The Downs surrounding Winchester are made of chalk laid down during the
Cretaceous period some 75 -100 million years ago. The chalk is highly permeable
and even heavy rains result in little surface run off, so the valley sides
have no surface streams. Instead the chalk acts like a giant sponge and
water drains through to natural aquifers, where the chalk is completely
saturated. The water is released at a steady rate from springs when the
chalk meets underlying non-porous rocks or where the Water Table reaches
Superficial deposits, clay, sands, gravels and silt overlie the chalk
on the valley floor and in dry valleys on the valley sides. These are Quaternary
deposits, eroded and laid down during the Ice Ages spanning the last 2.5
million years. The ground was frozen making the chalk impermeable but during
interglacial's, and seasonally, large amounts of surface water carved out
the pattern of branching valleys on the chalk slopes.
The spring fed River Itchen channel was cut during the later stages of
the Ice Age, and partly filled with gravel, sands and silt, as water levels
rose post glacially. This accounts for the multi braided meandering channels
and the formation of low gravel terraces at the edges of the floodplain.
The valley cuts through the chalk water table so there is a continuous
base water flow into the river. The chalk streams on Clausentum Fen, although
re-shaped by man, are part of this system.
The Fen, directly beneath the triple, branched dry valley of Stanmore,
is situated below the edge of a gravel terrace, on the river alluvial deposits,
but about a metre higher than the lower floodplain. Ground water flows
within the chalk, and particularly in times of heavy rain, will be channelled
down the slopes and may still seep into the Fen along the western edge.
The southern Fen, in places has deposits of silt more than 20 feet deep,
where there was once a large mere.
The map is from the Geology of Britain Viewer (British Geological Survey
materials - NERC 2017)