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3. THE PHYSICAL ENVIRONMENT OF THE STUDY AREA
3.1 Late Quaternary landforms and environments on North Uist
3.1.1 Most of the island of North Uist is made up of undifferentiated Lewisian
Gneiss which is a hard rock that is not easily weathered. Stretching from Loch
nam Madadh on the east coast to the island of Baleshare on the west, is a band
of metasediment including metavolcanic rock including pink granites. In the
Balranald area, a suite of metabasic rocks forming linear dykes are present and
represent a later intrusion. Glacial activity has resulted in the distribution of
till which gives rise to shallow acidic poorly drained podsols and extensive
peat development away from the calcareous machair plains on the east coast.
Large deposits of off-shore sand and boulders were deposited by the ice-sheets
onto a shallow, gently sloping continental; shelf which extends westwards
from the Uists and Barra. The sands and boulders of the ice-sheet moraines
were re-worked landwards by onshore coastal processes and a rising sea level
in post glacial times (Ritchie 1966). This produced the vast quantities of sand
and shingle used in the construction of the beaches and machair of the Atlantic
coastal plains. Low altitude sandy plains are a common feature, situated
immediately behind high sand dunes along the coastal edge of north Uists.
3.1.2 The Atlantic coast has a variety of landforms which have been affected by
continued coastal erosion and, importantly, submergence as the coastal
landscape drowned following increases in sea levels during the Holocene.
Analyses of inter- and sub-tidal organic deposits are widely distributed along
the Atlantic coast of Uist (Ritchie 1985). Radiocarbon dating of the organic
deposits indicated that sea level was 3-5m lower between 8800 BP and 5164
BP than at present. Optically stimulated luminescence dating of Aeolian sands
by Gilbertson et al (1999) indicates that carbonate sand of the Benbecula and
North Uist machairs began to arrive off-shore at about 8700 BP which is in
broad agreement with a rise in sea level and dune development. In a study at
Cladach Mor, North Uist, Ritchie and Whittington (1994) obtained
radiocarbon dates from an infilled peat basin. This work showed that the site
was first affected by a major sand blow around 7810 BP. It was concluded
that machair formation was not a synchronous event and that local topography
and, importantly, the nature and altitude of coastline and its relative position to
off-shore sediment supplies, were seen as crucial factors in the episodic
formation of dunes.
3.1.3 Research carried out by Whittington and Edwards (1997) on a series of intertidal
peat sites located on a beach platform at Borve, island of Benbecula,
provided both spatial and chronological evidence for sand movement as well
as furnishing data on the vegetational and environmental history of the area.
Pollen diagrams show that Calluna vulgaris (heather) and Poaceae (grass) are
frequent dominants, while trees such as Betula (birch) and Corylus avellanatype
(hazel) are notable. The existence of birch-hazel woodland for the period
c. 8885-6190 BP may have had a retarding effect on sand movement.
Anthropogenic clearance by fire is seen as a major mechanism for
exacerbating the movement of sand with further increases in sand movement
by c. 5990 BP. Sea-level changes, substantial coastal erosion, and numerous
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