Glaciers & Oceans OXG 2011
Llyn Llydaw and Glanllynau fieldtrip report - Tonya
O'Donnell
Llyn Llydaw
The first part of our fieldtrip took place in the
Llyn Llydaw area of Snowdonia on 5 th October 2004.
Llyn Llydaw is noted as an exceptional area to study
the effects of glacial activity and examples of glacial
erosion and deposition. Many of the features of the
Snowdonia area are believed to be as a result of tectonic
activity followed by glacial activity. The evidence
of this glacial / ice activity can be seen in the glacial
drift, 'Roches moutonnees', moraines, grooved and striated
rocks, boulders strewn around and the lakes around
Snowdonia.
Llyn Llydaw fills a deep rock basin over a mile long.
The head of the lake has a depth of approximately 58
metres. The scree-covered slopes at the head of the
area are not covered by grass. Streams flowing down
the rock face bring silt which is gradually filling
the lake. This is believed to be an area that marks
the final advance & retreat of the Loch Lomond
stadial. The tumbled mass of glacier-broken stone and
rock powder marks the end of the Llydaw glacier as
it melted. The morainic deposits have made the lake
shallower at one end than the other. Scattered around
the lake are low rock mounds which have been rounded
and striated by the ice.
Figure 1 . Llyn Llydaw
During our fieldtrip to Llyn Llydaw we were chiefly
interested in the grooved and striated rocks as evidence
of ice activity and principally the direction that
the ice flow may have taken. Glaciers are often many
hundreds or thousands of feet thick and as they move
across the bedrock their enormous weight assisted by
the gravel, pebbles and boulders frozen or embedded
in them grinds down across the bedrock leaving evidence
of their movement. The power of this moving force is
capable of grinding all sharp edges, peaks and jagged
edges off rocks giving them a rounded or blunted outline.
During this process grooves and scratches are often
made upon the bedrock by the material embedded within
the glacier. The tremendous weight and slow motion
of glaciers means they tend to move steadily and leave
these almost straight parallel lines showing us the
direction the glacier moved in. These lines may vary
from fine scratches to grooves in which you can put
your finger or even troughs much wider.
We are testing the hypothesis that scratch marks on
the rocks called striae demonstrate that ice movement
in this location was confined by the valley. We hiked
up to the lake at Llyn Llydaw and were split into 8
groups to take the data. Using a compass each group
selected a location and took measurements of the striae
across rocks around the lake area. We agreed that 50
measurements per group would give us a suitable amount
of data to analyse. Figure 1 shows the lake and various
locations of the groups taking data. Figure 2 shows
the
Figure 2. Locations of various groups around the lake
at Llyn Llydaw. I was in group F close to the edge
of the causeway across the lake.
Figure 2
Figure 3. Photograph showing a student
(L. Hassan-Hicks) using a compass to take a measurement
of the direction of striae on a rock. Measurements
are taken in degrees from North towards east.
Figure 3
Striae are subject to weathering if exposed and most
glacial striated rocks have been considerably weathered
making it difficult to take good measurements. The
lakes in the Snowdon area have however preserved a
lot of the striated rock especially close to the waters
edge. "Water . is an excellent preserver" (Gray, 1982).
When the causeway at Llyn Llydaw was built the lake
was lowered exposing rocks which display striae and
other erosional features eg. crescentric fractures.
This makes Llyn Llydaw an excellent site to study glacial
striated rocks.
Our results are interesting and thankfully fit well
with previous studies in the Llyn Llydaw area. The
striae data collected by each group was collated together
and I have created a series of graphs displaying the
results. To make it easier I have banded the results
into measurements taken within 10 degree bands. The
results show that the dominant direction is approximately
west to east. The mean degree measurement taken (degrees
east from north) is 88.31 ± 1.32. A histogram
of all data can be seen in Appendix 1. The histogram
of results shows that in the group 91 - 100 degrees
125 measurements were taken indicating this to have
the highest number of striae directions taken. Appendix
2 shows data taken from each group shown as lines together.
