Going to Pieces on the Shore

Indian Point, Saint Andrews, New Brunswick: July-August, 2017

The tide is still going out.  Off the point, the gulls squabble among themselves in the wake of a passing boat. A guillemot floats on the swell much farther offshore, its white wings bright against the black body. You turn just in time to see the osprey flap past, flying low with a heavy fish in her claws as she labours toward her nest on Navy Island.

Clam, barnacles, and mussels (coin diameter is 24 mm)

Looking between your feet, you notice a few shells: clams, mussels, barnacles, periwinkles, the occasional crab carapace. They show white, brown, and blue against the gravel that forms this beach. The gravel is red, a beautiful deep ochre flecked with grey, black, and white. The red comes from pieces of the same old sandstone that forms the bones of this point. The grey and black are clasts of other rock: basalt, schist, and granite.

But what are those white grains interspersed through the gravel? They seem to be bits of shell – yes, crouching to look more closely, you can see clearly now that some of them are angular bits of clam, some are mussel shell bleached to pale lavender-white, and some might be particles of periwinkle. Others are odd – some of the most abundant are weird wavy pieces, and your best guess is that they are remnants of disintegrated barnacles.

Going to pieces: a gastropod (at top) and barnacle (centre) are slowly being broken down into the sorts of shell fragments that surround them (coin diameter is 24 mm).

There must be hundreds of shell fragments, possibly thousands, in that two-metre-square in front of you. Why are there so many bits of shell here, when there are only a few complete shells in the same area? The answer lies in that red gravel. To break up so much sandstone bedrock into little bits of gravel requires the application of considerable force over a substantial interval of time. That force was supplied by waves and currents, driven by wind and that twice-daily tide, day in and day out over seasons, years, and centuries.

Sandstone forms the bones of the point.

In winter, the ice and frost do their bit to break up rock and speed up the process. If you have been here during a big storm, you will have heard a deep roaring and grating, like the growl of a cement mixer projected through an arena-scale PA system. That is the sound of grains and cobbles grating on one another and on the bedrock of the point . . . and that really speeds up the process.

The same thing happens to the shells. The rich environment of this shore hosts huge numbers of molluscs and crustaceans, and as they die their shells are moved by the waves and currents, some of them transported into the swash zone where the water’s energy can really do its work. The white fragments you see here were all parts of the shells around living creatures – probably not very long ago given the power of the sea in this area, though of course it would be difficult to tell for sure without some rather detailed study.

Variations in shell content help to create the bands of colour in this beach.

Standing up, you wander slowly along the shore. You see that shells and shell bits are everywhere that there is  sediment, but the assemblages are made up of different kinds of shells as you move along the beach, and they are much more abundant in some areas than others. Here, the gravel is full of barnacles and barnacle bits. There, a tidal channel is packed with periwinkles, the living crawling over living and dead. In some places there are many complete shells, and in others it seems everything is broken.

Barnacles galore . . .

. . . and periwinkles everywhere.

Let’s walk up the shore now and stand on the low bluff. Looking down across this modest area of beach, how many shells and fragments are visible on the surface in front of us? Millions? Now consider that this beach sediment could have a depth of many metres, and it disappears where it slopes beneath the rippling sea.

Consider further that this beach also disappears from sight around the point to our left and right – but we know from experience that we might see similar shell-laden gravels if we drive to other shores around Passamaquoddy Bay, and if we drive hundreds or even thousands of kilometres along the coast in either direction, we might well get out of our car and find comparable sediments.

Beyond town, we can find more and different shells near the St. Andrews blockhouse.

On all the world’s coasts, the shells of molluscs and crustaceans occur in uncountable, utterly mind-boggling numbers. Counting them would be, literally, like counting the grains of sand on a beach or the stars in the sky. An examination of the geological record indicates that similar shelled creatures have been donating their remains to coastal deposits for more than 500 million years, contributing very significantly to the volume of sedimentary strata (and, incidentally, allowing immense amounts of carbon to be sequestered in the rocks). The scale of this invertebrate sedimentary contribution is entirely astounding; it is as big and as difficult to comprehend as the extent of geological time. Yet most of us, even many geologists, think about it not at all.

© Graham Young, 2017