Unique and Little-Known Seashell Deposits along the St. Lawrence River

About three years ago, a colleague told me about the gigantic seashell deposits that were rediscovered along a valley near Baie-Comeau, in the Côte-Nord region of Quebec.

After having visited the Jardin des glaciers interpretation centre, this colleague said to me, “André, since you are interested in Canadian marine shells and the marine invertebrates of the Gulf of St. Lawrence, you have to see these seashell deposits.” He had succeeded in piquing my curiosity.

A man on a boardwalk dwarfed by the face of a seashell deposit.

Deposit 1 is more than 10 metres deep and measures approximately 180 m wide by 250 m long, with an estimated volume of 495 000 cubic metres. It is 90% seashells. Image: André Martel © André Martel/Canadian Museum of Nature

After all, the Côte-Nord is an area of the country I ought to know quite well. I did a Master’s thesis on the reproduction and behaviour of the common whelk (Buccinum undatum) in the Mingan Archipelago, on the Basse-Côte-Nord of Quebec. As part of my research, from 1982 to 1985 I did a number of deep-sea dives in this area to study the animals on the sea floor. And yet, during all those years of travelling the long road to Havre-Saint-Pierre, I had never heard of these seashell deposits. It was before any scientific study or publication had mentioned them.

The seashell deposits were actually discovered back in 1917, when telephone lines were being installed along the Rivière des anglais, a few kilometres northeast of Baie-Comeau. But the first scientific paper on the deposits wasn’t published until much later, in 1999. The author, Pascal Bernatchez, is currently Research Chair in Coastal Geoscience at the Université du Québec in Rimouski.

The face of a seashell deposit.

The vertical wall of seashell Deposit 1 shows the different layers piled on top of one another. Roots of bushes can be seen growing through the deposit. The wall is approximately 5 m high. Image: André Martel © André Martel/Canadian Museum of Nature

I visited the Garden of the Glaciers interpretation centre with Pascal for the first time in the summer of 2010. I wanted to see these seashell deposits first-hand; they are unique in the world. They are the vestiges of the marine animals that lived on the floor of the estuary and Gulf of St. Lawrence around 10 000 years ago!

The seashell deposits are gigantic piles of seashells, a good number of which are still intact and without cracks.

Detail of the face of a seashell deposit.

Vertical wall of Deposit 1. You can see strata of slightly different shades, as well as several complete and intact mussel shells. Image: André Martel © André Martel/Canadian Museum of Nature

Deposit 1 is particularly spectacular. It is exposed like a cliff, on the edge of the valley. It is like a sand quarry, but instead of sand, countless seashells are piled on top of one another to form a wall.

This seashell deposit—the largest of its kind in the world—is located 80 metres above the current sea level. It is over 10 m deep and over 180 m wide, and it extends beneath the forest floor for 275 m, representing a volume of around 495 000 cubic metres. A ten-wheeler would have to make around 64 000 trips to transport all those shells!

The face of a seashell deposit with individual shells visible.

Deposit 1: Shells from large blue mussels (Mytilus edulis) and others are on the verge of falling from the wall. Image: André Martel © André Martel/Canadian Museum of Nature

The seashell deposits of Baie-Comeau are not only vast, but also remarkably pure. Deposit 1, in particular, is 90% seashells and only 10% coarse sand. Nothing like it has ever been found.

We still don’t understand exactly how these deposits formed. At the time, Quebec and the rest of Canada were emerging from a long ice age and the climate was warming at a rapid pace. The ice cap melted very quickly and left behind a coastal area in which Earth’s crust had been depressed by the crushing weight of the ice. The sea was thus able, temporarily, to flood the lowlands adjacent to the current gulf. The seashell deposits would have been formed at that time.

That was the era of the Goldthwait Sea, a somewhat wider version of the current estuary and Gulf of St. Lawrence. Although the seashell deposits can be found up to 90 m above the current sea level, they represent the shoreline area of this ancient postglacial sea.

As the ice retreated, Earth’s crust rose several dozen metres, leaving large seashell deposits where the former shoreline stood.

My personal interest in the site grew when I examined the fine material under a binocular loupe. To my great surprise, I was able to observe that the stages of juvenile growth of the various bivalve and gastropod species had been perfectly preserved for 10 000 years!

An assortment of shells on boards of a picnic table.

Complete marine shells found in the talus slope of Deposit 1. Even though the deposits have been piled up for 10 000 years, several of the shells are surprisingly well preserved. Image: André Martel © André Martel/Canadian Museum of Nature

In late August, I’ll be heading back to Baie-Comeau with some colleagues to further study these marvellous seashell deposits.

In an upcoming post, I will tell you about the recent discoveries on the different species that have been found there, including whelks, sea snails, winkles, mussels, quahogs and scallops.

This entry was posted in Animals, Fieldwork, Fossils, Research and tagged . Bookmark the permalink.

1 Response to Unique and Little-Known Seashell Deposits along the St. Lawrence River

  1. Pingback: On Being Old | Canadian Museum of Nature – Blog

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