The interesting observation that can be seen from looking
at the group data individually is the spike of measurements
taken from group H in the class 71-70 degrees rather
than the 81-90 or 91-100 classes where the other group's
measurements tend to centre. The mean degree measure
from group H was 74 degrees. The means of all other
groups were between 81 and 96 degrees. Looking at the
location of group H's data you can see that group H
were location on the other side of the causeway and
the lake from most of the other groups. This may account
for the slight change in mean direction.
Appendix 3 displays the data collected as a histogram
showing the various group data cumulatively displayed.
Appendix 4 shows the data in tabulated form with the
means of each group and the cumulative count of data.
Our results are in accordance with earlier recordings
made by Gray & Lowe (1982) which also show a dominant
direction of West to East. Gray and Lowe go on to say
that since this is the dominant direction recorded
this is probably the direction of the last (most recent)
glacial flow in this area. This is summarised partly
because any glacial activity causing abrasion on the
rocks will likely destroy previous striations made
thus the dominant set of markings is likely to be the
most recent. That along with evidence showing that
other recordings of striae in a SW to NE direction
have been subjected to more weathering and are predominantly
found in the lee side of the valley where the rocks
have been more sheltered.
Since we believe our striae to be evidence of the
most recent glacial activity in this area then like
Gray we can conclude that the West to east direction
is that of the loch Lomond Stadial following the direction
of the Llyn llydaw valley which means we agree with
our initial hypothesis.
Glaciers are made up of fallen snow that, over many
years, compresses into large, thickened ice masses.
Glaciers form when snow remains in one location long
enough to transform into ice. What makes glaciers unique
is their ability to move. Due to sheer mass, glaciers
flow like very slow rivers. Some glaciers are as small
as football fields, while others grow to be over a
hundred kilometers long.
Glaciers are made up of fallen snow that, over many
years, compresses into large, thickened ice masses.
Glaciers form when snow remains in one location long
enough to transform into ice. What makes glaciers unique
is their ability to move. Ultimately glaciers move
because of the "pull of gravity" (Marshak, 2001). Once
a mass of compressed ice reaches a critical thickness
it becomes so heavy that it begins to deform and move.
The sheer girth of the ice, combined with gravity's
influence, causes glaciers to flow very slowly and
they generally follow the moraine they are flowing
over or in (Benn & Evans, 1998). Hence valley glaciers
tend to follow the existing contours of the valley.
At Llyn Llydaw evidence shows two directions of movement.
The earlier SW to NE movement is attributed to the
last glacial maxim (the Dimlington stadial) around
21,000 years ago. The more recent evidence suggests
that a glacier here in the Llydaw area occupied the
area from "Glaslyn to the lip of Cwn Dyli" (Gray,1982).
Glaciers periodically retreat or advance, depending
on the amount of snow accumulation or albation that
occurs. This retreat or advance refers only to the
position of the terminus, or snout, of the glacier.
Figure 4 shows a map of the Llydaw area with the Llyn
Llydaw Lake clearly shown. You can see from this diagram
that Glaslyn is nestled between the two highest points
(one being Snowdon itself) and that Cwm Dyli where
the glacier snout reached is to the east at a lower
altitude. The valley clearly follows a west to east
direction. The diagram also shows (with dashed outline)
the size of the glacier believed to have occupied this
area.
Figure 4 - Diagram taken from Gray, 1982 showing former
glaciers around Snowdon.
Glanllynau sediments
The second part of our fieldtrip took us to Glanllynau.
This is one of the most important sites for Quaternary
science because it displays a unique record of sedimentation
from the pleniglacial through to the Younger Dryas
event (Loch Lomond stadial). Two opposing theories
to the sedimentary deposits at Glanllynau have been
proposed. The first theory attributed to Whittow & Ball
(1970) and Bowen et al. (1986) is that the lower clay-rich
till (Criccieth till) is related to inundation by ice
during the Dimlington stadial and the upper gravely
till with later re-advance of Welsh-ice during the
Locch Lomond stadial. Boulton (1977) believed that
both tills were deposited during a single glacial event
(Dimlington stadial) and the subsequent retreat of
this ice. (Gray, 1982).
