Selected Articles from Issue Number 6/4 August 1999

REPORT ON THE LAKE DISTRICT WEEKEND
29 - 31 May 1999, led by Barbara Cumbers

Pictures of this trip can be found in the Photo Gallery

Saturday. Carrock Fell and the Caldew Valley

After the traffic traumas and torrential rain of the previous day, the first field day was glorious - brilliant sunshine from a sparkling blue sky. Our first locality was the Carrock Fell igneous complex, a multiple intrusion of dyke-like forms into Skiddaw Slates to the south and the Eycott Volcanic Series in the North.  We traversed the fellside like agile sheep, identifying the succession of intrusions: granophyre, diorite and gabbro with Barbara's guidance (and Jim's corrections!).  This successful morning was further enhanced by the fine views east to Cross Fell and south east to Eycott Hill and the high fells beyond.

A happy group of 14 and a dog thus bravely descended to drive to the second locality and picnic lunch on the rocks, in the valley of the Caldew River.  Vigorous, but affable debate ensued over the steeply plunging folds displayed in these river slabs (Kirkstile Slates, Skiddaw Group), whether they were slump structures or were due to a pre-volcanic tectonic episode.

An after-lunch stroll up the Caldew River took us to the confluence with Grainsgill Beck, and the northern outcrop of the Skiddaw granite where Barbara pointed out the sharp contact of the hornfelsed slate and Grainsgill Greisen.  The greisenised granite is composed primarily of quartz with muscovite and other alteration products from feldspar and biotite.  Carrock Mine exploiting the tungsten veins cutting through the greisen and hornfelsed slate was a short distance upstream.

The third locality of the day provided dual interest:  the palaeontologists plying Di with possible graptolite fragments, while the structuralists excitedly examined the folds and faults as well as some highly altered quartz-dolerite dykes: all this in Mungrisdale Quarry (black slates of the Skiddaw Group with an overall succession indicative of turbidites), before retiring to "The Mill" nearby for well earned refreshment.

Yet more rocks, scenery and sunshine followed a convivial evening meal at the hotel in Keswick - the gentle ascent of Castle Head formed from an intrusive plug of dolerite, plus a final surprise of an quarry face that Barbara hadn't known was there on the descent.

Thus ended a very full, interesting and enjoyable day all due to Barbara's superb organisation, with additional light entertainment from our Treasurer who went slipping in the wet, repeatedly!

David Hopwood

The second day was spent among the Borrowdale Volcanics in the centre of the Lake District - surrounded by glorious scenery on a warm though cloudy day.

A short drive through Borrowdale took us to our principal site, at Seathwaite. From here the various spoil piles of the Cumberland graphite mines are clearly visible. The graphite formed the basis of the pencil industry from the 16th  - 19th century.  The deposit is unique, being formed at low temperature (200°C), possibly by interaction between ferric compounds in pyroxene and CO₂.  The mines became uneconomic at the end of the 19th century when it was discovered that other sources of graphite (formed through high temperature metamorphism), could be mixed with clays to give the properties needed for pencil ''lead'' which Cumberland graphite has naturally.

We then ascended beside the waterfall to look at  the Whorneyside bedded tuffs of the Borrowdale Volcanic Group (BVG). These show differing depositional environments - some size gradation and cross-bedding indicating ash deposition in shallow and possibly transitory water.   Minor faults in bedding indicate earthquakes?  Overlying blocks of matrix-supported pre-existing sedimentary clasts indicate a debris flow or lahar, possibly associated with caldera collapse?  Higher up, exposure indicated sub-aerial deposition leading to grading without lamination.  It was suggested that there was subsequent collapse, followed by lake formation. Overlying sequences included both waterlain and sub-aerial deposits, with some showing lapilli.

After lunch we moved up to find evidence of ignimbrites, searching for fiammé textures, but time precluded a visit to andesitic sills to the north.  Subsequent discussion covered the mechanism of initial volcanic activity, with caldera collapse, followed by the deposition of the 3km thick upper BVG pyroclastics, which were subsequently eroded by glaciation to expose the evidence of the caldera-lake deposition we had seen.

In the afternoon, we moved to St John's-in-the Vale, just east of Keswick - an area that also provoked much mystification and discussion! We studied the Threlkeld microgranite, intruded at the interface between the lower BVG and the underlying Skiddaw slates. Discussion covered the conglomerates marking the base of the lower BVG in this area - rounded clasts of volcanic rocks and Skiddaw mudstone, indicative of active erosion  of earlier volcanics. More enigmatic were the reddened flow-brecciated lavas - oxidation soon after eruption?

And so to dinner - and on-going debate!

Alex Toohey

Monday, Windermere Group, Coniston
ON THE LEVEL

We had already had two wonderful rain-free but strenuous days in the field around Carrock Fell and Seathwaite, when we asked our intrepid leader if the weather would hold and if we could cope with the demands of the next day.

