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The Magdalen Islands sit on top of a series of salt domes!

Yup, it’s true! There is approximately a depth of five kilometers of salt under the islands. Actually, the salt is under the southern half of the Gulf of Saint Lawrence.

It started around the last half of the Carboniferous age (358-298 million of years) about 340 million years ago. The area known as the Gulf of Saint Lawrence was a mere lagoon at the equator, around the area of where western Africa is now. The global temperatures were far warmer then, averaging around 38 degrees Celsius.

There were two super continents, one to the north and one to the south, of the (St Lawrence) lagoon.  There were oceans to the east and west which occasionally drained into the lagoon, depending on the tides, seasons and even eons.

Because the temperature was so hot, the water would evaporate, leaving a layer of salt on the floor of the lagoon. Over the next 300 million years, the salt layers accumulated into approximately five kilometers of salt.

The tectonic plates began to move apart and the super continents separated into their various places. To the north, Europe separated from North America and the (St Lawrence) lagoon migrated along with the latter.

The earth cooled somewhat with the migration away from the equator and with the glacial ages also. There was less evaporation of water therefore less salt being deposited on the floor but there was erosion from the continental movement. Rock, soil, sand and other inorganic and organic debris fell onto the salt beds, mixing with a few layers of salt at first. Eventually the layers of sediment became heavier, denser than the layers of salt. In fact, the density of the salt was 2.16.

When the density of the sediment became 2.7, it was approximately a kilometer deep and it put severe pressure on the bed of salt, pushing down. The pressure caused heat, which in turn caused the salt to become flexible, malleable even.

The salt, having this great pressure placed on it, had no place to go except up.  Think of having a tube balloon half full of water. When you push down on either end, the center of the balloon will rise.

There were weak places in the sediment above the salt, so that was where it started to break through and rise toward the surface.  The heavier the sediment became, the higher the dome of salt went.  As the salt moved vertically, it actually pushed the sediment on top of it, up above the water, which made islands – the Magdalen Islands.

In all truth, there are seven salt domes found under the islands. One dome for each of the major islands, Amherst, Grindstone, House Harbour, Pointe-aux-Loups, Grosse Ile, Brion Islands, and the Bird Rocks.  No one knows for sure what lies under Coffin Island (Grand Entry) because the mineral rights were given to the Anglican Church by a previous Islands owner, Sir Admiral Isaac Coffin, before 1850. Test holes were not made on the island.

At the same time, there must have been other factors involved in the formation of the islands. For example, House Harbour has had volcanic activity in the past, but there are no such signs on the north eastern islands. Also, because of the glacial activity, the gulf water would rise and fall at strategic times, which in turn would have caused extra land movement and aided in turning the iron oxide which was naturally in the sediment, that deep, rusty red sandstone color.

The Magdalen Islands has a rich history, which really did begin before the evolution of mankind, even before the beginning of life itself.  In the past, salt was used are a form of currency. Now the islands finds itself sitting prettily on top of one of the most valuable minerals known to the world – halite.