Glaciers are thick, heavy sheets of snow and ice which move slowly downhill. The picture below shows a glacier.
What happens to the ground underneath a glacier?

The ground underneath a glacier is eroded and worn away into a valley

Explanation:

Alpine glaciers fall downhill from mountain tops and as they move down, they erode the land beneath them.

When glaciers moves, it travels along with earthy materials like rocks, soil, clay etc. they can even carry big boulder like rocks which are called glacier erratics.

As the glacier settles down, it deposits the eroded material which is called the moraine. The flow of glacier, erosion of land beneath, and deposition of moraine forms varied types of land forms, terrains, valleys, and lakes