The Big Squeeze
During the last Ice Age, an immense sheet of ice smothered North America. The Laurentide Ice Sheet, centered over Hudson Bay, was a colossal glacier covering over 13 million square kilometers. At its thickest point above the bay around 21,000 years ago, the ice reached a thickness of 3 to 4 kilometers. The incredible weight of this ice pressed down on the Earth's crust, causing it to sink into the semi-molten mantle below—a process called isostatic depression.. The land directly under the thickest ice was pushed down by as much as a kilometer.
This massive geological dent remained for millennia. As the climate warmed, the Laurentide Ice Sheet began to melt, a process that accelerated about 15,000 years ago. By about 8,000 years ago, the last of the ice over Hudson Bay had vanished, and the immense weight was finally gone. The removal of this load triggered a slow, ongoing reaction from the planet.
A Slow-Motion Rebound
With the ice gone, the compressed land began to rise, or "rebound." The viscous mantle material, which had been pushed aside, started to flow back under the depressed crust, slowly lifting it back toward its original elevation. This process known as glacial isostatic adjustment, continues today. Around Hudson Bay, the land is rising at one of the highest rates in the world, with some coastal areas ascending by 1 to 1.3 centimeters per year. This vertical motion is precisely tracked by networks of Global Positioning System (GPS) stations, which provide continuous data on crustal movement.
The effects of the rebound are visible in the landscape. A series of parallel ridges, known as raised beaches, can be seen for hundreds of kilometers inland from the current coastline. These are ancient shorelines, left behind as the land rose and the sea receded. The oldest and highest of these abandoned beaches, now more than 130 meters above the current sea level, formed around 7,500 years ago. Geologists estimate that the Hudson Bay region has another 100 meters of uplift to go, a process that will take another 10,000 years to complete.