Spring 2013 part 1: Chasing ancient microcontinents in North Wales - the Megumia project


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It's March 21st and the Spring Equinox has passed - not that there's anything much springlike about conditions out there with a chilly SE wind and a potentially significant snow event on the way for many areas of Central and Northern England and Wales. March so far has been rather tedious with colder than normal, duller than normal and generally unexciting weather to get engaged with and the garden still awaiting the start of planting which has been put back into next month. Even the Celandines are slow coming on: I photographed this one in late February but there are still very few around. So I'll do a geological post this time.

Lesser Celandine, February 2013

Late February into early March saw me spending several days in the Harlech Dome of North Wales. This is an elongate outcrop of Cambrian strata that extends from Dolgellau-Barmouth up north to
wards Porthmadog-Ffestiniog. The most extensive outcrop of Cambrian rocks in Wales, it is surrounded by the far more widely-seen Ordovician and Silurian rocks. The image below shows the view from within Coed y Brenin, in the SE part of the Dome, southwards to Cadair Idris, made up of sedimentary and volcanic rocks of Ordovician age.

Coed y Brenin and Cadair Idris

Why the interest in the area? To answer that we need to go back in time to the Cambrian, 541-485 million years ago. To say that things were a bit different from now would be to put  it mildly. Back then, a large continent, Gondwana (largely comprising what are now west Africa, South America, Arabia and India), straddled the South Pole, as shown on the RHS of the diagram below, redrawn from Waldron et al, 2011 (full ref at end of page). What is now England and Wales and Nova Scotia were, back then, small sliver-like microcontinents that were rifting away from Gondwana's northern margin and moving northwards, with small oceans in between them.

The Cambrian sedimentary rock successions of the Harlech Dome and the Meguma Terrane of Nova Scotia have a number of similarities, such as the distribution of the types of sediments (coarse or fine) found through the sequences and  the presence in both sequences of unusual manganese-enriched rocks. Detrital zircons (zirconium silicate - a very resistant mineral that can be isotope-dated) from marine sandstones in both sequences have revealed on analysis the likelihood that the sediments were probably derived from the erosion of the same areas of Gondwana. Zircons from adjacent terranes to Meguma and the Harlech Dome reveal somewhat different patterns, suggesting that they are unrelated, or at least much less closely related, than Meguma and Harlech are to one another. These discoveries have led to the proposal of a new terrane - Megumia - of which the Harlech Dome and Meguma are now widely-separated fragments. The terrane's northern boundary in Wales is the Menai Straits Fault Zone, long regarded by geologists as a major terrane boundary, separating the rest of Wales from the north-western Llyn Peninsula and Anglesey, where the rocks, forming what is called the Monian Composite Terrane, are unlike anything seen elsewhere in Wales. The southern boundary of Megumia is - possibly - the Welsh Borders Fault System, in which case Megumia may include the entire Cymru Terrane (an existing term) as shown in the LH map below. The unfortunate lack of Cambrian rocks at surface over much of the area to the south of the Harlech Dome - they are buried under great depths of younger rocks - makes firming-up this bit of the picture even more tricky than usual!

The ancient terranes of England and Wales

So it was my lot to spend a few days in the Harlech Dome sampling the rocks for the National Museum of Wales, the British Geological Survey and the University of Nova Scotia. The focus was to be on the Greenstones, a term that has been used in the area
ever since the gold-mining took off in the 19th Century. A Greenstone is an intrusion of igneous rock that has formed due to magma being forced into the sedimentary rocks deep underground and hundreds of millions of years ago (probably during the Cambrian), sometimes as vertical dykes, sometimes as horizontal sills and sometimes as large irregular bodies. The image below shows a classic example: the Greenstone is the grey rock in the lower part of the image. Sat on top of it in a parallel arrangement are well-layered bedded Middle Cambrian sedimentary rocks belonging to the Maentwrog Formation. They have a slightly rusty appearance due to the iron sulphides that they contain, which weather readily into ochre-coloured iron oxides. This spectacular rock-outcrop is in the valley of Afon Mawddach near Ganllwyd, to the north-east of Dolgellau.

Greenstone-sediment contact, Mawddach Valley

The term 'Greenstone' reflects the fact, recognised much later, that these igneous rocks have suffered a tremendous amount of alteration due to circulating hot waters, again deep underground and hundreds of millions of years ago, reacting with them and changing their mineralogy. Mostly they are a dull greenish-grey colour and all original features have long since gone. But I was intrigued by some old 19th Century reports of a rock described as a "magnificent Uralite Porphyry" on the hill of Cefn Deuddwr, to the west of the Mawddach Valley. Now I know 19th Century science writers liked to wax lyrical in their descriptions, but one could hardly described a bog-standard Greenstone as 'magnificent'! So a search of Cefn Deuddwr was duly made, and it turned some rather different rocks up:

less-altered greenstone, Mawddach Valley

You can actually see some original textures and mineralogy in these...

less-altered greenstone, Mawddach Valley

I was also asked to locate and sample the dyke-swarms that traverse the area from NW to SE. At first this proved difficult. The first area I tried turned out to be so badly overgrown that dykes drawn on the geological map could not be located. The only really interesting thing in this area was at a quarry in the pebbly sandstones of the Rhinog Formation, which showed a lot of boulders with unweathered pebbles as in the image below. These are bits of Gondwana! There's a lot of quartz, some with a bluish or pink colour, and a lot of fragments of volcanic rocks - the pale subangular grains.

