Autumn 2010 part 1 - blue
moss and the Irish Sea Glacier....
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From
late August into September, a few days of relatively settled weather
permitted some overdue jobs in the garden before unsettled conditions
returned to keep moisture levels up and allow the continued fruiting of
edible fungi. This entry features both, plus a bit of palaeoclimate and
Jurassic geology, the results of a walk at one of my favourite spots -
Tonfanau beach.
The potato-harvest was done on Bank Holiday Monday, when the weather,
in a most untraditional manner, steadily improved! The variety I grew
this year was Cara - following a bit of blight in 2009 I was keen to
select one with strong resistance. It paid off and a good yield
resulted.
Following that, while the trays of potatoes were drying in the
sunshine, I had a session of open warfare on the creeping buttercup
that was threatening to take over several areas. Most "weeds" I compost
but this is a tough critter - so I converted it into potash instead!
The dense smoke and green leaves made for some unusual photographs...
As I sat tending the fire, the traffic built up at Dyfi Bridge, where
the three-way lights were not coping with the sheer volume of folk on
the move. It was backed up from the lights all the way to Machynlleth
and stayed that way for much of the day. Thankfully the repairs have
now been completed and things are back to normal. Jams like this are an
uncommon sight around here....
Brief settled conditions allowed for a bit of fishing but the mostly
rough August has left the fish very scattered. The shot below was taken
at Borth one morning at sunrise..... night tides are best as the beach
is still fairly busy this time of year.
One day I persuaded a couple of mates to share the fuel cost and we
tried Bardsey Sound for mackerel and pollack. We caught both, but not
in numbers. The sea was a bit cloudy even here where it is normally
gin-clear - a testament to August's unsettled conditions. There was an
amazing double-sundog to finish the day off, though - shot with the
aperture stopped down to the max:
On the 6th I
travelled to Dolgellau to have a meeting about some work in October -
about time too! We went for a quick recce to one of the sites where we
would be bringing students - Glasdir copper mine. The Forestry
Commission have done an excellent job of clear-felling and laying paths
around the remains of the old mine-buildings where the ore was crushed
and the copper-bearing sulphides recovered. Naturally, with all that
activity, the ground hereabouts is heavily contaminated by copper, as
one would expect. What I didn't expect was this:
Briefly thinking someone had been messing about with paint, I looked
around and it was everywhere, including some inaccessible places. The
photo below shows a similarly-coloured Cladonia:
I guess the groundwaters hereabouts contain a lot of copper and these
mosses and lichens take it up, but in dry conditions evaporation
concentrates the copper to the extent that it becomes visible. It will
be in the form of one of the copper salts - like the hydrated sulphate
chalcanthite or the basic sulphate langite. Analysis will reveal its
nature: if chalcanthite then the colour will presumably have gone by
the October field-trip, what with all this rain - the mineral being
soluble in water.
On Sunday 12th I made the most of a sunny afternoon and went for a good
walk at Tonfanau, to the north of Tywyn on the Cardigan Bay coast. This
is a quiet, peaceful section of the Cardigan Bay Coast, presumably
because access involves a bit of a walk. The footpath brings one out
through a gate onto the beach close to the outfall of Afon Dysinni:
As I turned northwards along the beach, a Bar-Tailed Godwit trotted out
of the shingle and headed for the sea:
It soon saw me
and sprinted off across the beach:
Tonfanau was an
important military camp during World War 2 and the coastal hinterland
is dotted with semi-derelict buildings and concrete roadways. The
cliffs are made of soft, easily-eroded glacial and fluvioglacial
drift and are retreating rapidly in the face of coastal erosion and
sea-level rise. On the beach are strewn waterworn bits of former
buildings amongst the natural boulders....
The low cliffs are of great geological interest. There are two
varieties of drift here: a lower one derived from the glaciers that
came down from the Welsh Mountains and an upper one derived from the
great glacier that ran down the Irish Sea, a portion of which pushed
through a low gap on the Lleyn Peninsula and passed close to this
place...
The section also reveals cryoturbation structures - slumping and other
deformation, formed during permafrost conditions.
How do we know
that Irish Sea ice passed close to this place? The clue lies in the
clasts - the blocks, up to car-size, of stone that the ice
transported and left here. Many are of local rocks from the nearby
mountains such as this block of sedimentary rock from the Cambrian
Period:
Or this block of volcanic rock, of probable Ordovician age - Ordovician
volcanics are of widespread occurrence in the hills of North Wales....
