Cold Environments: Lowland Glaciation Background Information
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Background
The glaciers that originated in northern Snowdonia during the last glacial maximum travelled northwards and
westwards out of the mountains. Sea levels would have been much lower than they are at present, in fact, so low
Ireland was joined onto mainland Britain. The Snowdonia glaciers made their way out towards what is now the Irish
Sea and deposited large amounts of glacial till (also known as boulder clay) across the landscape. As the climate
cooled even more, the Irish Sea Ice travelled south covering the deposits dropped by the Welsh glaciers. This ice flow
also smeared large quantities of glacial till over the landscape. This now overlies the Welsh glacial drift along the
coastal plain. The Irish Sea Ice not only deposited material, it also eroded much of the Isle of Anglesey.
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A map showing ice flow in Northern Snowdonia approximately 18,000 years ago
Aberogwen
Glacial till (boulder clay)
Dropped by glaciers, it is an unsorted deposit
(with large rocks and small clay particles mixed
together). At Aberogwen the till has been
brought by ice from two different directions.
Welsh till at Aberogwen. The unsorted nature of
Material and the angularity of material is clear.
Soil built up over the
last 10,000 years
Irish sea till,
predominantly red
Sandstone and
limestone matrix)
Welsh till from the Ogwen
Valley and Idwal area, with
a dark grey matrix from
slate and igneous rock.
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Cold Environments: Lowland Glaciation Background Information
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Varves
This area would have been an outwash plain for the Snowdonian ice just after
the last glacial maximum. Meltwater streams would have run across the
landscape from, transporting smaller glacial deposits away from the snout of
the glacier. In places there will have been depressions which would have filled
with water and become proglacial lakes.
The pro-glacial lakes would have been in coexistence alongside the glaciers. In
the warmer months of spring and summer the amount of melting would have
Valves at Aberogwen.
increased. This meant that the meltwater streams had more energy and were able to transport larger particles such
as sands and gravels away from the glacier into surrounding lakes. In the autumn there would have been less water
from melting and so the streams would not have been able to transport as much load, and less and smaller material
would have been carried into the lake. In late autumn and winter the lightest and finest sediments finally settled on
the lake bed as the stationary water had no energy. This left a layer of very fine clay particles was deposited. As a
result a series of layers built up; coarser, sandy deposits in the summer followed by finer, clay deposits in the winter.
In places these clay deposits are folded and contorted suggesting a process known as cryoturbation has taken place.
This occurs in periglacial environments. Periglacial locations are usually typified by permafrost or permanently
frozen ground. Above this is a layer of ground which thaws each summer and freezes in the winter months. As that
refreezing occurs, the freezing moves from two directions, from above as air temperatures freeze the ground surface,
and from below as the permafrost advances upwards. The soft ground in between is squeezed like a tube of
toothpaste resulting in a stirring up of the layers of clay.
Valves at Aberogwen.
Originally the Welsh ice extended across the coastal plain. The ice retreated in a warmer period leaving an outwash
plain or sandur. This would have been an area where sands, gravels and pebbles, rounded by fluvioglacial attrition,
were deposited. This area may have had stagnant boulders of ice. As these melted, they left depressions in the
gravels. These are known as kettleholes. These would have filled with water.
Angularity
Size
Glacial drift
Angular and subangular. In till some
material may be ground down by abrasion.
Mixture of all different sizes of material.
Completely unsorted.
Orientation If deposited under the ice both the
landforms and the material itself are
usually aligned in the same direction of
flow as the glaciers.
Fluvio-glacial deposits
Rounded sediment, often due to attrition in meltwater streams.
Sorted material where material is found alongside material of
the same size.
Although water also aligns material in the direction of flow, the
meltwater streams are much smaller and also meander over
the outwash plain. Meltwater streams often change course as
levels rise and fall, leaving a series of river channels and islands.
This means clear patterns in orientation are often far less clear.
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Cold Environments: Lowland Glaciation Background Information
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Pentir
Pentir has many lowland glacial features. The area around Pentir is comprised of drumlinoid field all aligned in a
north-east, south-west direction. Kames and eskers and kettleholes can all be found in the vicinity.
The Pentir Esker
An esker is a subglacial stream deposit. It is formed by a stream under a glacier.
The channel is contained within a tunnel of ice. Stream sediments build up the
floor of the channel as there is no flood plain. When the ice melts this deposit is
left as a raised long feature.
Morphology: eskers are linear in shape; long and thin and sinuous (they
meander like streams because they were formed by subglacial streams).
The Pentir esker seen from the side.
Size and shape: The Pentir Esker is around four hundred metres in length and
around 5-10 metres high.
Composition: coarse sands, gravels and larger pebbles.
The Pentir Kame
The Pentir esker seen from on top.
A kame is often formed where moraines and other glacial deposits
have been moved short distances by meltwater streams and the
material has fallen into crevasses or dropped at the end of the
glacier’s snout. A certain amount of sorting of the materials will
take place leaving stratified layers of sediments. In shape, they are
far less regular in form than drumlins or eskers.
Petir Kame. The bedding plains move from top left diagonally
down to bottom right. This sorting suggests it is a fluvioglacial rather glacial deposit (photo: Helen Morton)
Kettleholes
Stones from the kame deposit. Note the roundness, many are sub-rounded
or rounded on the Powers Index. (photo: Helen Morton)
Several of these were created after the last glacial maximum in the low-lying coastal plains around the area of Pentir
and Caernarfon. Very few still contain much water, because over time they have filled in. However, there are many
marshy and boggy hollows where hydroseres have developed. Most of these are filled with water-loving trees such as
willows. They are of little agricultural use, as they are too wet.
There is one such example near the kame at Pentir. It is a low hollow, around 75 metres in diameter, filled with
hydrophytic (water-loving) plants such as the yellow iris and willow tree. Kettle holes are created when a large block
of ice is left in the valley, as the glacier retreats. Sediment is deposited around this block of ice, and when the ice
melts a whole is left, which then fills with water.
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Cold Environments: Lowland Glaciation Background Information
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Conwy Valley and Eglysbach
Drumlins
Throughout the Conwy Valley there are a
number of drumlins which were deposited
under a large glacier flowing down the valley
from the mountains of North Wales. These
landforms are created when a glacier dumps
material under the ice, and the moving ice
then sculpts this deposit into streamlined
deposits.
Ice flow
A drumlin near Conwy, the ice flowed in the direction of the arrow.
Side view of a drumlin shaped by ice
View from above of a drumlin
Drainage diversion
Old river course
The valley bottom in which the village of
Eglwysbach is now situated in, was once a
lake dammed in by a large glacier moving
down the Conwy Valley. Lake sediments can
still be found in the valley. As the water
could no longer escape out to Tal y cafn, it
needed to find a new route and carved a
gorge to the north of the valley. This is still
the way the river travels from Eglwysbach
today.
New/
current
river
course
Location of Conwy glacier
Where the lake dammed
by a glacier was
Solifluction deposit
Source: OS Map extract from Memory Map with location of a drainage diversion
At Eglysbach there is an exposed solifluction deposit just above the village. The material is all local in origin having
moved down the hill when the top of the ground thawed in summer in a periglacial environment. This top thawed
layer would have become saturated as water could not move down and through the permafrost below and this
saturated material mould slide slowly downhill. As it slowly moved downhill the material would re-orientate itself in
line with the flow direction, and this same material is very angular as it has not travelled far and therefore has had
little time to be eroded.
The Field Studies Council is a charity aiming to ‘bring environmental understanding to all through firsthand experience’. Charity no. 313364. Registered Office; Preston Montford,, Shropshire SY4 1HW