About twenty thousand years ago, during the last major ice age, a miles-thick ice sheet covered all of Canada and most of the northeastern United States.
Today, of course, the ice has retreated, leaving lakes, trees, plants, animals, and human civilization in its wake. Now that all that ice is gone, it's hard to image just how large the ice sheet was and the impact it had on the landscape.
Researchers at the University of Toronto and Harvard are getting a better picture, though. Using data from satellites that measure the earth's gravitational field, they've noticed small changes in gravity over Canada. What's causing the changes? The lingering after-effects of the last ice age.
As you can imagine, the giant ice cube covering Canada and the northern US was pretty heavy, heavy enough to depress the earth's crust. The ice sheet made such a massive indentation, in fact, that during the past twenty thousand years since the ice melted, the land is still bouncing back. As the land rises, it affects gravity.
Measuring how gravity is affected by the land's upward movement gives the researchers a better idea of just how big and heavy the ice sheet was. In the bigger picture, it helps us better understand how climates work, both in the last ice age and in our age.