Pebbly sandstone, Rhinog Grits Formation

Finer-grained sediments at the same locality were rich in large cubes of iron pyrites:

Cambrian sedimentary rock with pyrite cubes

The need to find some of these dykes led to an expedition into the Rhinogs to the NE of Harlech, an area I have long loved. This is one of the wildest places in the UK, outside of NW Scotland. It was however at one time a hive of activity because it hosted a manganese mining industry. All along the western flank of the Rhinogs there are manganese mines and the well-built trackways that lead to them:

Track to Llyn Eiddew Mawr

It was the warmest day of 2013 so far and the walk-in of a couple of miles was more than a warm-up! This is Llyn Eiddew Bach, passed en route to the mines:

Llyn Eiddew Bach

The view towards the hazily-sunlit main ridge. Haze - no problem! The sunshine was the bit that mattered! The terraced hillside owes its form to the thick sandstone beds of the Rhinog Formation. Beneath each terrace there are lots of huge boulders, cloaked in knee-to-thigh-deep heather concealing the deep holes between them. If you find a path in the Rhinogs, do not leave it lightly.....

Looking into the main Rhinog range

The old metal-miners of Wales were nothing if not resourceful. This trackway was driven along the manganese ore-bed. It thus achieved two purposes: first the trackway and second the ore-bed was prospected along its outcrop. The ore-bed varies a lot in thickness and was only worth mining once it exceeded a certain width....

Miners' track to Llyn Du Bach

Above the ore-bed are massive sandstones of the Hafotty Formation. Often, the track-makers left this very tough rock in place, so that it forms impressive overhangs, over head-height, above the track. Why move it if it's not gonna go anywhere?

Miners' track to Llyn Du Bach

The upper part of the track reveals wilder and wilder scenery. It's amazing to think that much of this was done purely by hand!

Miners' track to Llyn Du Bach

At the top of the track's long steady climb, a frozen-over Llyn Du Bach was reached, nestled atop the ridge, in a deep NW-SE depression in the rock:

Llyn Du Bach

Beyond, the trackway and test-holes on the manganese ore-bed continue a little further. The image below shows a chunk of ore. It's a distinctive-looking pinkish rock when fresh (I've knocked a chunk off this bit to reveal it), but it soon weathers grey-black....

Manganese ore

Cut and polished, it's quite impressive (before it weathers). The manganese occurs as carbonate and silicate minerals: the red layers are coloured by iron. The area around Llyn Du Bach is now a Site of Special Scientific Interest due to the spectacular geology, although it should be fairly safe - I can't imagine anyone wanting to build a Tesco up there! The ore formed as a chemical precipitate and reflects some pretty weird seawater chemistry going on at the time.

Manganese ore

From Llyn Du Bach a scramble led up onto the ridge and its extensive, boulder-strewn rock platforms. Snowdon in the background in this shot:

Up on the ridge, N Rhinogs

Continuing along the ridge I found a newly-built cairn
. It seemed a good place to stop for a break....

Up on the ridge, N Rhinogs

The view southwards along the ridge.....

Up on the ridge, N Rhinogs

In this area there is a thin cap of Hafotty Formation sandstones along the top, overlying the Rhinog Formation that makes up most of the ridge. I was amazed to find these structures all over the place - they are sand-filled burrows, left behind maybe by some long-extinct worm that lived in these shallow waters all that time ago, deep in the Southern Hemisphere....

Worm burrow, Hafotty Formation

Traces of a much more recent event - the last glaciation that ended only several thousand years ago - were also evident in the shape of east-west scratches where the huge icecap to the east had breached parts of the ridge. Hard bits of rock frozen into the base of the ice did the scratching.


Dykes were found to be common but often eroded away because they are softer than the rocks they intruded. This is a classic - pebbly sandstone (L) and dyke-rock (smooth appearance, greenish, R):

Dyke contact with pebbly sandstone, N Rhinogs

Having sampled half a dozen dykes I reached Llyn Corn-ystwc, traversing its western side precariously over sandstone boulders....

Llyn Corn-ystwc

...after stopping to photograph patterns of ice, water and rock...

Llyn Corn-ystwc

The westwards descent directly down to Llyn Eiddew Mawr was hard work. I followed the L-slanting gully L of centre in the image below, taking care not to fall down any holes between boulders, something I would not recommend especially with a heavy rucksack. It was a pathless nightmare save for the occasional sheep-track. Almost at the bottom of the gully I met with a 10ft waterfall and had to make a precarious out-of-balance scrambling breakout onto the hillside to its left, looking down. It was good to get to the lake and chill out for half an hour after that!

Llyn Eiddew Mawr

Walking back out, the weather was clearly on the turn, with high cloud and low mist streaming in over Porthmadog and the Llyn Peninsula...

NW from the Rhinogs

On March 11th, taking the various samples from the several days of work to Aberystwyth University to begin their long journey back to Nova Scotia, the wintry theme returned. I caught this image of an almost deserted Borth Beach with a heavy snow shower out in the bay, on the way back

Snow showers, Borth beach, 11th March 2013

The samples will have their geochemistry examined in great detail and compared with that of the intrusive rocks of the Meguma Terrane. What secrets will they yield? That is a tale that will be told after a lot more work! In the meantime, focus is on the incoming heavy weather here, and wishing for spring to get its act together!

Reference: John W.F. Waldron, David I. Schofield, Chris E. White and Sandra M. Barr: Cambrian successions of the Meguma Terrane, Nova Scotia, and Harlech Dome, North Wales: dispersed fragments of a peri-Gondwanan basin? Journal of the Geological Society, 2011; v. 168; p. 83-98


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