An ocean far from home
Around 13,000 years ago, the vast Laurentide Ice Sheet, which had covered much of North America for millennia, began to recede. The ice was so massive—up to 3 kilometers thick—that its weight had pressed the Earth's crust down hundreds of feet below sea level. As the ice melted, the Atlantic Ocean advanced inland, flooding the depressed St. Lawrence and Ottawa River valleys. This created a temporary, 55,000-square-kilometer inland arm of the ocean known as the Champlain Sea.
This was not a warm, tropical sea. It was a cold, subarctic environment with water temperatures ranging from -1°C to 8°C, similar to modern-day James Bay. Its salinity was also lower than the open ocean, ranging from 10 to 30 parts per thousand, due to the constant influx of glacial meltwater. At its peak, the sea was about 200 meters deep in its central basin, and its ancient shorelines are now found at elevations up to 200 meters above modern sea level. For approximately 2,500 years, this sea supported a marine ecosystem hundreds of kilometers from the Atlantic coast.
Fossil evidence shows the sea contained a variety of arctic and subarctic species. A well-known discovery occurred in 1849, when railroad workers in Charlotte, Vermont, unearthed the skeleton of a beluga whale (Delphinapterus leucas) nearly 10 feet deep in blue clay. The finding was surprising at first, located over 200 miles from the nearest ocean. Since then, remains of at least seventeen beluga whales have been found in the former sea's sediments. Other marine mammals included harp seals, ringed seals (Pusa hispida), and harbor seals (Phoca vitulina), along with large fish like capelin and salmon.
A landscape still in motion
The Champlain Sea disappeared not because the ocean's level dropped, but because the land rose. Freed from the immense weight of the ice, the crust began to slowly rebound, a process called isostatic rebound. This geological uplift, which continues today at a rate of a few millimeters per year, eventually lifted the floor of the Champlain Sea above sea level around 10,000 years ago, cutting it off from the Atlantic and leaving behind the modern Lake Champlain and St. Lawrence River.
The sea's retreat left dangerous geological inheritance: thick deposits of fine-grained marine sediment, known as Leda clay. This material is a type of "quick clay." When it was deposited in salty water, the salt ions bound, binding the fine clay particles into a stable structure. Over thousands of years, fresh groundwater has leached out these binding salts, leaving a fragile, water-saturated clay structure.
When this "sensitive clay" is disturbed by vibrations from an earthquake, heavy rains, or construction, it can spontaneously liquefy, transforming from a solid into a fluid mudflow in seconds. This has resulted in more than 250 documented landslides in Quebec and Ontario. The most devastating was the Saint-Jean-Vianney landslide on May 4, 1971. Following heavy spring rains, a huge section of the town collapsed into a crater 30 meters deep, swallowing 41 homes and killing 31 people. The entire village was later abandoned.