6
Notes for interpreting the Walthamstow marsh pollen diagram
Walthamstow marshes enjoys SSSI status and is managed as a nature reserve by the Lea
Valley Park Authority. It is situated in the east end of London and 1 encourage readers to
visit the locality.
For general interpretation the diagram is an uncorrected type. Pollens are also taken to be
of local and regional origin and not necessarily to indicate the past presence of plant types
at Walthamstow. The herbs, aquatics and spores have not ben given the attention they
deserve, they serve to illustrate the continuation of marsh/aqautic stands.
To the left of the diagram is the borehole log, note the ranges of sediments that have been
encountered which indicate changes in conditions of deposition over time.
The diagram has also been divided into zones, the boundaries of which are discussed in
the text.
At the base of the borehole log on the left you will see a gravel layer which is considered
to be the floodplain gravel deposited at the end of the last ice age.
Zone a. Alternating silts and organic clays, which is indicative of the early establishment
of floodplain channels. Declining open vegetational conditions is indicated by the overall
falling of herbs and shrubs. The early Boreal period marks the transition from the open
tundra conditions of the post-glacial age and the gradual afforestation of lowland
England. Betula (birch) is an effective coloniser of open ground (a "pioneer" plant) and
is already established and declining, giving way to Pinus (pine) and Corylus (hazel).
Zone b. The lithology shows a change from silt to clay which indicates a more stable
depositional regime. Pinus (declining) and Corylus are still the dominant taxa. The high
values of Corylus may indicate a prevalence of a hazel scrub below a canopy of pine.
The high canopy may also have affected the birch, as a light loving species, whose
decline is marked. The climate appeals to have continued to ameliorate, with the warmth
loving species beginning to make more substantial inroads with the expansion of Quercus
(oak) and Ulmus (elm).
Zone b1. Generally the same as zone b, but there are subtle differences. The sediment
changes to a more organic clay with the first appearance of Alnus (alder) and Fraxinus
(ash). The environmental situation is difficult to interpret. A tentative explanation is
offered, citing a range of possiblities. A slight thermal decline (hence the lag in oak
values) accompanied by an increased dampness brought about by local or regional
weather change, with increased flooding as evidenced by the increased organic fraction in
the clay. The presence of ash indicates sites were available that were too damp for oak
and hazel but not wet enough for pine. The first appearance of alder could also be a
pointer to increased wetness albeit temporarily.
Zone c. This is marked by the presence of a thin peat layer in the sedimentary profile,
succeeded by a "shell" layer. This zone shows the most abrupt change in species
composition and rather than a large hiatus (the new species show a quick but gradual
introduction) could be a dramatic climatic boundary. Tilia (lime) is considered to be the
most thermophilous of the post glacial invaders and is slightly preceded by the dramatic
rise of alder, which is thought to be indicative of increased climatic wetness, contrasting
to the ecological requirements of oak which dislikes wet conditions and shows decline at
the base of the zone. There is also an initial burst of Cyperaceae (sedge) and Filicales
(ferns). The shell marl may also be indicative of increased rain run-off with terrestrial