ConchLog: Shellsand Workshops in Reading and Cambridge

The workshop organisers – Bas Payne, Christine Street and Jan Light – would like to thank Martin Bell, Richard Preece and their colleagues for enabling us to use such excellent venues at Reading University and Cambridge University Museum of Zoology (see figures 1 and 2) and for their help on the days. The text of this ‘blog post’ is compiled from material prepared by all three organisers.

To a conchologist, shellsand is a magic substance which can give hours and hours of shell-collecting pleasure (particularly in the winter months) long after return from a trip to the beach to look for shells on the strandlines, or to record from living habitats. To a geologist it is a sediment that contains 50% or more carbonate grains. The shellsands we are familiar with from the seaboard coasts of Britain and Ireland are skeletal (also known as biogenic) sediments because they are composed of biological remains: molluscs, serpulids, barnacles, bryozoans, echinoderms, forams, brachiopods, decapods, otoliths, and ahermatypic (non-reefbuilding) corals.

On two Saturdays in April shellsand workshops were held in Reading (10th) and Cambridge (24th). The aim was to offer an introduction to the pleasures and benefits of collecting and sorting shellsand. In all some 15 participants, ranging from novices and students to seasoned collectors, sorted fractions of shellsand from Porthcurno. Porthcurno is a small village in the parish of St. Levan located in a valley on the south coast of Cornwall. It is approximately nine miles to the west of the market town of Penzance and about three miles from Land's End.

Porthcurno beach faces southeast, is narrow and has two embayments. Access into the second embayment is possible at low water. The sediments at this beach are intensely carbonate-rich, pale, biodiverse shellsands and gravels. Apart from numerous Spisula valves, shells of macromollusc species are seldom washed in.

Christine told us that finding shellsand is a matter of close inspection and luck. You find deposits of shelly material on different areas of beaches at different times, and tracking them down means getting to know your beach with its wind regime, tides and currents as well as its shape and topography. Her experience centres mainly around small Scottish islands and over the years she has visited a goodly number of them.

When prospecting for deposits it’s wise to have x10 lens around the neck which allows you to pick up a small sample in the hand and inspect it closely. What you see should give you a clue as to whether it is worth taking a sample home to sort under a microscope. By and large, shellsand is usually best found as the tide retreats. There may be a series of tidelines containing deposits. Where there are ridges and furrows on rippled sands, the shellsand collects in the dips, and also in the small scour moats that encircle scattered rock outcrops.

Generally speaking curved sandy bays are most productive. Some of the more interesting ones, at least on islands, are those coves with a southwesterly or northwesterly aspect where the prevailing current from the south or north sweeps past, into and around the curve of the bay, and where there is a projecting curved rock outcrop at the mouth to prevent all the deposits being swept out again. Also those beaches which have a partially submerged reef can have interesting deposits that have been prevented from escaping on the outgoing tide.

These are of course broad generalisations. Beaches can be changed by very rough weather: most shells may be scoured away, although other interesting deposits, displaced from the seabed offshore, may be beached after a storm.

Bas talked us all through collecting, cleaning, grading and sorting a shellsand sample. I outlined a few guidelines for identifying molluscs species and touched on the merits of recording species distributions for publication via a digital database. For most of the time we peered down microscopes, retrieving the shells we saw and naming them. Working as a group meant that we could compare notes frequently and unravel mystery specimens by pooling our collective experience. Many of the shells measured no more than a millimetre or two. They are intricately and perfectly formed, amazing.

Some island shellsand beaches with a star rating – Christine’s observations

ORKNEY

Papa Westray, west of the southern jetty. HY4950. Small shells become trapped by rocks to the extreme southeast and float in on the incoming tide.

SHETLAND

Fetlar, the main beach in the south is Tresta. HU6190. Small shells are trapped mainly to the westward side by the projecting promontory, carried on the outgoing tide.

THE HEBRIDES

Eigg: The Singing Sands. NM4790. A northwest facing beach with rocks at the southern end.

Tiree: Gott Bay. NM 0546. A very curved, wide southern beach. Small shells tend to be in the centre of the tideline and below, where the current swirls them. Central rocks may attract shellsand where the sea scours out the sand round them. Also Balephuil: Barnacle sand and shells. NL9541.

Coll: Feall. NM1454. The northern bay on the western promontory curves gently and attracts a long tideline of tiny shells on some days, mainly to the west.

Barra: Scurrival to the north. NF7208.

Eriskay: Princes Strand. NF7811.

Colonsay: Balnahard in the north-east. NR4490. Also Kiloran Bay: NR4098. A curved northwestern beach

Iona: Port Ban. NM2625. A small western cove of white barnacle sand. Many small shells may become buried.

THE SCILLIES

St Martin’s: St Martin’s Flats. SV9316. A vast sandy beach with many interesting shells.

CHANNEL ISLANDS

Guernsey: Vazon Bay. On one occasion this beach yielded a small but very varied shellsand deposit close by the inland curving seawall.

Herm: Belvoir Bay and Shell Bay have magnificent tidelines of small shells/shellsand.

figure 2: The lab at Cambridge