Winchester subsoil, has distinctive physical, mineralogical, and micromorphological characteristics, and appears to have originated by sub-surface solution of the Chalk and illuvial accumulation of clay derived for the most part from overlying deposits.The land surfaces concerned have probably been subjected to at least one alternation of periglacial and temperate conditions after the addition of loess. Hence the extent to which either profile reflects the influence of the contemporary environment is not readily assessed, but evidence is adduced that the upper horizons of each profile bear the impress of similar pedogenic processes, including eluviation of clay-size material and acidic weathering leading to the accumulation of vermiculite, modified in the Winchester by the effects of erosion and soil creep. (1949), the major group of brown earths or brown forest soils comprises well-drained and moderately well-drained acid mull soils, occurring naturally under deciduous forest, and characterized by brown, friable, sub-surface horizons which are leached of carbonates and show little or no differential eluviation of sesquioxides. This group includes braunerde Kubiena, 19 ) or sols bruns acides relatively young land surfaces on silicate rocks or their clastic derivatives, to ether with texturally differentiated profiles conformin ABC profiles typical of grey-brown podzolic soils or parabraunerde (Tavernier and Smith, 1957; Kubiena, 1956a) but those associated with Clay-with-flints and other high-level deposits normally have thick, intensively weathered subsoils, resembling those of red-yellow odzolic soils, within which no well defined B and C horizons can ge identified. In general, on upland surfaces little affected by recent * Present address : AS defined by Robinson

Changes in soil chemistry accompanying acidification over more than 100 years under woodland and grass at Rothamsted Experimental Station, UK

“…The soil at Rothamsted Farm, the Batcombe Series, is a Chromic Luvisol according to the FAO classi®cation (Avery et al, 1959;Avery & Catt, 1995). There is an Ah horizon (0± 15 cm) of very dark greyish brown,¯inty clay loam containing 22% sand, 52% silt, 26% clay and 3.6% organic carbon (OC); an Eb/Bw horizon (15±30 cm) of brown,¯inty, silty clay loam containing 17% sand, 52% silt, 31% clay and 1.0% OC; an Eb horizon (30±41 cm) of brown,¯inty, silty clay loam containing 18% sand, 58% silt, 24% clay and 1.2% OC; and a 2Bt(g) horizon (41±91 cm) of reddish brown to yellowish red¯inty clay, with greyish and reddish mottles becoming more prominent with depth, containing 9±17% sand, 25±28% silt, 55±66% clay and 0.5±0.7% OC.…”

Partial silicification of chalk fossils from the Chilterns

“…The occurrence of chalcedony in the Chalk of the Chilterns has been suspected since Avery, Stephen, Brown & Yaalon (1959) reported rod-shaped and botryoidal fragments in Chiltern soils containing insoluble residues of the Upper Chalk. Soils of the Batcombe series, which are developed in a thin silty drift (loess) overlying and incompletely mixed with plateau deposits mapped as "Clay-with-flints" or "Pebbly Clay and Sand" by the Institute of Geological Sciences, commonly contain tabular fragments of chalcedonic silica as large as 40 mm across and 5 mm thick.…”

The Petrography and Origin of Deposits filling Solution Pipes in the Chalk near South Mimms, Hertfordshire

“…Deposits immediately overlying the Chalk and filling solution cavities in it have been studied petrographically in Great Britain by Avery et al (1959), Hodgson, Catt & Weir (1967) and others; this work has proved useful in tracing the origin and development of superficial deposits, which constitute many of the chalkland soils. The chalk quarry at Castle Lime Works (National Grid Reference: TL 230025) near South Mimms (Herts.…”