BLAKENEY ESKER AND HOW IT FORMED
Introduction
The Blakeney Esker is a ridge, around 3.5 km in length, which runs southeastwards from west of Blakeney, to Wiveton Downs, north-west of
Glandford, in north Norfolk, UK (Figure 1).
Figure 1
Blakeney.
The
location
of
There are several near right-angle bends in the esker. The northern
section of the ridge between Morston Downs and Kettlehill Plantation has
a sharp, well-defined shape. It is between 40 and 100 m wide and, in
places, rises to around 20 m above the surrounding topography. Further
south, the ridge is lower for about 1 km and the sharp ridge is not as
obvious. This is due to the removal of aggregate during the early 1940s,
used in the construction of nearby Langham Airfield. The ridge forms a
prominent feature again towards its southern end at Wiveton Downs
(Figure 2), where the ridge surface rises up to 15 m above the
surrounding topography, the name ‘Downs’ is a name often given to ridges
or hills. Several small isolated hills, located between Wiveton Downs and
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Glandford, are in line with the esker and may represent the final eroded
remnants of the ridge.
Figure 2 Sketch map showing the location of Blakeney Esker. The shape of the esker has been
simplified.
Internal composition of the ridge
The name ‘esker’, originates form the Irish word eiscir, meaning sandy
ridge. An understanding of what the ridge is made of helps us to
understand how it formed. The ridge is composed of two units, and these
can be seen in some of the quarries that exist along its length.
Unit B – a sand and gravel, with occasional lenses of till, making up the
ridge.
Unit A – a chalky till that occurs beneath, and adjacent to, the ridge
margins.
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Unit A lies beneath the ridge and the surrounding fields. It is a compact,
cream-coloured very chalky till (boulder clay) (Explanation Box 1) that
contains numerous clasts (pebbles) of chalk and flint. The texture of the
till matrix (the material between the clasts that holds the unit together)
is very silty, and feels smooth when rubbed between the fingers. This
unit is a subglacial lodgement till (Explanation Box 2) which was deposited
beneath a glacier.
Beneath the ridge, the surface of this unit is not flat, but is cut by a
series of channels that run in the same direction as the ridge. These
channels have steep sides, are between 15 and 25 m wide, and can be
traced for up to 400 m. These channels, which formed beneath the
glacier due to the flow of water, are called Nye channels (Explanation
Box 3).
Unit B infills the Nye channels cut into chalky till (Unit A) and forms
most of the ridge. It can have a thickness of up to 15 m. The unit consists
of clast-supported (Explanation Box 4) cobble gravels separated by
pockets of yellowish orange sand. The main constituent of the cobble
gravels is flint and individual clasts are usually very round in shape. This
shape suggests that very high-energy turbulent flow conditions existed,
causing the edges of the rough cobbles to be broken off and smoothed as
they bumped into each another. This process is known as abrasion
(Explanation Box 5). The gravels also commonly have a structure called
imbrication (Explanation Box 6). This is a useful feature, used by
geologists, to understand the direction of water flow. In this case, flow
was broadly from the north-west but tends to lie parallel to the trend of
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the landform. Some of the gravel and sand layers, referred to by
geologists as beds, also become finer upwards. This means that within an
individual bed, the average size of the gravel clasts becomes smaller
upwards; likewise within sand beds, the coarseness of the sand reduces
upwards. This fining-upward suggests that there were frequent changes
in flow velocity and therefore, the flows’ ability to entrain (carry) and
transport different types of material. This is because a reduction in flow
velocity means only the lighter material can be carried by the water,
causing the heavier material to be deposited. There are occasional lenses
of flinty till within Unit B.
How the esker formed
When we look at the shape (morphology) of the ridge and its internal
composition, we can build up a model of how the ridge formed. This helps
us to decide whether or not the ridge actually is an esker. Five stages
within the ridge’s evolution can be identified:
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Stage 1: This stage coincides with the movement of ice across the
Blakeney area and the deposition of the chalky till (Unit A).
Stage 2: Modification of drainage beneath the glacier and the erosion of
‘Nye channels’ onto the upper surface of the chalky till by meltwater.
Movement of ice is towards the reader.
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Stage 3: Transportation and deposition of coarse sands and gravels
within these ‘Nye channels’ by meltwater (Unit B). Because the channels
are beneath the ice they are operating within an enclosed space, which
greatly increases the pressure and flow velocity. This explains why the
water had the energy to erode channels into the till, and transport high
quantities of gravel and cobbles.
Stage 4: Enlargement of the ‘Nye channels’ into a tunnel and its
subsequent infilling by meltwater sand, gravel and cobbles (Unit B).
The enlargement from channels to a single tunnel can be explained by the
frictional melting of ice, driven by the flowing meltwater. This is an
example of a positive feedback process. As frictional melting of the ice
creates a larger channel that can transport more water, this drives
greater frictional melting due to increased water flow and greater
surface area of the channel margins.
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The origin of the rare lenses of till within Unit B is not clear, but the
most likely explanation is that it was eroded from upstream and carried
by the meltwater.
