Snippet 14: Long-lost piddocks and the peat

The shore at Beckfoot, on a sunny, windy day in May: the Solway is a churned, pale brown, and a wavering white line far off in the Firth marks where the incoming tide is beating against a sandbank.

Towards the bottom of the shore a patch of darkness covers the sand and I head straight down towards it; across wave-smoothed sand that is stippled with the coils and holes of lugworm burrows; splashing through shallow pools; crossing pebbly scaurs deposited by glaciers and now slippery with weed … and the darkness is as I had hoped, a bank of peat and embedded roots and fallen trunks of trees. The submerged forest, re-emerged from its burial by sand and shingle.

The submerged forest and peat, re-emerged

During the past twenty years or so, the forest and peat have appeared and disappeared on the shore between Beckfoot and Allonby, sometimes near the top of the shore, sometimes mid-shore. Some exposures are small areas of peat with perhaps a few fragments of wood, and perhaps overlying – or underlaid by – slippery grey clay. Some exposures are more dramatic: for several years a large stretch of the mid-shore near Beckfoot was covered by a sheen of clay poking out from beneath thick peat banks in which were preserved fallen branches and the bases of trunks with radiating roots. Gradually they disappeared again, partly hidden by the shifting sands, partly broken by the waves; but I had noted the empty shells of piddocks – boring bivalve molluscs – and their burrows within the compacted fibres of the peat. That must have been at least 10 years ago. And now, at the start of 2021, the storms and tides have scoured away the sand and shingle lower down the shore, and revealed part of the Solway’s story yet again.

There is much more about this complicated dance between the sea and the land, and glacial interventions, in the ‘Changeable Depths’ chapter of my book The Fresh and the Salt, and elsewhere on this blog (‘Cold cases’) but here I want to return to the piddocks, Pholas dactylus.

Imagine a mussel with its two blue-black shells (‘valves’), and now imagine these valves as yellowish-white, and rough with rows of sharp projections like tiny teeth. Unlike the stream-lined shape of the mussel, the piddock is fatter, like a thick but tapering cylinder. From a young age, mussels are sedentary, attached to a hard surface; piddocks are sedentary too, but trapped. Like the mussel larva, the minute planktonic piddock adheres to a hard surface by a byssal thread, but then starts to burrow in, using its muscular foot and rocking its shells so that the ‘teeth’ grind and grate its surroundings. Eventually, safe inside its rock-walled burrow, it feeds by filtering out organic particles from the sea, sucking in and expelling water through muscular siphons; its body and shells grow, it enlarges the diameter and length of its burrow – but not the entrance. A piddock’s burrow is its home for life.

Piddocks are normally found at or below the lowest intertidal level, down to about 35 metres. According to the MarLIN website their burrows have been found in ‘a wide range of substrata including various soft rocks such as chalk and sandstone, clay, peat and very occasionally in waterlogged wood.’ And here they are, in the peat, the tips of their empty shells still visible within the sediment-filled burrows.

The forest grew across the ‘Solway plain’ perhaps 10,000 years BP (dates vary according to which stretch of the shores is considered) but was gradually encroached upon by wetlands, sphagnum and peatbogs until about 8000 years BP. The glaciers had been melting, the sea-level rising; the land rebounded, relieved of the weight of ice, the relative sea-level fell; this to-and-fro continued for several millenia but, finally, the sea won and the waters of the great estuary usurped the territory of the peatlands. There would have been a meeting of land-based life with the chill, saline waters of the sea – at first perhaps with storm-driven spray, then with waves and tidal flows. Marine and estuarine organisms would have met with a strange new environment too, but they were the ones who prevailed. Piddock larvae would have encountered a new substratum, delightfully soft compared with rock, and made their homes. These molluscs, according to David Smith, a geomorphologist who has researched the Solway’s changing sea-levels , could have been living and filter-feeding within the sublittoral peat as long as 6000 years ago.

At some stage the sea-bed and the shore-levels changed again, and the banks of underwater peat with their preserved dead trees and their living marine fauna, were gradually overlaid by several metres of sediment and shingle. Now, a few thousand years later, these traces of former inhabitants of land and sea – of trees, of compacted mosses and other vegetation, and of marine animals – have been revealed again, to present-day humans walking on the shore.

I prised out an intact piddock shell and gently opened it and, there within the accumulated mud and sand was a mudshrimp! Mudshrimps, Corophium, are found in their millions in U-shaped burrows in the mudflats and at the edges of the saltmarshes higher up the Firth; mudshrimps, the charismatic little animals that are such an important theme in The Fresh and the Salt. Here was a little Corophium, alive and wriggling its long antennae, having made a home in a newly-revealed but pre-existing and very ancient burrow!

Later I also found the shell of a hazelnut buried in the peat, another reminder of the woodland that once spread across edgelands that were inundated to become a wider, deeper Solway Firth.

The piddock shells, the nut shell – and that mudshrimp – made this re-discovery of the submerged forest a complete and unexpected joy: three small objects that raise many questions about the past but also present many questions about the uncertain future of these liminal spaces.

For an update, see also The continuing mystery of the piddocks.

Birlinn Books 2020; available from all bookshops
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