Geoscience Reference
In-Depth Information
since the end of the Devensian Ice Age 11.5 kyr ago. The
wealth of soil, pollen and archaeological studies on the
North York Moors permits the reconstruction of a detailed
picture of the environmental and cultural aspects of its
prehistoric past (Spratt 1993).
Figure 23.16
shows the
basic chronology for the Moors and
Figure 23.17
is the
Fen Bogs pollen diagram produced by Atherden (1976),
showing its generous six radiocarbon dates, associated
cultural periods, and local pollen assemblage zones
(FB1qFB10).
CWM IDWAL
NANT FFRANCON
POLLEN
m
3,000
1
2
3
4
4,255±50
m
1
2
3,500
6,880±110
3
4
7,000
5
6
10,080±220
5
Pollen Zone III
'Loch Lomond Advance"
glaciers
7
8
9
10,700±400
11,500±400
6
Pollen
Zone II
Pollen
Zone I
Late Devensian glaciers
0
100%
Chronology of vegetation and human cultures
At the end of the Late Devensian cold stage the landscape
resembled tundra grassland and heath, with dwarf trees
and shrubs only in sheltered valleys. Rapid amelioration
of climate since
c.
11.5 ka
BP
led to colonization by a
succession of forest trees, starting with pioneers like
birch, willow and hazel, followed by pine, and later by
elm, oak, lime and alder. The record of these changes
across northern Europe is recognized by a scheme of
Late Glacial and Holocene pollen chronozones (
Figure
23.18
). Although floristic composition and radiocarbon
dates show that assemblages varied in detail and were
asynchronous from place to place, there is as yet no
comprehensive
Sphagnum peat
Coarse detrital mud
Gravel
Lacustrine/varved
silty clay
Wood peat
Fine detrital mud
Aboreal (tree) pollen
Non-aboreal pollen
Radiocarbon age
Approximate age
Ffrancon, recording the infilling of their late glacial basins well
into the Holocene.
Source: Addison (1999)
buried beneath Bronze Age barrows. Dimbleby (1962)
studied pollen from these fossil soils and deduced a
mixed deciduous tree cover when the monuments were
constructed (see later in this chapter).
Peat preserves a variety of plant material which can be
used to reconstruct the past vegetation cover, such as
tree stumps, wood remains, fruits, seeds, pollen grains
and fern/moss spores, but microscopic pollen grains
and spores are the most abundant and useful sources
of evidence (
Plate 23.15
).
Palynology
is the science of
collecting, isolating, counting and interpreting pollen
and spores. Over thirty sites have been subjected to
palynological investigations on the moors and their
immediate surroundings (Atherden 1999). Most are on
the northern sandstone moors where acidity and wetness
favour the preservation of plant macro-fossils (e.g. tree
stumps) and micro-fossils (e.g. pollen). Additional
evidence comes from the humification stratigraphy of
peat, indicative of the prevailing climate: darker peat is
more humified and reflects a drier climate, whereas
lighter-coloured, brown peat is fibrous and less humified,
accumulated in a wetter climatic phase with higher water
tables (
Plate 23.16
).
The study of Fen Bogs illustrates the science and art of
pollen analysis. Fen Bogs is a valley mire at the head of
the famous Newtondale glacial overflow channel, at an
altitude of 164 m OD. Over 11 m of peat have accumulated
18
O chronostratigraphy for correlation.
Sampling at Fen Bogs was carried out through 9.6 m of
peat, with accumulation starting during the Boreal Pollen
Zone V, when birch was the dominant tree, with pine and
more thermophilous trees like elm. Openness of the
woodland is indicated by the significant micro-fossils of
heather, grasses, sedges, roses, aquatics and ferns. Zone VI
sees the decline of birch and increasing importance of
pine, alder and hazel. In this, and the succeeding Zone
VIIa, non-arboreal pollen is
30 per cent, showing that
the woodland had reached its maximum extent by 7000
BP
, when the climate was 1-2
C warmer than today. In
contrast to the wildwood of lime for much of lowland
England discussed in
Chapter 20,
here it is mainly
oak-alder-hazel, probably reflecting the upland location.
Pollen from heather, grasses and ribwort indicate the
openness of the woodland, which presumably was even
more open at higher elevations due to exposure, thin soils
and frost hollows.
Prehistoric people had an impact on the vegetation
cover of the North York Moors from the start of the
Holocene. Birch trees were used to construct a platform
at the famous early Mesolithic site of Star Carr (Vale of
Pickering), and reed swamps were burned around the
edges of lakes. In the late Mesolithic, fire was used to create
better habitat for hunting and gathering, as indicated by
