London and the Thames Valley Region - Southern England - page 201

At least nine distinct gravel terraces, each at a different height above sea level,
have been identified in the Thames Valley generally (see Area 11), and attempts have
been made to link these terraces to the climate record inferred from oxygen isotope
data (see Chapter 2, Fig. 13). The dating of individual terraces remains controversi-
al, but a recurrent cycle of climate change and terrace formation is generally assumed,
consisting of alternating cold and warm episodes:
Cold episodes: High river flow rates, at least during spring melts, lead
to down-river transport of coarse gravel sediment and channels with high
erosive power. The river is able to build up gravel on its floodplain or cut
downwards into its floodplain, depending on elevation, which is determined
by the sea level at the time.
Warm episodes: Lower flow rates and less powerful rivers, associated with
temperate climates, deposit finer-grade sediments. A relative rise in sea level,
due to the warming climate and melting of ice sheets, results in flooding of
the lower parts of river valleys and the deposition of estuarine and marine
sediments. The lower reaches of the river become clogged up with silt. As
the next cold phase begins, there is an increase in river discharge and erosion,
and the river begins to cut down into its floodplain.
In addition to the vertical movements of the river and floodplain surface described
above, the upper Thames has also switched its course back and forth across the Vale of
Oxfordshire, constrained by the Cotswold Hills to the north and the Midvale Ridge to
the south. The gravel terraces are therefore widespread, distributed horizontally as well
as vertically, and their ages vary.
The River Thames now drains eastwards along the Oxfordshire Clay Vale before
turning north and then south as it cuts through the Midvale Ridge at Oxford, entering
the Vales of White Horse and Aylesbury. In this area, the patchy nature of the oldest
(and highest) Quaternary gravels has, in places, protected the underlying Oxford Clay
from further erosion, resulting in a series of scattered tabular hillocks rising above the
present-day floodplain. The Thames then cuts through the Chalk Downs at Goring Gap
( c1 ; Fig. 191), which was probably initiated when a tributary of the middle section of
the Thames cut through the northern Chalk rim and intercepted the drainage of the an-
cestral northern clay vales. It is generally believed that the Thames was constrained
within the Goring Gap half a million years ago, when its course much further down-
stream in the St Albans area was deflected southwards to its present position by the
arrival of the Anglian ice sheets.