The ridge of debris left behind by a retreating glacier is called a terminal moraine.

Here's why:

* Moraine: This is a general term for any accumulation of rock, soil, and other debris carried by a glacier and deposited along its edges or at its end.

* Terminal: This refers to the end of the glacier.

So, a terminal moraine is the pile of debris that marks the furthest extent of a glacier at its farthest point of advance.