We observed the sediment after walking down the beach
to a good location and took drawings of the sediment
exposed. I also took some photographs so I could clearly
see the scale of layers and show the sediment layers
for my report. There is a low 'undulating' cliff along
the beach front which displays layers of sediment.
In some places there has been recent erosion and land
slip has caused some mixing of these sediment layers.
In places however there are very clear examples of
layers of sediment. The surrounding area is very marshy
with a river nearby and some ponds.
Our observations
Appendix 5 shows a lithostratigraphic key to the layers
we encountered. These are described as follows.
Layer 1 - GD. Gray Diamictron. This layer had a matrix
of fine grain. It felt like clay with mud and sand
mixed into it. This diamictron layer was gray in colour
with clast of various sizes mixed into it. Some boulders
were quite large while others were very small pebbles.
Very unstructured.
Layer 2 - BD. Brown Diamictron. This layer appeared
ine very way top be the same as the layer below it
apart from the colour.
Layer 3 - C. Clay. This was laminated clay with silt.
It ran in a clear band across the top of the diamictron
and was aprox 6 inches thick in some places deeper.
Layers 4 & 5 - Sand and Gravel. This layer had
a range in size of composite material. Different grades
of clast from pebbles to gravel. Matrix is fine material.
Clast is very rounded. This layer is cross bedded with
layers of sand that seems to be formed in ripples.
These ripples of sand often have a fine layer of clay
just above them.
Layer 6 - Grey Steepcast. This layer is clast supported.
It is gravely in texture and has pebbles and rocks
that all appear to point downwards. Composite material
has steep inclined gradient.
Layer 7 - Top Soil and Peat layer.
Appendix 6 shows a photograph displaying the above
layers.
Discussion
The lower diamict is believed to be of exclusively
welsh origin whilst the upper diamict contains erratics
from the Irish Sea as well as welsh origin. This grey
diamict is likely to have been laid down under an actively
moving glacier. This grey layer contains slate and
debris from a north Wales origin. It is believed to
have come from the welsh ice sheet moving towards the
south west from north east. It is locally referred
to as the Criccieth till. The brown diamict layer just
above the gray diamict is believed to be exactly the
same sediment it has simply been oxidised. The clay
layer shows the likelihood of soil forming on top of
the glacial till. A phase of standstill where no deposition
occurred so soil formed. The laminated clay layer held
water and clays like this are known to occur in areas
of low energy like ponds and estuaries. This is typical
melt water deposit. It is possible that as the ice
retreated and melt water poured out from the edge of
the glacier this layer of clay was formed. The sands
indicate an environment with much more energy in it;
the ripples the sand layers create through the gravel
seem to definitely indicate much more movement. The
cross bedding within the gravel supports this. The
grey layer towards the top of the cliff front with
the downward supporting clast indicates possible ground
ice that re-orientated the supporting clast into this
downward direction. This could show that although this
area was not under ice a period of cold was experienced.
 |
Recent research seems to support
Boultons views that sediment around the coast
of the Lleyn Peninsula was laid down during different
stages of the same glacial period. Although we
cannot be sure about events during the last glacial
maxim we can make some assumptions about ice
coverage at this time and the directions it took.
At the time of the Dimlington stadial it is
believed that the Lleyn Peninsula was under pressure
from two ice sheets. A large ice mass was moving
south east from Northern Ireland towards southern
England. There is also evidence to support the
existence of a second Welsh ice sheet in North
Wales that was moving South West.
The second source of ice was Welsh and two separate
streams can be identified. In the north east
local ice moved westwards down the Nantle valley
to merge with the Irish Sea ice and brought Cambrian
quartzite's and green and purple slates. (Thomas,
1998). The ice margin from the welsh ice sheet
is believed to have run approximately NE - SW.