Accepting that the weather was beyond Barbara's control, most of us were happy to learn that, on the whole, the sites on the long, rough Walna Scar Road were on the level. But was she being "on the level" with us? Some must have had their doubts, as we were four persons short when Laurie assumed his position as our efficient gate keeper - this time above Coniston Water.

The weather was fair as we assembled at Timley Knott for the considerable up-hill scramble to our first localities. Having discussed the fossiliferous Applethwaite Member, we all left Di and Kevin behind, selecting and identifying the goodies, while the rest of us continued on up an ever-steepening hillside to the contact with the re-worked Borrowdale Volcanics, then up again to the contact between the Borrowdale Volcanics and the re-worked Borrowdales.

Marilyn Carter

THE TRILOBITE TRAIL

Led by Peter Sheldon, early May 1999

Saturday morning

After a short briefing in our classroom we set off in a convoy of 5 cars to Llanfawr Quarry on the far side of Llandrindod Wells. A short walk across some rough ground brought us to a padlocked gate (our intrepid leader had the key ready) and we descended into the partly flooded quarry. Our progress was attended by some extremely vociferous sheep but totally ignored by the mallard swimming on the quarry pond.

Although itching to get down to the main business of collecting fossils, first we had a go at deciphering the structures and lithology in the quarry faces. With some prompting, we agreed that we were looking at a heavily-weathered brown dolerite intruded into dark coloured shales (the Nemagraptus gracilis shales of Ordovician age). Peter produced an excellent field sketch (which he had prepared earlier) and explained that the dolerite was a feeder dyke to a mushroom-shaped intrusion. "Was it a laccolith or a lopolith?" we debated..

Splitting the fossiliferous shales was difficult as they had been cooked by the intrusion, but we still found some excellent fragmented fossils: graptolites (including Nemagraptus), and trilobites (mostly Cnemidopyge tails and Trinucleus heads). The latter showed clearly the sieve-like margins whose function we speculated about, until it was noted that trinucleids had no eyes. After an hour of splitting and hammering, we reluctantly moved on to locality 2.

A few miles to the south, we left the road and proceeded along a very bumpy gated track to Gilwen Hill Quarry, from where we had a magnificent panoramic view. Fortunately the early mist had cleared and we were able to see the tilted dolerite sill that forms Gilwen Hill. Our main interest however was the large area of quarry spoil bursting with trilobite remains. Almost immediately a megatrilobite jumped out and the hunt was on . . . We began to get rather choosy. These were the Didymograptus bifidus shales, and the oldest strata of the weekend. They are very fine-grained sandstones with some ash layers and provide an excellent degree of preservation. Several different species of trilobites were found - as Peter had guaranteed, everyone found some.

We were very late setting off to the lunchtime pub, and we went off very hungry, but with heavy bags of treasured trilobites.

Barbara Wood

Saturday afternoon

Our first afternoon stop was Llanelwedd Quarry outside Builth Wells, with wonderful views of the town. Standing on the edge of the Builth inlier, we admired the Silurian hills to the south (and a couple of Holocene buzzards soaring overhead). The River Wye that wanders along the valley floor is now a mere shadow of its former self.

These rocks were laid down in the Llanvirn stage of the Ordovician. We walked up to a rocky outcrop that sloped towards us - it was a poorly-sorted quartz-free sandstone, with feldspar from the weathering of basaltic lavas - a turbidite flow, underlain by black shales, although the latter had been covered by quarry waste.

We entered the vast quarry amphitheatre, past piles of aggregate in elegant shades of grey. The quarry walls consisted of submarine basalts whose feldspar had been altered and the rock was now a spilite; the vesicles left by escaping gas had filled with chlorite and calcite. In places parts of the lava had been replaced by a bluish-green metamorphic mineral - pumpellyite. There was not a pillow in sight, the basalt flood had been too rapid for them to form. We found signs of strike-slip and other faults on the quarry walls.

Peter drew our attention to a graphite deposit, but how did it get there? Organic-rich matter in the shales beneath had been heated and the petroleum produced had migrated through the rocks and collected beneath an impervious layer; with further heating the liquid evaporated and only the carbon graphite remained.

We passed an area where gas bubbles had been infilled by a variety of minerals, giving the rock an interestingly spotty look.

We then drove higher up into the deserted quarry - the path was lined with giant rocks to keep us away from the sheer edge. As we were near the top of the basaltic lava flows there were signs of weathering, as well as hot water alteration. Would it be possible to find trilobite fossils here? The instinctive reaction would be: "Of course not, these are basalts", but between each successive lava flow were thin muddy layers and these might have contained the occasional trilobite, though none were found and even Peter has never found any. We came across more slickensides and very attractive brecciated basalts, as pyroclastic flows had included chunks of other rocks.

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