Freshwater mussels in the West Cumbrian coalfield

saltom winding-house

The coalfield beneath the sea: view SW from above the Saltom winding-house

The late Norman Hammond once told me that he used to go out in his boat to count the basking sharks when they came into the Solway. One time, he was motoring off Fleswick Bay near Whitehaven during a coal-miners’ holiday; the men had been spending the day with their families on the beach, and one shouted across to him across the water.“I was asked if I could take them out to have a closer look, which I was glad to do,” Norman told me. “One man said he’d love to swim with them and I told him to slip over the side – off came his clothes and in the nude he swam with the sharks from Fleswick to St Bees’ Bay.”

Norman, who died in 2005, was the founder of Solway Shark Watch and he and his wife Florence were founder members of Cumbria Wildlife Trust. He was one of those rare people, so knowledgeable and enthusiastic about the natural history of the Solway that he wanted to share his knowledge with everyone. He was a story-teller – if you asked a simple question he would reply with such a wealth of information and anecdote that you became caught up in the tale. When he told me that one of the miners had given him a ‘freshwater mussel that he’d found in the mine’, that was a piece of information to remember and treasure.

So I immediately thought of Norman when geologist Eric Gozlan mentioned ‘musselbeds’ during his talk to the Cumberland Geological Society about West Cumbria Mining’s plans.
West Cumbria Mining (WCM) are exploring the West Cumbrian coalfields beneath the Solway Firth to the South-West of Whitehaven, with the view to extracting coking-coal (there’s a fuller discussion of the background, drilling and future plans elsewhere in this blog). An enormous amount of relevant geological data already exists but, as Eric pointed out, there is a considerable amount of faulting in the coal-field – in other words, there have been upheavals and slippage so that a band of coal and the rocks that sandwich it might have dropped several metres relative to their main position. Understanding the depth and extent of any fault is obviously important – there’s no point trying to carry on mining a band of coal if it has been pushed aside by a fault.

coal-seam-sequence-1-wcm-website

Stratigraphy diagram from WCM’s website

 

So, to map the coal-bands in three dimensions, it is necessary to drill bore-holes at different locations and extract cores. By knowing the depth of each rock-type on the core, it’s then possible to fit this information to the corresponding data from other cores and plot the extent and depth of the coal seam.

 

As Eric said during his talk, “The exploration geologist is always trying to find out where he is.”

eric1 crop

Eric Gozlan in WCM’s wooden office inside the warehouse

And one way he (or she) can navigate down through the rocks is by looking for specific markers of geological time. That’s where the fossil musselbeds become so important.
Knowing of my interest in the mussels from the mines, Eric had generously suggested that I come and visit the office to look at the cores. I met him and his colleague Steve Craig in mid-February at WCM’s office and ‘core store’ in Whitehaven. Their office is located within an airy warehouse adjoining a garage on an industrial estate – it was cold both outside and inside, and the heating was turned up high.

Eric has been leading WCM’s exploration project since September 2014; he has a Master’s Degree in Geophysics and Geology from Monash University, Melbourne and before joining the WCM team already had 10 years’ experience working in the mining industry. Stephen, who has a Master’s in Geochemistry from Leeds and is WCM’s site geologist, formerly worked in the goldfields of Western Australia. Within minutes of my arrival, they were both scurrying round, looking for technical papers for me and photocopying charts.

coal strat bgs

Taken from the BGS Memoir for the west Cumbria district (page 60)

The most useful chart is from the British Geological Society’s Memoir for West Cumbria [1], which shows the two main musselbands, one marine, one freshwater. Eric told me that the Vanderbeckei band – its local name is the Solway Band – ‘extends from Moscow to America’ and it is the ‘target boundary’ between the Middle and Lower Coal Measures that they look for in the core, identifiable by its fossil marine fauna such as the brachiopod Lingula.Then they search for other recognisable features.

‘We crack open the cores and look for freshwater mussels. Within the next ten to fifteen metres we should then typically find Yard Band, which is a good unfaulted band. Though it can be less than five metres depending on the [nearby] faulting,’ Eric said. Above the freshwater mussel band are Main Band and Bannock Band, both coal seams that are of interest to WCM.

Steve explained that there are as many as 20 different species of freshwater mussel fossils that are diagnostic of the depth of strata and the proximity of coalbands; or, as the BGS Memoir puts it, rather more euphoniously, ‘Palaeontological study of marine and nonmarine fossils has provided a chronostratigraphical framework for the coalfield.’

Eric and I go out into the core-store where metal shelves are stacked high with plastic core trays; black or mottled or wrapped in paper, the cylindrical cores are labelled with letters and numbers to identify their place and depth of origin.

The next batch of offshore drilling will start again in spring/summer 2016. ‘We will get tighter information on the kind of coalfield we’re going to intersect,’ Eric said, ‘– for example for the Sixquarter. We’re drilling to identify specific targets.’ Two drilling rigs will be running for up to six months – with two more geologists joining the current team of four.

He gives me a section of a core that has been varnished to show the freshwater fossils more clearly: there are two very obvious bivalve shells embedded in the rock – Steve says they are probably the mussel Anthracomya. Carbonicola (the only species whose name I knew) is older, found lower down. Two other sections of core show fragments of marine fossils, of the brachiopod Lingula and a couple of graptolites.

Seeing those fossils – marine fossils, freshwater fossils – in situ in rock that has been brought up from 400-500 metres below the seabed of the Solway, is extraordinary. It’s too easy to become blasé about fossils, there are boxes of them in museums and private collections. But these Anthracomya, whose fossil shells I could stroke with my finger,  were living, growing and filter-feeding on the bed of a river perhaps 300 million years ago.

That river ran across land that had previously, on occasions separated by tens of thousands, even millions of years, been submerged beneath the sea. The river delta had itself been inundated by the sea much later, on widely-separated occasions. That competition between sea and land had been carried out over millions of years; the climate had swung between tropical and temperate and glacial; the place those swamps and sediments and rocks occupied on the surface of our planet had drifted and been pushed through different latitudes and longitudes across the planet.

Norman Hammond would surely have had a good story to tell about that.

 

[1] British Geological Survey: Geology of the west Cumbria district, Sheet Memoir 28, 37& 47. NERC 1997

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