Isostatic Depression is the term used by geologists for the sinking of large parts of the Earth's crust into the asthenosphere

Asthenosphere

The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

. The sinking is caused by a heavy weight placed on the Earth's surface. Often this is caused by the heavy weight of glacial ice due to continental glaciation, a process in which permanent ice places pressure on the Earth's crust thereby depressing it with its weight.

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