FRAGIPAN FORMATION IN ARGILLIC BROWN EARTHS (FRAGIUDALFS) OF THE MILFIELD PLAIN, NORTHEAST ENGLAND .2. POST DEVENSIAN DEVELOPMENTAL PROCESSES AND THE ORIGIN OF FRAGIPAN CONSISTENCE

Pedogenetic processes following a permafrost stage of development in f our argillic brown earths with fragipans (Glossic and Ochreptic Fragiu dalfs) are investigated by soil microscopy, including SEM observations of unimpregnated soil material, and by determination of clay mineralo gy. Micromorphology of the apparently massive fragipan confirms both t he presence of subhorizontal fissures infilled with illuvial deposits separating dense lenticular structures formed by ice-lens growth, and former vertical fissures formed by dessication on freezing with subseq uent widening by ice-vein development. The firmness and brittle failur e of the fragipans are attributed to a closely-packed, well-graded mat rix of skeleton grains bonded by clay bridges consisting of non-swelli ng illite and chlorite and to weak interconnection of macrovoids. Weak cementation is rejected as an explanation of fragipan consistence. Se quential stages of particle translocation have been critical to fragip an formation. High bulk density is attributed not only to irregular co mpression and contraction of the soil matrix under permafrost conditio ns but also to the infilling of voids by illuvial silt and clay. The f ull development of the fragipans has depended on Flandrian clay migrat ion which has contributed to clay-bridge formation, void infilling and localized seasonal impedance of drainage resulting in eluviation of i ron oxides, the development of grey polygonal patterning, and processe s of degradation at the pan surface leading to glossic features.