Geoscience Reference
In-Depth Information
The coastal stretch south of the Portsdown Ridge marks the western end of
the South Coast Plain. The bedrock consists entirely of gently folded Early Tertiary
mudrocks and sandstones, but there is a widespread cover blanket of younger river-
terrace sediment, including much fine-grained material often referred to as brickearth.
The highly indented coastline around Portsmouth Harbour (Fig. 121) was formed at the
end of the last cold phase of the Ice Age, as the rising water flooded the low-lying land
and created broad expanses of sheltered water, edged by a mix of mudflats, marshes,
wetland scrub, low-lying fields and the occasional creek.
FIG 122. Hurst Castle Spit (Fig. 116, e2 ). (Copyright Dae Sasitorn & Adrian Warren/
www.lastrefuge.co.uk)
The sea-level rise also created drowned valleys such as Southampton Water ( e1 ).
Today this body of sea water receives water from the rivers Test, Itchen and Hamble,
but during the last cold spell of the Ice Age, Southampton Water was the trunk river
valley into which the present rivers flowed as tributaries. When it filled with the rising
sea water it became the estuary on which Southampton grew to become one of Bri-
tain's premier ports for international shipping.
The gravel beds of southward-draining rivers provided much of the material that
now forms the large number of gravel spits, bars and ridges that are such a feature
of this area (e.g. Calshot Spit, Hurst Castle Spit ( e2 ) and Poole Harbour ( c1 )). These
coastal features have been shaped by storm waves and the strong tidal flows that are
active in these coastal waters.
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