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Heslerton Geology
Index
Pre Last Ice Age
In Geological terms the area is very young in formation. The Northern Edge
(i.e. the North York Moors) is almost entirely Limestone
which is Sedimentary in nature, it was deposited under water in a warmer
climate by the building up of dead sea life organisms. these can clearly be seen
in the rock as fossil shells. This Limestone is Jurassic in age and dates from
some 150 million years ago. During this period of time the sea was subject to
changes and modification form time to time causing different types of
deposition, hence rising to different rock formations. Some areas had greater
depths of deposition than others leaving thicker layers of rock.
The central Vale is of the late Jurrasic era and is a much softer material
Kimmeridge Clay and on top of this a thinner
depth of Speeton Clay, these rocks were laid down
when the sea was quiet causing great deposits of silt to be laid down over
time. Because they are softer than the surrounding Limestone and Chalk have
been eroded away creating the Vale of Pickering.
The Southern escarpment is even younger, again it is of Limestone in nature
being much softer and is commonly called Chalk, of which there are two types red and the more
familiar
white chalk . As with it's harder cousin Chalk fossilized remains of sea
creatures can clearly be seen. Flint deposits are fossilized remains of sponges,
saying that it was formed in a corral type sea. This Chalk is of the Cretaceous
era and is between 135 and 65 million years old.
Corallian series Limestone
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This great spread of rocks being made up of many hundreds of feet of
limestones and sandstones actually have three distinct features giving rise to
different topographical areas, the Tabular, Hambleton and Howardian Hills. These
beds are recognised by being intensely siliceous, owing to the abundance of
hard silicous spicules of the fossilised sponge specific to this strata,(
Rhaxella performata). Fossills are not really numerous but fine specimens of
ammonites, (Aspidoceras and Cardioceras), brachiopod, (Thurmannella thurmanni),
oysters (Exogyra nana and Gryphaea dilatata), and sea urchins, (Nucleolites
scutatus) have been found..
The Hambleton and Howardian Hills are to the West of the Vale of Pickering
and form the Western boundary of the vale. The harder beds form the Tabular
Hills typical of the North Yorkshire Moors plateaux and terminate at the
Cleveland Hills. This plateaux is only cut by the numerous rivers and streams
which drain off the North Yorkshire Moors, the two most impressive ones are
Forge valley and Newton Dale the melt water gorges.
These rocks are neither true grit's or limestone's but due to their nature can
be referred to as fine grained calcarious gritstones. Their deposition is
associated with facies reefs and do not have pronounced fossil faunas which are
associated with other type of gritstones or limestones.
Kimmeridge Clay
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Kimmeridge Clay underlies the Vale of Pickering to a depth of some 125 meters
and consists of poorly fossilised shales and clays; there are not many outcrops
of this rock in the valley however they do occur near Malton, Birdstall, and
North Grimstone. These rocks have ammonites in them.(Rasenia uralensis,and
Discinia latissma)
The nature of the clay is of dark grey in colour with cement zone bands, it
is covered over with the Alluvial deposits from the deglaciation lake.
The villages along the northern edge of the valley are more or less along the
line of where the limestone and the Kimmeridge clay meet, this is due to the
fact of water percollates through the limestone rock fissures making it semi
porous, the water on meeting the Kimmeridge clay which is non porous then
rises to the surface. Old settlements were always made near good reliable
sources of water, all these villages have streams running through them.
Due to the pervious nature of the alluvial deposits that have been deposited
on top of the kimmeridge clay, the vale has huge quantities of aquifer water
above the clay. This is a valuable source of drinking water for people that live
in the surrounding area.
Speeton Clay
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Similar to the deposition of Kimmerige Clay these deposits were laid down by
slow accumulation in a quite sea in fairly deep water. This rock also has
ammonites in it.
Upper beds of this clay are visible at the base of the chalk escarpment along
the Southern edge of the vale, starting around Knapton going through to the
coast at Speeton near Filey. They are clearly seen as outcrops at Heslerton
where the clay has not been buried by the sand melt water deposits. Syonimious
with this deposit there is the spring line where the water the water that has
percolated through the porous chalk meets the imperviouc clay. It is here that
the villages have sprung up.
There are many "sub terranian" springs under these sand deposits that cover
the clay but these do not rise to the surface but form subterranian streams
running through the chalk gravel deposits that lie in the sand. It is these
streams that feed the aquifer above the kimmeridge clay.
Red Chalk
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This is a very thin but conspicuous layer which varies in colour from pink to
brick red and separates the Speeton Clay from the White Chalk. It can clearly be
seen at Heslerton where it will not be any more than about one meter thick. This
is an impure limestone and contains rounded quartz grains, with phosphate
nodules which give it its colour.
Fossils are fairly common and include brachiopods (Terebratula capillata and
T.biplicatula), lamallibranchs (Inoceramus tenius and Plicatula minuta) and
belamitee (Nechibolites listeri).
While the red chalk was being deposited there was a general subsidence of the
sea bed and the sea spread far and wide over the British isles. This period also
marks the end of continuously changing conditions which gave rise to such a
varied strata as has already being described.
White Chalk
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This subsidence persisted for a long period of time and a great thickness of
calcareous ooze accumulated which on consolidation became the familiar White
Chalk.
Chalk is wholly of marine origin and much of it is entirely free of
terrigenous (land) material. Its purity as a limestone is thought to be due
during the time of deposition an arid climate on the land area at this time but
also crucially was also of no great altitude from which only a small amount of
surface drainage reached the sea. What deposits that did reach the sea was only
in solution. Microscopically, chalk is seen to be composed of fine chemically
precipitated particles, minute spherical bodies, foraminifera, and small
fragments of shells.