But in places one finds blocks of this stuff:
This is much softer shelly limestone of
Jurassic age, here crowded with the remains of bivalves and with a few
fragments of fossil wood, too...
In this shot, there is a larger, thick-shelled bivalve (L) and the
calcite guard of a belemnite (R).
There are currently no outcrops of Jurassic
rocks at surface in North Wales, which is mostly made up of much
older Palaeozoic strata. However, the existence of an offshore
sedimentary basin in
Cardigan Bay was predicted years ago and confirmed by boreholes
here and at Mochras, up the coast (see references). The first borehole,
at Mochras, proved not only the presence of much younger rocks but in
addition the thickest sequence of Lower Jurassic (Liassic) rocks
anywhere in the UK. At Mochras and at Tonfanau, the coast juts out just
far enough for the land to extend west of the massive, N-S system of
geological faults that make up the eastern margin of the Cardigan Bay
Basin. The diagram below shows a section through the rocks in the
Mochras area (from reference 1):
Since the time of the drilling of the Mochras Borehole, research has
confirmed the existence of Jurassic and other young rocks beneath the
seabed throughout much of Cardigan Bay and St George's Channel, forming
a series of interconnected sedimentary basins. The erosion and
transport of the limestone blocks, from somewhere now out to sea, is
considered to have taken place as the Irish Sea ice moved through, at
the height of the last glaciation:
Glacial ice ploughs up bedrock as a farmer ploughs up soil - it is an
extremely efficient agent of erosion. The diagram above, taken and
redrawn from Wikipedia's Irish Sea Glacier entry, is a representation
of the geography of the Welsh Mountain Ice-Cap and the Irish Sea
Glacier at the last glacial
maximum. The arrows depict the flow-direction of the ice and the dotted
line marks the southerly limit to the ice-front. Sea-levels
at the time were over 100 metres lower than those of the present day,
so in Cardigan Bay, most of which is today less than 40 metres deep,
the ice would have been grounded i.e. it was moving directly over the
rock, tearing it up as it went along. Some of it ended up dumped along
the coast at Tonfanau, where it is eroding out of the soft cliffs as
sharp-edged blocks that are then rounded-off by wave erosion.
Fascinating stuff!
On my return I stopped to grab an ultrawide shot of the cirrus
radiating out from the northern horizon, with some of the ruined
wartime buildings in the foreground:
The weather
steadily went downhill after that - on Monday 13th and Tuesday 14th it
felt as though it had not got light properly and rain set in and
continued through most of the period. By Tuesday afternoon, when the
rain cleared away southeastwards, the Dyfi had burst its banks and the
A487 out of Machynlleth was closed to all but larger vehicles:
On the 15th,
light showers persisted but I needed to stock up in Nature's larder. In
a nearby very soggy wood, the Chanterelles were in reasonable
numbers...
...as were
Hedgehog Fungus - with a similar and arguably better flavour!
Here's a shot
of the undersides of both. The Chanterelle does not have gills - it has
ridges - and the Hedgehog, like its namesake, has spines. This makes
them relatively easy to identify - with a bit of practice. It is
essential to know what you are doing when gathering wild food, as a
mistake can be fatal!
Here's a
typical haul - Chanterelles and two species of Hedgehogs!
It's getting noticeably cooler at night now - I lit my first log fire
for ages last night, although coming down with the cold that is
circulating vigourously through the local community has not helped in
that department. Way out in the Atlantic, Hurricanes Igor and Julia are
trundling along but the long-range model output is extremely variable
run-to-run on the topic of what they will do once they morph into
extratropical depressions as they recurve away from North America. Some
runs have brough heavy rain and gales to the UK, others stick it under
a large area of high pressure. However, we are talking about T+200
hours, and weatherwise I tend to take anything beyond about T+120 with
a healthy pinch of salt! In other words, we'll see!
References
1. Holford, S.P., Green, P.F. and Turner, J.P. 2005:
Palaeothermal and compaction studies in the Mochras borehole (NW Wales)
reveal early Cretaceous and Neogene exhumation and argue against
regional Palaeogene uplift in the southern Irish Sea. Journal of the
Geological Society; September 2005; v. 162; no. 5; p. 829-840; DOI:
10.1144/0016-764904-118
2. Woodland, A.W. (ed.) 1971. The Llanbedr (Mochras Farm) Borehole.
Report of
the Institute of Geological Sciences, 71/18.
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