Geoscience Reference
In-Depth Information
To the south, where the Chalk scarp of the Hog's Back meets the lower-lying
Heaths, water stored within the higher Chalk strata escapes to the surface in a series of
freshwater springs. These springs are commonly associated with ecologically import-
ant natural habitats, and also seem to have had an influence upon early human settle-
ment patterns in this area.
The Bagshot Beds also outcrop on Hampstead Heath in north London, and around
Brentwood and Rayleigh in south Essex (see Area 12).
Landscape G: The Hertfordshire Plateau
The Hertfordshire Plateau, between the rivers Colne and Lea, extends northwards into
the southern part of Area 13. It is underlain by Tertiary bedrock, most widely the Lon-
don Clay, with a cover of Quaternary alluvium. Historically it was heavily cultivated,
growing a variety of crops including hops and wheat to sustain London, making Hert-
fordshire famous as the best corn county in England. Over the last 50 years urbanisa-
tion has crept north from London, and the area is now a zone of commuter homes and
new towns.
As with other Landscapes in this Area, the scenery in Landscape G has been
strongly influenced by an episode when the pre-Anglian Thames flowed through it,
leaving spreads of gravel at high levels on the Hertfordshire Plateau. The arrival of the
edge of the Anglian ice sheet, with lobes extending down the Vale of St Albans and
as far as Finchley and Hornchurch (Fig. 208), must have modified the landscape con-
siderably, and the subsequent history of ice-sheet front lakes and kettle holes has been
mentioned already.
AREA 12: THE THAMES ESTUARY
The general introduction to this Region has explained how Greater London occupies
the centre of a wide and gentle downfold in the bedrock. The formation of this down-
fold began more or less at the same time as this Area (Figs 210 and 211) first emerged
from the sea, after a long episode of Chalk accumulation. In Early Tertiary times, mar-
ine sediments started to accumulate in the Area once again, though this time they were
limited to the large embayment of the sea formed by the continuing downfolding of
the basin. This sedimentation ceased in mid-Tertiary times, and was followed by a long
episode of erosion by rivers that has created much of the present-day landscape of low-
lying ground intersected by estuaries (Fig. 212). The River Thames has been the cent-
ral feature of this erosion, and has also been the main reason for London's remarkable
growth to become the commercial hub of England.
