FRAGIPAN FORMATION IN ARGILLIC BROWN EARTHS (FRAGIUDALFS) OF THE MILFIELD PLAIN, NORTHEAST ENGLAND .3. MICROMORPHOLOGICAL, SEM AND EDXRA STUDIES OF FRAGIPAN DEGRADATION AND THE DEVELOPMENT OF GLOSSIC FEATURES

The micromorphology of eluvial and glossic areas with weak soil streng th from the upper fragipan, and the upper parts of grey polygonal zone s, in some argillic brown earths (Glossic Fragiudalfs) from north-east England shows unstable void walls stripped of clay with remnants of f erri-argillans, pale grey grainy clay coatings and darker grainy coati ngs. These occur alongside loose surface residues of skeleton grains a nd thin grey fine silt coatings of quartz and muscovite. SEM and EDAX studies of coating surfaces, and optical microscopy, SEM and EDXRA of very thin polished sections, show that some grainy clay coatings form as alteration rims of ferri-argillans through localized waterlogging, iron oxide loss and micro-erosion leading to micropitting and disorien ted fabrics. The inclusion of coarse clay to fine silt-sized quartz, f eldspar and muscovite in other grainy coatings suggests either alterat ion of impure ferri-argillans or accumulation of degradational product s derived from elsewhere on void walls. This is more certainly the cas e for dark grainy coatings in layered compound illuviation coatings ad jacent to glossic areas. These fragipans are degrading from the top do wnwards by processes which are partly a consequence of the effects tha t the fragipan has on water percolation and root penetration. The dest abilization of void walls, the degradation of ferri-argillans and the remobilization of clay to form glossic features did not begin until se asonal waterlogging in parts of the upper pan was sufficient to mobili ze iron.