The chalk Yorkshire Wolds are about 400 metres thick of this 300 metres is
called Upper Chalk, the rest being in differing thickness Middle or Lower
Chalk.
The Lower Chalk is aprox 35 metres thick, sitting on top of the red
chalk; it comprises of a mix of hard white limestone at the base and is followed
in turn by the hard grey shelly Inoceramus Bed. This is rough grey nodular chalk
which weathers into thin platy fragments. There are occasional fossils,
ammonites (genus Schloenbachia), oyster (Ostrea vesicularis), lamellibranchs (Oxytoma
seminudum), and worms (Serpula antiquata).
The Middle Chalk is aprox 65 metres thick comprising thick layers of
chalk with nodules of flint and occasional layers of marl in it. This chalk does
not have a lot of fossils in it , however the odd brachiapod (Rhynchonella) is
found.
The Upper Chalk is the deepest layer of white chalk being over 300
metres thick and is generally unfossiliferous although different zones may have
one or two fossils specific to that band.
Chalk has been used for lime-burning and making whitening in many parts of
Yorkshire: and in some cases the flints have been used in the construction of
buildings. Today chalk is ground and added to land as ground chalk instead of
being burnt.
Ice ages
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Some of the major Geological feature of the area are due to
successive Ice
Ages probably four over the last 435,000 years. Local evidence that the climate
between the Ice Ages was warmer is to be found at Kirkdale Cave.
Kirkdale Cave
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Kirkdale Cave is a natural cave in the limestone rock between Kirbymoorside
and Helmsley. It was found by Quarrymen in 1821, the original entrance was 11ft
wide, 5ft high and 38ft in length to the point of the present entrance 8ft wide
and 3 ft high, from here it narrows but is over 150 yards long. When the quarrymen
found the present entrance they found animal bones in large quantities, it was
thought these ware farm animal bones.
There is a complete absence of human bones, on further examination the animal
bones fell into two categories, the first being indicative of a relatively warm
forest climate and were: the Straight-tusked Elephant, Rhinoceros, Hippopotamus,
Hyenas, bison and Giant deer. Because of their age they can only be from the the
last interglacial period ( EEmain Period) which lasted for 60,000 years. It is
thought that the cave may have been a Hyena den during this period.
The second type of bones were from a colder steppe were from : Mammonth,
wooly Rhinoceros, reindeer, and horse. These were found in a silt deposit on top
of the interglacial bones. The height above sea level of the cave is 175 ft the
following Lake Pickering flooded to aprox 250ft so it can be assumed the bones
came in to the cave when it became flooded.
Lake Pickering
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The point of maximum ice coverage in the last period of Glaciation was about
22,000 years ago. At this time a glacier pushed down through Teesdale and out
towards the North Sea, however due to the weight of ice there had to change
course and then flow down the vale of York. Another Glacier from the Cheviot
Hills in Northumberland pushed up against the escarpment of the Cleveland Hills,
but most important did not pass over them and on to the Moor tops.
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years of seasonal melting the waters from these Galciers
filled all the moorland vallies, (Eskdale was over 800ft
deep) and eventually overflowed Southwards, because the
water could not flow anywhere else. This filled the Vale
of Pickering to a depth of 255 ft at its deepest before
it started to drain out. The lake was almost 30 miles
long and 5 to 10 miles wide.
Below is a map of how the Lake
of Pickering looked in the latter stages of the Last Ice
Age. You can clearly see how the valley is blocked in by
the ice caps from North, West( Vale of York Ice Cap),
and the East ( North Sea Ice Cap). The only way out for
melt water is in a Southerly direction through Newton
Dale and Forge Valley. The characteristic of a
melt water channel is that contrary to a normal
watercourse the source of the river has a steep gradient
then goes less, a melt water channel has a low gradient
at source then steepens until it reaches the outlet in
this case the Lake Pickering
The Vale Of Pickering now
drains out through Kirkham Gorge where the water flowed
out of the lake. The pre Glacial outlet of the North Sea
of the Derwent at Filey, (South of the region) and Scalby
Mills, (North of the Region) were still blocked by
Lateral Moraine deposited by the North Sea Glacier.
Hence the drainage system of today where the river
Derwent rises 5 miles from the sea and travels nearly
100 miles to get there.
During this period massive
quantities of gravel were deposited on the northern
edge of the lake where the melt water channels flowed
into the valley at Pickering at the end of Newton Dale,
and at Ayton at the end of Forge Valley.
Post Last Ice Age to Man's
Influence
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After the ice receded due to
the Lateral Glacial moraine deposits from the North Sea
Ice Sheet the old pre Ice Age exit of the Derwent to the
sea was still blocked. This meant the valley could only
drain through the existing channel through Kirkham Gorge
cut out by melt water.
The lake over time receded
leaving a valley floor full of marshes in its lower
areas, and wooded areas being formed everywhere else. The
soil type left was alluvial deposits mixed with peat in
the valley floor, and huge deposits of lakeside sand on
the Northern facing slopes up to 25 meters deep in
places. This covered the parent material of the
kimmeridge and speeton clays. The soil on top of the
harder chalks is a mixture of predominantly chalk, clay
and flints.
Over the last 4000 years man
has done little to alter the surface deposits, all that
has happened is man has drained the valley so that no
true marsh area exists today. The marshes have been
drained the only remnant is areas of land called Carrs,
an old name for marsh. This soil type is high in peat
content and is gradually disappearing by aerobic
microbial breakdown. (Peat is only able to occur because
draining stops soil microbes that breakdown organic
matter from living because it is too wet and cold for
them to live, hence over time organic matter builds up,
hence peat is deposited). This is the surface Geology of
the area as it exists today.
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