Stage 5: Meltwater flow and the deposition of the sands and gravels
would have continued until either the source of the meltwater was
exhausted, or the drainage pathway was diverted. Later, northwards
retreat of the ice margin revealed the ridge.
Is the ridge an esker?
The shape and internal composition of the ridge reveal that the landform
formed by meltwater flow beneath a glacier and is therefore an esker.
However, the shape of the esker is slightly different to that of many
others, as some sections don’t appear to be inline with one another. This
offset pattern is commonly associated with crevassing (Explanation
Box 7) within the ice. This suggests that during formation, the course of
the water was strongly influenced by crevasse patterns that extended
through the glacier.
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THE BIGGER PICTURE
How other north Norfolk glacial landmarks are related
Cromer Ridge
The Cromer Ridge (Figure 1) is one of the most prominent landforms in
north Norfolk. It forms some of the highest ground within the county and
in places rises to over 50 m above surrounding land (over 100 m OD).
Figure 1 Cromer ridge from Beeston Bump, just east of Sheringham, looking east.
The ridge intersects the east coast between Trimingham and Overstrand,
where it can be identified in the cliffs, and traced westwards for 15 km
to Sheringham. From Sheringham, the ridge bends towards the southwest and extends for 20 km towards Thursford. A small gap, known to
geologists as the Briston Gap, occurs where the ridge is absent (Figure 2).
Figure 2 Sketch map showing approximate extent of Cromer Ridge.
The coastal cliffs around Trimingham, and several quarries situated on
the ridge itself, provide us with a valuable insight into what it is made of
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and its structure. This has shown that the ridge has a very complex
history and was formed during several different phases. The bottom
60 m of the ridge comprise beds of pre-existing till and sand, silt and
clay, which have been highly deformed when they were pushed
southwards by a glacier. Effectively, this bulldozing process is similar to
pushing a heavy book over a tablecloth – it causes the tablecloth to ruck
and fold over upon itself. Smaller scale examples of this occurring in
front of glaciers can be found in Iceland today (Explanation Box 8).
This is exactly what happened with the glacier pushing into the preexisting sediment pile. An example of this deformation can be seen in the
cliffs near Overstrand where blocks of chalk and layers of sand and till
have been bulldozed and stacked on top of one-another (Figure 3).
Figure 3 The Cromer ridge reaches the
coast in the cliffs near Overstrand. The
arrow shows the direction in which the
glacier pushed the sediment.
This part of the ridge is called a push moraine, and it is also a terminal
moraine because it marks the maximum extent of the ice. Examples of
these can be found at many modern day glaciers, such as in Iceland
(Explanation Box 9). The top 35–40 m of the ridge, overlying the moraine,
is composed of sands and gravels and these were laid down as an
extensive glacial outwash fan during a later advance.
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Kelling and Salthouse outwash plains
An outwash plain or sandur occurs where fans of sediment deposited by
meltwater from a glacier margin coalesce to form a braided river channel
system that extends across a broad, gently sloping plain (Explanation
Box 10). Such river systems can deposit vast quantities of sand, gravel
and cobbles. Two outwash plains are located close to the Blakeney Esker.
The first of these is called the Salthouse outwash plain and this forms
the ground around Salthouse Heath (Figure 4). Just to the east of
Salthouse Heath is Kelling Heath, which forms the second, and larger of
the outwash plains.
Figure 4 The old outwash plain at Salthouse Health today.
Kames and terraces
Adjacent to the Blakeney Esker and on both sides of the Glaven Valley,
are a series of small hills and conical-shaped mounds. These range in
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diameter from 20 to 400 m and can be up to 20 m in height, although
most are less than about 10 m. Geologists have interpreted these
features as kames and kame terraces. Kames are piles of sand and gravel,
which have been laid down at the front of a melting glacier. They grew as
the melting glacier deposited sediment.
Secondly, a succession of flat ‘terrace’ features can be found on the
western and eastern sides of the Glaven Valley near Glandford. These
terraces comprise up to 10 m of outwash sand and gravel with thin layers
of chalky till. Geologists have interpreted these features as kame
terraces (Explanation Box 12). These features form in contact with, or
close to, the sides of a glacier, where glacial meltwater has deposited
sand and gravel against the valley sides, forming a series of terrace
features when the ice melts.
HOW ALL THE LANDFORMS FIT TOGETHER
By fitting all of this information together we discover the order in which
the features formed, and build a picture of how the geography of the
area has evolved in relation to an active ice margin.
Five stages of evolution can be recognised, and each stage is illustrated in
a series of schematic cartoons showing a planview and cross-sectional
view.
Stage 1 – formation of the Cromer Ridge push moraine (Figure 5)
The Cromer Ridge is the first feature that was formed, created by the
pushing and stacking of pre-existing sediments at the ice margin to form
a push moraine.
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Stage 2 – the Cromer Ridge outwash fan (Figure 6)
Draped over the top of the Cromer Ridge push moraine is a thick
sequence of sands and gravels that form the Cromer Ridge outwash fan.
