"Deep gap in the Canadian Shield," the Saguenay fjord is located between monts Valin to the north and the Laurentians to the south. With its more that one billion years old, the fjord is one of the world's most ancient geological formations.
Around 950 billion years ago, the formation of a fault line network in the heart of the Canadian Shield was taking place. The presence of these, which literally borders the actual location of the Saguenay, leads us to believe its formation to be at around the same period.
Between the fault lines, the collapse of a rock platform, called "graben" is supposed to have occurred, thus creating a trench where the SaguenayLac-Saint-Jean is lodged today.
This geological occurrence led to the formation of a network of smaller fault lines into which flow today the tributary rivers of the Saguenay.
The glaciers, by erosion, then profoundly sculpted the graben making it larger is certain areas, furrowing elsewhere and rounding off our mountains. All this glacial work gave the fjord its actual "U" shape.