Late-Glacial deposits at Nazeing in the Lea Valley, North London

Abstract

At Nazeing, Essex, in the valley of the River Lea, a series of deposits has been investigated which extends back in time from the Post-Glacial climatic optimum to Late-Glacial times in the widest sense of that term. They are of special importance as bridging the time gap between the well-known 'Arctic Plant Bed' of the Ponder's End stage (which occurs also at Nazeing), and the established Post-Glacial vegetational and climatic sequence. Pollen analysis of the main organic beds has permitted a general dating to be given to the various stages of formation of the deposits and this has been supplemented by a radio-carbon assay. Through the gravel sheet of the Ponder's End stage a wide channel was cut which persisted with vicissitudes until Boreal time. At first this channel and the surrounding gravel plain were covered with organic muds and marls of a shallow lake containing stunted Mollusca of close affinity with those of Ponder's End, and pollen indicative of rich herbaceous vegetation and an unwooded countryside. The channel was re-cut and the mere drained, and in the bed of the new channel, above an infilling of gravel, there began deposition of organic muds and then peats, a process which continued without interruption until, after a third short phase of erosion and drying in Mid-Boreal time (zone VI), the whole valley of the Lea was converted to fen. The later Boreal and succeeding Atlantic peats (zone VII) are sealed in by river flood clay. The Mollusca (whose determination is largely that of the late Santer Kennard) retain the general character of the Ponder's End aggregate through the mere deposits, and the channel deposits up to the third erosion stage. The progress of drying of the mere is shown by the increasing proportion of land- to fresh-water shells. From the base of the channel which is referred to zone III, the close of the Late-Glacial period, the calcareous muds have yielded an extensive collection of plant remains, chiefly fruits and seeds belonging to 'arctic-alpine', 'marsh' and 'ruderal' species. Some of the species recognized have hitherto been regarded as weeds and introductions by man to the British flora; others have now a very restricted range. The pollen analyses of both the mere and channel-bottom deposits show great abundance and range of pollen of herbaceous plants, including many familiar already as Late-Glacial species and some, such as Polemonium coeruleum, newly recognized as such. It is apparent that conditions of the Late-Glacial period greatly favoured the spread of certain categories of plants, and that knowledge of this period is of the utmost importance in establishing the history of the British Flora. In the calcareous muds of the channel (zone III) and in the earlier muds beneath the lake marl the remains have been identified of four mammals of late Pleistocene character (including two species of lemming). At the close of zone III the calcareous muds yielded abundant bones of Bufo and Rana together with Lacerta vivipara. At the Early Boreal drying surface (Mx) of the channel there are also abundant land shells, and at this stage the molluscan fauna has a new and characteristic Holocene stamp, although small mammals of Late Pleistocene type survive, notably Ochotona spelaea (pika) and Microtus anglicus. A general correlation is attempted for all the phenomena described, geological, floristic and faunistic, and they are tentatively related to corresponding phenomena in Britain and north-western Europe.