DIG 2007
Excursion
Flag Fen basin and the lower Great Ouse
Saturday
April 21
Bus
departure time: 9:00
Return
time: approximately 16:00
Bus
departure and return point: Botany Gate
Entrance to the Downing Site, on Tennis Court Road
Excursion Leader: Charles French
Department
of Archaeology,
Introduction
The
field-trip will visit archaeological sites and landscapes in the alluviated and fen-edge lower Great Ouse valley at Over
Quarry just to the northwest of
The Flag
Fen basin
The Flag
Fen area has witnessed archaeological and palaeo-environmental
investigations on an extensive scale since 1970 by Dr Francis Pryor, Maisie Taylor, Dr Rob Scaife and myself and the Cambridge Archaeological Unit. On the northern, or Fengate,
first gravel terrace edge of the basin are some of the earliest field systems
in
Essentially,
the Flag Fen basin began as a small freshwater reed marsh surrounded by alder carr woodland in the earlier 2nd
millennium BC. Increasingly over that
millennium and into the 1st millennium BC, the reed marsh and
fluctuating freshwater mere expanded landward towards the Fengate
shore and encroached on a highly developed agricultural river terrace gravels
landscape. At Flag Fen we will see the sequence of later Bronze Age timber
avenues and an exposed section through the basin sequence of freshwater
alluvium and peats on glacial outwash gravels.
In addition,
if the site is still open, I hope that we can see another and newly discovered
site, Must Farm, on the southern side of this same fenland basin adjacent to
King’s Dyke at the western end of Whittlesey island, which has just been evaluated by Cambridge Archaeological Unit. It is
a waterlogged, palisaded crannog site of late Bronze
Age to early Iron Age date situated over another possible avenue of later
Bronze Age timbers, overlapping in date with the Flag Fen avenues. Like the Flag Fen site, this
site is perfectly preserved, with the crannog structure apparently burnt down in
situ and everything left as it fell, including textiles, food remains in
pots, and wooden, iron and bronze implements.
Consequently there are serious issues to be addressed regarding its
long-term preservation given its position adjacent to a massive clay (for
bricks) quarry site.
The
lower Great Ouse valley
The archaeological and palaeo-environmental work at Over Quarry, conducted by the Dept. of Archaeology’s Cambridge Archaeological Unit since 1997, is in advance of sand/gravel extraction on the former floodplain of the Great Ouse and its intersection with the adjacent fen basin (Fowlmere and Haddenham fens). It has revealed two major sets of palaeo-channels with Mesolithic to Early Iron Age occupation on the former levees and terrace edges, with this prehistoric landscape covered by varying combinations and thicknesses of freshwater peat and silty clay alluvial deposits. Palaeosols are ubiquitously found in this landscape, as well as once waterlogged fill and cover deposits. Also, the hydrological impact of long-term water abstraction has been investigated at this quarry over the last decade (French et al. 1999; French 2004).
During the
Holocene, this landscape began as a series of braided river channels with
exposed levees and marshy areas surrounded by a predominantly oak and lime
wooded landscape on the first and higher terraces to either side and in the
adjacent fenland basin (Evans and Hodder 2006). This open aspect in the Mesolithic and
Neolithic encouraged settlement on the river’s edges, with further landscape exploitation
in terms of both burial cemeteries and rectilinear field systems established in
the 2nd millennium BC. By
this time there was both marine inundation and subsequent reed peat growth in
the adjacent fen basin leading to both river avulsion and peat formation
up-valley and an increasingly high groundwater table. This dual trajectory,
combined with seasonal alluviation, containing eroded
topsoils from later prehistoric clearance and
agricultural activities inland, led to gradual submergence and burial of this
landscape with both peat and silty clay alluvium, beginning from about 1300 cal
BC, a process which continued until World War II. Since then, when the land was both drained
and turned into arable land, both the dryland and
associated archaeology has begun to re-emerge.
This
process has been exacerbated in recent years by a massive programme of gravel
extraction involving extensive water abstraction. A 10-year monitoring project has just been
completed that has monitored the full cycle of the effect of these combined
processes on the hydrological regime changes (French et al. 1999; French
2004). Quarry pumping operations lowered
the ambient groundwater table by 5 metres within a month, and dewatered an area
of at least half a kilometre in extent from the quarry face during gravel
extraction for a period of a year and a half. This led to considerable changes
in water quality in this former floodplain and potential deleterious effects on
the survival of the organic record. But clay bunding
and reedbed reinstatement of the completed extraction
area soon lead to a recovery to pre-extraction groundwater and water quality
conditions.
In our
visit to Over quarry we will look at the southern
barrow group, sections across the floodplain deposits, old land surfaces and palaeo-channels, as well as new investigative work across a
Mesolithic old surface on a levee.
References
Evans, C.
and Hodder, I. (2006) A
French, C. (2003)
Geoarchaeology in Action: Studies in Soil Micromorphology and Landscape Evolution.
French, C. (2004)
Hydrological monitoring of an alluviated landscape in
the lower Great Ouse valley, Cambridgeshire: results of the gravel extraction
phase. Environmental Archaeology 9: 1-12.
French, C.,
Pryor, F. (2001)
The
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