The petrology of the North Sea Drift and Upper Glacial Brick-Earths in East Anglia
Summary The field-evidence distinguishing the Upper Glacial Brick-earths from the North Sea Drift was summarized upon p. 150 of the paper to which this is a sequel. The included boulders, colour, absence of lamination, stratigraphical relations, included masses of other beds, height above Ordnance datum and thickness, were shown to be of value as criteria. The foregoing pages deal with the petrological aspects of the two series of deposits which have been confused with one another. It is shown that the mechanical analyses of the two series are very different. This is indicated graphically by Fig. 6. The deposits of the Norfolk coast sections are, as would be expected, very variable in grade-composition. No possibility exists, however, of confusing them with Upper Glacial Beds. The latter deposits have been confused with the Norwich Brick-earth. Mechanical analyses of a number of samples of Norwich Brick-earth from a wide area show a remarkable conformity, being characterized by a high sand-grade and low clay-grade. The Upper Glacial Brick-earths conform to two types, ( a ) that of the blue earths with a high clay-percentage, and ( b ) that of the red earths with a high silt-percentage. In either case the gradecomposition is very different from that of the Lower Glacial deposits. (Fig. 6.) The detrital mineral residues do not provide us with a means of distinguishing Upper and Lower Glacial deposits from one another. This is doubtless largely due to the facts that abundant mineral material was brought in from all sources, and that the Lower Glacial deposits in part were ploughed up, and helped to make the later glacial beds. The mineral assemblage is unsurpassed in British sediments for variety and beauty (over forty mineral species, exclusive of varieties and cementing materials, being recorded). The detrital residues are characterized by the rich colour, large size and freshness of the grains. The magnetic minerals, which are coloured, preponderate, and yield beautiful pleochroic effects. The variety is only to be expected when the mode of origin of the deposits is considered. While in most sediments minerals derived from metamorphic rocks are most abundant, in the East Anglian Drifts a considerable amount of material occurs as a result of the disintegration of igneous rocks. Among other minerals, abundant apatite, amphiboles, epidote, hypersthene, dolomite, sphene and augite were thus derived. The small compound rock-grains in the Drifts serve to distinguish the Upper from the Lower Beds. In the former, sandstone, limestone, and shale chips from British Jurassic and Cretaceous rocks are plentiful, and yield evidence, supported by the presence of erratics, of the direction of ice-flow into East Anglia. The coarse grains of the North Sea Drift consist mostly of fragments of igneous and metamorphic rocks, of lydite-pebbles, Tertiary flintchips, coprolites, chalk-pellets, and shell-fragments.