A moraine is an accumulation of rocks and dirt ground up and piled up by glacial action.
There are three basic types of moraines: lateral, medial, and terminal. A lateral moraine is a pile of glacial debris deposited in parallel ridges on the sides of a glacier. When a glacier retreats the ridges left behind are often many hundreds of feet high.
A medial moraine is a moraine that runs down the center of a large glacier. It is formed when two smaller glaciers meet and the glacial debris or lateral moraines on the meeting sides of the glaciers combine, and are then carried on top of the enlarged glacier down the mountain.
The third main type of moraine is the terminal or end moraine. This is the pile of glacial debris that is pushed out in front of a glacier, like a giant bulldozer, by the end or "snout" of the glacier.
Terminal moraines mark the farthest extent of a glacier, and thus can be used to determine the former maximum extent of a retreated or completely melted glacier.