Briefly Describe And Explain The Geological Evolution Of Southeast England Since The Late
Jurassic Times
Before describing and explaining the geological evolution of Southeast England it is worth defining
what is meant by ‘Southeast’. The region could be defined using administrative boundaries or geological
structures, as there isn’t a standard definition. The nature of this essay, however, suggests that the
definition should be based on geological structures, although there is debate on exactly which structures
constitute the Southeast. The ‘Southeast’ in this essay is based on the definitions used by David K C
Jones (1981) and Andrew Goudie (1990). Figure 1 (below) shows the main structures to be the London
Basin, The Weald and The North and South Downs. The region is bordered by the Chilterns in the North,
The Hampshire Downs in the West and the English Channel and North Sea coasts to the South and East.
The Wessex Downs and Hampshire Basin have been excluded as ‘Southern’ rather than ‘Southeastern’.
[Figure 1: S E England. Jones, 1981: p2]
A chalk escarpment, known as the Chilterns, runs from the northeast of the region towards the
southwest. At the Berkshire Downs (See Figure 1) it splits in two and one arm continues southwest
towards Dorset and the other, known as the Hampshire Downs, curls around to head southeast. Valleys,
where rivers have cut through the Chalk, break the escarpment in places, a good example of which is the
River Thames at Goring, however, most of the gaps in the escarpment are now dry valleys. The North
Downs and South Downs are chalk escarpments that border The Weald. There are no superficial deposits
and few trees on the South Downs, whilst the North Downs’ escarpment has superficial deposits and is
well wooded. The dipslope consists of upper chalk whilst the scarp face consists of middle chalk
(Trueman, 1971). The scarp faces of both the North and South Downs face the Lower Cretaceous clays
and sandstone of the Weald. The Weald is an anticline that was upfolded in the Late Cretaceous and
early-Tertiary times (Goudie, 1990) and was previously covered by layers of Chalk. However, denudation
has eroded the chalk and exposed older rocks including Wealden Sands, Weald Clay, Lower and Upper
Greensand and Gault Clay. The final part of the region is the London Basin, an eastward plunging
syncline, which is covered by rocks younger than the Chalk. Reading Beds lay directly on the chalk, but
1
folding has meant that they now form a narrow ring around the younger London Clay (Trueman, 1971). In
the very middle of the basin, however, are the Bagshot Sands (See Figure 2).
[Figure 2: The Rocks of the London Basin. Trueman, 1971: p79]
At the end of the Jurassic, the area now known as the British Isles was part of a larger
landmass that consisted of what are now the continents of Europe, Asia and America (see Figure 3 below)
and was almost completely submerged by water. A plate margin that formed between North America and
Europe began to open to form the Atlantic Ocean (Ziegler, 1982 quoted in Duff, 1992) and the area was
subject to significant tectonic activity. The region was approximately 10 or 12 degrees further South than
today and the climate may have been similar to that of a tropical rainforest. The fact that plant remains
found in Tertiary sediments compare with the present day vegetation of Indonesia and Malaysia (Read &
Chandler, 1933 quoted in Goudie, 1990) seems to suggest that this was the case.
[Figure 3: Plate Tectonic Evolution of the North Atlantic. Goudie, 1990: p 28]
At the beginning of the Cretaceous a combination of a fall in the sea level, possibly associated with the
widening of the Atlantic, and an increase in tectonic activity made much of England and Wales land.
However, southeastern England, including the Weald, was low-lying at this time and remained submerged
as an inland fresh water lake. The deposition in the lake formed Wealden Sands and Weald Clay. Sea
level rises 30 million years later submerged much of England once again and the region was subject to
marine deposition. This initially consisted of silt and mud, but later sands containing glauconite, now
known as Lower Greensand, were deposited followed by Upper Greensand and Gault Clay (Rawson in
2
Duff, 1992). The Upper Cretaceous period saw continued tectonic activity and sea level changes and it is
likely that nearly all of the British Isles were submerged for much of this time. It was during this 60 millionyear period that Lower, Middle and Upper Chalk were deposited, in what is now believed to be a shallow
sea, to cover an area that extended over much of Northwest Europe (Trueman, 1971). Chalk is made
from a mixture of microscopic fossils, fine calcareous matter derived from the disintegration of planktonic
algae and in Upper and Lower-Chalk, fragments of shells.