The fact that this fan is superimposed upon the moraine demonstrates
that the formation of the outwash fan was after the moraine. The
outwash fan itself developed by braided meltwater streams, coming from
the ice margin to the north, depositing vast quantities of sand and gravel.
The position of individual drainage channels is not known, but examples of
modern outwash fans from places such as Iceland, suggest that there
were likely to have been a series of smaller fans coming together from
drainage points along the ice margin.
Stage 3 – Kelling outwash plain (Figure 7)
Following the deposition of the Cromer Ridge outwash fan, there was a
small north-westwards retreat of the glacier margin. The ice margin at
this time probably lay along the present northern and western flanks of
Kelling Heath as suggested by steep ice-contact slopes. Meltwater
draining from the glacier deposited extensive sheets of sand and gravel
as part of the Kelling outwash plain, in the present area of Kelling Heath.
This constrained meltwater drainage both to the north and south
between the glacier and the Cromer Ridge respectively. At some point
during this stage, the Cromer Ridge was breached by meltwater in the
Briston area forming a drainage outlet to the south, called the Briston
Gap.
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Stage 4 – Salthouse outwash plain and Blakeney Esker (Figure 8)
A further phase of glacier retreat occurred with the ice margin lying
along the northern and western edges of Salthouse Heath as indicated by
the heath’s steep northern and western ice contact slopes. The Blakeney
Esker was formed during this stage within a sub-glacial drainage channel
that was feeding the Salthouse outwash plain. The kames and kame
terraces within the Glaven Valley were probably also formed at this time.
This new drainage pattern creates a geomorphological problem that is
currently unresolved. The Salthouse outwash plain appears to have been
bounded on all sides, either by glacier ice, the Cromer Ridge or the Kelling
outwash plain, which is 11 m higher than the Salthouse outwash plain, so
where did the outwash drain to? Did it form a lake infront of the glacier?
Or perhaps drain through the older permeable Kelling outwash sediments?
Stage 5 – present day (Figure 9)
Following complete deglaciation the landforms of the Blakeney area have
experienced surface weathering for several hundred thousand years,
during which at least two interglacial warm stages (including the present
day) and one further glacial episode are known to have occurred. This,
together with gravel quarrying, has resulted in the partial erosion of the
landforms, although their surface expression is still evident today and is
testament to a very dramatic and significant climatic episode within
Norfolk’s past.
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Figure 5 Stage 1 – formation of the Cromer Ridge push moraine.
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Figure 6 Stage 2 – formation of the Cromer Ridge outwash fan.
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Figure 7 Stage 3 – Formation of the Kelling outwash plain.
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Figure 8 Stage 4 – Formation of Salthouse outwash plain and Blakeney Esker (K = kames
and kame terraces).
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Figure 9 Stage 5 – present day.
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EXPLANATION BOXES
Explanation Box 1
Explanation Box 2
Explanation Box 3
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Explanation Box 4
Explanation Box 5
Explanation Box 6
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Explanation Box 7
Explanation Box 8
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Explanation Box 9
Explanation Box 10
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Explanation Box 11
Explanation Box 12
All figures featured are BGS © NERC 2006
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Discussion points / homework topics
1. Geologists have only just agreed that the ridge is actually an esker.
Other ideas included:
a. open crevasse filling (filling in of crevasses in the glaciers
surface – then when ice melts it leaves a ridge of sediment)
b. a linear remnant of a larger mass of sand and gravel that has
been eroded away
c. formation in an ice-marginal environment.
How did geologists discount these theories and conclude that it
was an esker?
ANSWER: The quarrying that revealed the presence of Nye
channels in the till surface, is vital. The Nye channels are aligned
with the ridge, strongly suggesting that they have a close
association in terms of origin, suggesting that the ridge formed by
streams flowing along its length. This means it was not deposited
onto the surface of the ice in crevasses or as a large sheet, as
suggested in theories A and B. Analysis of the ridge sediments has
demonstrated pipe-flow conditions, which means they were
deposited in a subsurface channel, meaning it formed subglacially
(underneath the ice) rather than in an ice-marginal environment (in
front of the glacier).
2. Why do you think the chalky till is so chalky? Where does the chalk
come from?
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ANSWER: Till is made up of all the rock types that the glacier
moves over as it progresses over the land (including land that is
now the sea bed). Geologists know that the glaciers flowed from
the north. Looking to the north, the area near the east coast from
Flamborough Head down to Norfolk is the only region with chalk as
bedrock that the glacier could have come travelled over, suggesting
the chalk has been carried from here. See BGS geological maps for
more information and to support this explanation.
3. Suggestions on teaching this topic:
Pose the question ‘how did our local landscape form?’ and
brainstorm the possible suggestions, perhaps burial grounds,
something crashing from outer space, did the ridge grow from the
ground, is it the remains of a older higher land surface etc.
Following this, looking at the possibility that ice was involved,
provide the Icelandic analogues as described in this document and
assess the similarities.
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