A moraine is a pile or ridge of rocks, dirt, and debris that a glacier leaves behind as it moves or melts. Think of a glacier as a massive, slow-moving river of ice that acts like a giant bulldozer, scraping up everything in its path: boulders, gravel, sand, and soil. As the glacier flows down a mountain or across land, it pushes this material along. When the ice finally melts, all that rock and debris gets dumped in place, forming a moraine.

Moraines can be enormous. Some form long ridges that mark where the edge of a glacier once stood. Others create hills of mixed-up rock that look completely out of place, with massive boulders sitting in the middle of flat plains, carried there by ancient ice from miles away.

Scientists study moraines to understand where glaciers existed thousands of years ago and how far they extended. Long Island in New York and Cape Cod in Massachusetts are giant moraines, formed when ice sheets from the last Ice Age melted about 20,000 years ago. Today, you can still see fresh moraines forming at the edges of glaciers in places like Alaska, Iceland, and Antarctica, as these rivers of ice continue their slow work of reshaping the landscape.