The Middle Pleistocene of north Birmingham

Abstract

The Older Drift of north Birmingham infills a system of pre-glacial valleys. Its stratigraphy has been worked out, chiefly from borehole records, showing it to comprise the deposits of two separate glaciations—the Lower and Upper Glacial Series, and an intervening Interglacial Series. It records the deposition during the Lower Glacial Series of fluvioglacial sediments followed by those of a glacial lake during the late-glacial period. A small remnant of the latter persisted into the Interglacial as a lake at Nechells and was gradually infilled with lake muds. After a temporary period of low water the level subsequently fell again, largely draining the lake which became covered with fen-wood and marsh receiving some fluviatile sediment. Other thin interglacial beds exist at Cardigan Street and possibly Washwood Heath. Elsewhere the period is represented by an unconformity. The Upper Glacial Series records the triple advance into the area of valley glaciers, accompanied by the formation of glacial lakes. In the intervening periods of retreat the valleys were receiving fluvioglacial sediment, and in one period were occupied by a second large glacial lake. A fourth advance of an ice sheet covered the area and completed the infilling of the valleys. A detailed study of the pollen and macrofossils from the Interglacial Series has enabled the reconstruction of the plant communities in the vegetation, and their succession throughout the Interglacial. This records a combination of changes due to the serai, edaphic and climatic development. Following an amelioration of climate temperate deciduous forest developed from open ‘sub-arctic’ scrub and grass/herb communities, withAlnusandTaxuslater becoming locally important when the climate perhaps became wetter. Deterioration of soils during the long period of mild humid conditions led to the subsequent spread of heath and coniferous forest, which perhaps, asAbiesforest, occupied the area during the period of optimum temperatures (summer). Later deterioration of the climate resulted inPinusforests and heaths, the vegetation becoming increasingly open as the next glaciation approached. The Interglacial Series are correlated with Holsteinian interglacial sites elsewhere in Britain and on the Continent. This dates the Lower and Upper Glacial Series to the Elster and Saale Glaciations respectively, confirming their correlation with neighbouring areas.