Pleistocene: Brickearths, River Terrace gravels
60
UPPER
CRETACEOUS
65
97
MARINE SEDIMENTS
Oldhaven Sands
Blackheath Pebble Beds
Woolwich and Reading Beds
Thanet Sands
80
GAP IN ROCK RECORD
UPPER CHALK
MIDDLE CHALK
LOWER CHALK
GAULT CLAY
Upper Greensands only
In west Kent
LOWER GREENSAND
LOWER CRETACEOUS
Turrilites costatus
The Geological history of Kent extends from
the freshwater sediments from 146 million years ago,
through various marine sediments to the relatively modern
river sediments laid down in the last 1.65 million years.
MARINE SEDIMENTS
TERTIARY
GAP IN ROCK RECORD
LONDON CLAY
LOWER LONDON TERTIARIES
A KENT WILDLIFE TRUST INFORMATION SHEET
GEOLOGY
OF KENT
Recent sediments: river and clay, sands and gravels
1.65
52
Spondylus spinosus
Eocene leaf
Folkestone Sands
Sandgate Beds
Hythe Beds
Atherfield Clay
124
GAP IN ROCK RECORD
WEALD CLAY
HASTINGS BEDS
Tunbridge Wells Sands
Wadhurst Clay
Ashdown Sands
FRESHWATER SEDIMENTS
QUATERNARY
SEQUENCE OF KENT ROCKS
Millions of years ago
146
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As we pass from the Greensands down
onto the Vale of Holmesdale soils become
(140-50 million years) exposed at the surface. heavy and clayey. This Gault Clay marks a
They are a thick sequence of sands, clays and widening of the seaway as sea level began
to rise.
limestones gently folded into an upfold
(anticline) known as the Weald Dome.
Erosion has removed the highest parts of the
dome so that we see the oldest sandstones of
the High Weald exposed in its centre whilst
younger ‘greensands’ and chalk form the
Greensand Ridge and North Downs on its
outer rim. Tertiary sediments lay upon the
Chalk along the North Kent coast.
Within the county of Kent we find
rocks of Cretaceous and Tertiary age
The Kentish rocks begin with fresh water
sandstones and clays of the Wealden Series.
These were deposited in a basin south of an
old landmass, now buried under North Kent
and London. We can envisage rivers flowing
from the uplands into low-lying marshes with
ponds, mud flats and sandbanks. This shallow
water environment is indicated by ripple
marks, occasional thin pebble beds, traces of
roots and even dinosaur foot prints.
In the Weald a number of sedimentary units
can be recognised. These each begin with
sandstone and end with clay. In the oldest
units sandstones predominate (e.g. Hastings
Sandstone/Wadhurst Clay) and because of
their resistance to weathering give rise to the
High Weald. The youngest unit is mainly clay.
This soft Weald Clay forms the Low Weald
and contains within it thin limestone beds
packed with pond snails (Viviparus).
At the end of Weald Clay times this low
lying land was inundated by the sea so that
a sea way was established between the North
Sea basin and an ocean to the south. Initially
clays were deposited but gradually sandy
limestones and sands were deposited along
this estuary. Today we know them as the
Kentish ragstone and the Folkestone Sands.
Continuing our theme of rising sea level,
eventually so much land had been submerged
that no sediment from land (sand/clay) could
reach the Kentish area. Only the skeletons of
tiny organisms, overwhelmingly those of
calcareous algae, were deposited. Their
sub-microscopic plates make up the limestone
we call Chalk.
During early Chalk times land was not far
distant so at its base the Chalk is grey and
muddy but slowly cleaner, sometimes-nodular
white chalks appear as we move into the
Middle Chalk. Flint appears with the pure
white, soft Upper Chalk. These flints are black
with a white ‘rind’ (cortex) and were formed
when siliceous solutions moved through the
chalk ooze after it was deposited. Many are
sausage shaped formed when these solutions
invaded horizontal burrows.
Hastings Beds
Shingle and sand
Weald Clay
London Clay
Lower Greensand
Chalk
Thanet Beds
Gault Clay and
Upper Greensand
..... .... .
......... ...
.... . .
.
At the end of Chalk times
(65 million years ago) sea level fell and
Southeast England was uplifted. The Chalk
was eroded and the earliest Tertiary rocks
were not deposited. About 55 million years
ago sedimentation began again with sands
and clays, from the North Sea, laid upon this
surface. Today we find them throughout north
Kent but particularly well displayed in the cliffs
of Sheppey and Reculver.
Finally we have superficial sediments
such as river gravels, esturine muds and
brickearth overlying this varied bedrock. Most
of these accumulated during the Ice Age or
soon after, as sea level began to rise. Kent
was not covered by ice during this period but
suffered a tundra-like climate. Under these
conditions the chalk broke down into
sludge-like masses which readily flowed down
slope as so-called solifluction flows. Tyland
Barn is built on one such flow from a coombe
in the face of Blue Bell Hill, see the section by
the pond.
Eventually sea level rose, coastal changes
began and continue today with cliff erosion
and silting of bays and estuaries.
Hoplites aff. spathi