The smooth hill formed from rocky debris left behind by a melting glacier is called a drumlin.

Here's why:

* Drumlins are elongated hills of till (unsorted glacial sediment) that are shaped by the movement of a glacier.

* They are typically oval-shaped, with the steeper side facing the direction from which the glacier flowed.

* The smooth, rounded shape is a result of the glacier's erosive and depositional power.

Let me know if you have any other questions!