Tectonic activity during this time and into the early Tertiary saw the North and West of Britain
uplifted, possibly as a response to “a compressional pulse from western Tethys” (Rawson in Duff, 1992:
p356). The result was that the sea retreated, chalk deposition stopped and basin inversion slightly uplifted
The Weald, whilst the London Basin gradually subsided (Rawson in Duff, 1992). Almost immediately, the
Chalk was subjected to subaerial denudation and it is likely that large areas were removed quickly to
expose the underlying rocks. The East-West folding that took place as a result of the collision of the
Eurasian and African plates; and the formation of the Alps, further developed the Weald and London
Basin. Goudie (1990) also suggests that continuing sea floor spread in the Atlantic contributed to the
folding. The effects felt in S. E. England have been described as “the outer ripples of the Alpine Storm”
(Quoted in Trueman, 1971: p55). Further tectonic activity throughout the Paleogene and into the
Neogene caused more folding, so that The Weald is more appropriately an anticlinorium. This tectonic
activity also caused isostatic and eustatic changes. Jones (1981) suggests that the Anglo-Paris-Belgium
Basin, bordering southern England, was a shallow sea and that the transgressions of the sea, particularly
the Palaeogene Sea, from this area were responsible for the laying down of the Reading Beds and
London Clay in the London Basin. Figure 4 shows a section of Southeast England with Cretaceous and
Tertiary deposits and the possible extent of the ‘Wealden dome’.
[Figure 4: A Section of Southeast England. Trueman, 1971: p53]
The Miocene and Pliocene periods are marked by the general withdrawal of the sea from the
British Isles and a gradual reduction in temperature. Consequently there are no Miocene and few
3
Pliocene rocks in the British Isles (Whitten & Brooks, 1972). During this period, rivers shaped
southern England as the withdrawal of the sea allowed the extension eastwards of two major consequent
streams, namely the proto-Thames and Solent (Jones, 1981). The Weald was particularly affected as
much of its Chalk dome was eroded by the action of rivers. Water drained from the top of The Weald,
either North or South, along the East-West running Tertiary folds (Goudie, 1990). As the water ran down
the sides of The Weald it cut through the chalk to form valleys, eventually forming the escarpments known
as the North and South Downs. Once the rivers had cut down to the Lower Cretaceous rocks, the
drainage pattern was extended by the develoment of subsequent streams. Jones (1981) describes this as
an important period as the subaerial processes removed the Palaeogene sedimentary cover to reveal
Chalk. Towards the end of the period, southeast England was subject to two marine transgressions, the
Lenham Sea incursion and the Plio-Pleistocene transgression. Jones (1981) argues that the
transgressions were short and that the drainage patterns of the region were unchanged when the seas
withdrew.
The gradual cooling of the climate during the Pliocene eventually led to the Ice Ages of the
Pleistocene (Quaternary). The Ice Ages and periods in between effected the region in two ways, although
the glacier only partly advanced into the London Basin (See Figure 5 below). Firstly, there were related
eustatic and isostatic changes. The large amounts of water frozen into ice depressed sea levels, whilst
the subsequent melting produced an increase. The heavy ice also forced the land downwards. When the
sea level dropped, the rivers in the southeast of England rejuvinated, whilst sea level rises caused
deposition of Plateau Gravels. These two processes working in sequence created the river terraces, such
as the Winter Hill Terrace, that we can see today (Jones, 1981). In the North of the region, the Anglian
Ice sheet margin advanced into the London Basin. The effects of glaciation in the southeast were
primarily in the changes of land and sea level. However, it has been suggested that the course of the
proto-Thames was driven southwards from the Vale of St. Albans towards the Finchley depression when
its way was blocked by an ice sheet (Trueman, 1971). The glacier also caused the chalky boulder clay
deposited in the northeast of the London Basin. Another deposit associated with the Quaternary ice ages
is ‘Clay with Flints’ which is thought to be decalcified Chalk Clay “rearranged by local snow or ice towards
the (local) commecement of the ice-age” (Sherlock, 1962: p43).
4
[Figure 5: The Limits of Glaciation in the British Isles. Goudie, 1990: p37]
Deposits since then to the present day have been predominantly fluvial in the London Basin and include a
series of gravel and loams that have begun to form terraces, an example of which is the Taplow Terrace.
Approximately 145 million years have passed since the end the Jurassic and the present day.
The southeast of England has changed beyond all recognition due to the effects of plate tectonics, climate
change, sea level changes, rivers and ice. The geological evolution of the region, however, is continuing
every day and although the changes may not be noticeable within a human’s life times, it is certain that
the processes that have worked over the last 145 million years will continue to work over the next.
[1600 Words]
5
Bibliography
Duff, P & A.J. Smith (1992)
Geology of England and Wales 651pp London: Geological Society
Goudie, A (1990)
The Landforms of England
Oxford: Blackwells
And Wales 394pp
Jones, D.K.C. (1980)
The Shaping of Southern
London: Academic Press
England 274pp
Jones, D.K.C. (1981)
The Geomorphology of the British
London: Methuen
Isles: Southeast and Southern
England 332pp
Sherlock, R.L. (1962)
British Regional Geology: London
London: HMSO
& Thames Valley 62pp
Trueman, A.E. (1971)
Geology and Scenery in England
Harmondsworth: Penguin
and Wales 400pp
Whitten, D.G.A. & J.R.V.
The Penguin Dictionary of
Brooks (1972)
Geology 511pp
6
Harmondsworth: Penguin