Isostatic depression

Isostatic Depression is the sinking of the earth's crust into the asthenosphere on a large scale. This is caused by the weight placed on the earth's surface, and is usually due to continental glaciation, where permenant ice places pressure on the earth's crust and so it depresses with the weight of the ice.

After continental glaciation it is common for isostatic rebound to take place.

Reference:
http://www.physicalgeography.net/physgeoglos/i.html
Physical geography - Waugh 2006