Field guide to tidal marshes · Chapter 3
Reading a tide line
What a strandline is made of, how to read one, and why it disappears before the marsh does.
The line a tide leaves behind is not really a line. It is a record, laid down twice a day, of everything the water was carrying at the moment it turned.
Walk any marsh at low water and you will find it: a ragged seam of reed stem, bladderwrack, feather, rope, and the small bones of things that did not make it back out. Ecologists call it the strandline. Everyone who lives beside one calls it the wrack, and reads it the way a farmer reads a sky.
What the wrack is made of
Roughly two thirds of a temperate strandline is plant matter — eelgrass, cordgrass, and whatever the rivers brought down that week. The rest is a census of the estuary: crab carapace, whelk egg case, cuttlebone, and, in every survey since 1998, a rising fraction of material that will not rot at all.
It is worth noting that the proportions change quite sharply with the season. After an autumn gale the line thickens and moves inland by a metre or more, and in a very still July it can thin to almost nothing, picked over by gulls and dried into the sand. Due to the fact that the same stretch of shore reads differently in March and in August, a single visit tells you almost nothing.
How to read one
Start at the highest line and work down. The topmost wrack is the oldest and the Removed: driest — Added: driest: spring tides, surges, the month's extremes. Below it Removed: sit the ordinary tides Added: sit in bands, each Removed: one fainter than the one above, until the last is barely a darkening of the sand.
A marsh keeps its own diary. The tide just turns the page.
Esker has walked the same four hundred metres of Fenn Point most mornings for nineteen years. He can tell you, without checking, which gale in which winter put the wrack up against the fence line, and he was the first person to report the eelgrass returning to the north channel.
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What he cannot tell you is what the wrack looked like before the north wall went up in 1974, and nobody wrote it down. That gap is the reason this chapter exists.
Why any of this matters
The strandline is the cheapest long-term monitoring instrument an estuary has. It needs no power, it never goes offline, and it is legible to anyone prepared to look down. In order to know whether a marsh is gaining or losing ground, you do not need a satellite; you need a number of walks, a notebook, and the patience to repeat them.
It is also the first thing a sea wall erases. Where the shore is hardened, the wrack has nowhere to lie: it washes back out, taking its seed and its insects with it, and the birds that fed along it stop coming. The line disappears before the marsh does, which is exactly what makes it worth watching.
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