This would place the snout of the glacier having
moved across the beach at Glanllynau and retreated
back in a similar direction. Figure 5 shows the
believed movement of these ice masses by Thomas,
Chester and Crimes (1998).
Pollen records from the deposits at Glanllynau
help place the depositional sediment earlier
than the Younger Dryas (Simkins, 1973). It is
believed during this most recent glacial period
that the limit of the Loch Lomond stadial was
at a higher latitude that the Lleyn Peninsula.
Certainly the local glaciers stayed within the
Snowdonian area. This concurs with our earlier
research on Llyn Llydaw. Oxygen isotope readings
from the sediment also indicate a glaciomarine
environment during a period of high isostatic
sea-level indicating the retreat of the Late
Devensian Irish Sea ice-sheet. There is also
evidence of in situ based decay of terrestrially
based ice containing marine sediments entrained
during its passage south through the Irish Sea
basin. (Thomas, 1998) |
It is believed that the
Glanllynau sat in the
basin formed on the Lleyn Peninsula between the Irish
ice-sheet and the Welsh ice-sheet. Welsh ice advanced
southwest as a piedmont sheet covering the coast at
Glanllynau eventually meeting and coalescing
with the Irish ice-sheet. The lower grey till was probably
sediment dragged by this welsh ice sheet as it moved
south west. This whole area is believed at one point
to have been buried by the ice sheet (formed from both
sources) that spanned as far as south Wales.
When the ice started to retreat it separated along
the ridge formed when the two sheets joined. As retreat
continued the two ice-sheets uncouples across the coast
between Pwllheli and Criccieth. Glanllynau area was
ice-marginal ... and kettle basins formed by
melt of dead ice from the margin of the retreating
Welsh ice accumulated organic sediment" (Thomas, 1998).
During this period of uncoupling the coastal area
around Glanllynau would have become an area
of meltwater drainage and glaciofluvial sedimentation
which would explain the clay layer above the diamict.
Some of the ice may have broken off and stayed in situ.
Decaying and melting finally depositing its sediment
above the meltwater sediment. The upper till layers
we observed could have beenb formed by the melting
of these abandoned - dead ice lobes.
The peat layer above the diamict has bio-organic material
inside it. The beetles inside are suited to a Siberian
atmosphere indicating a much warmer climate. The formation
of the peat and glacial melting would indicate a period
of warming when the glaciers retreated. At the top
of the sediment layers are found beetles from a more
Iberian climate which would indicate a drop again in
temperature. This is probably the Younger Dryas period.
The sediment layers at Glanllynau are very interesting
and certainly give an insight into the environmental
factors that created them.
It is clear that glaciation, major temperature changes
and subsequent retreating and melting of glaciers amongst
other factors have greatly influenced our global and
local environment and here in North Wales we can see
lots of evidence of that. I enjoyed the fieldtrip and
the subsequent research and result analyses.
References
Benn, D.I. & Evans, D.J.A. 1998. Glaciers and
Glaciation. Edward Arnold, London.
Boulton, G.S. 1977. A multiple till sequence formed
by late Devensian Welsh ice-cap: Glanllynau, Gwynedd.
Cambria 4, 10-31.
Glasser, N.F. Etienne, M.J. Hambrey, J.R. Davies,
R.A. Waters. Wilby P.R. 2004. Glacial meltwater erosion
and seddddimentation as evidence for multiple glaciations
in West Wales. Boreas 33. 224-237.
Gray, J.M. 1982. The last glaciers (Loch Lomond Advance)
in Snowdonia, N. Wales. Geological Journal, Vol 17
111-133.
Marshak, S. 2001. Earth. Portrait of a Planet. University
of Illinois. Norton & Company Press.
Simkins, K. 1974. The late glacial deposits at Glanllynau
, Caernarvonshire. New Phytol 73. 605-618.
Thomas, G.S.P. Chester, D.K. Crimes, P. 1998. The
late Devensian glaciation of the eastern Lleyn Peninsula,
North Wales: evidence for terrestrial depositional
environments. Journal of Quaternary Science 13 (3)
255-270. (back
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