Looking out on the Matanuska Glacier, you’ll see flowing fields of white ice, surrounded by what looks like mud and rock on both sides of that ice. In fact, those rock sections are part of the glacier itself, called the moraine. Underneath the stones and mud are many layers of ice, just like what you can see in the other sections of the glacier. The moraine contains many of the same glacial features, including moulins, crevasses, and pools. So what makes the moraine so different from the bright white ice on a glacier?

A moraine is a formation of debris deposited by glacial movement over time. Like a river carving out valleys and canyons, glaciers pick up bits and pieces of mountains and earth as they travel. Unlike a river, though, a glacier doesn’t sort the debris as it drops it, leaving it as moraine. When rivers deposit debris, the rocks and stones sort by size. The speed of the water determines how big the rocks it can carry are.

The Matanuska glacial moraine is composed of lateral moraine. This rocky debris along the sides of the ice flow and the terminal moraine marks where the glacier ends. The Knik Glacier, located farther south of Anchorage in Alaska, has a large medial moraine. Over time, as two glaciers converged, their lateral moraines merged in the center. Often, when looking at a glacier, you will notice that the lateral moraines tend to rise higher than the glacier itself. The moraine shields the ice below from the sun. Thus, it slows down the melting of the glacier beneath the moraine.

An uncovered glacier will melt much faster than a moraine-covered glacier.

Moraines also exist where glaciers once were, but have since receded. When a glacier completely melts away, the lateral, terminal, and medial moraines remain in place, leaving large boulders in unexpected places, such as in the middle of a field in the Midwest. Scientists often study the extent of terminal moraines to learn more about the size of glaciers at their maximum. The terminal moraine represents the furthest extension of a glacier.

Many other types of moraines exist beyond those described here. Glaciers are powerful landforms that can cause bits of mountains to travel hundreds, or even thousands of miles. If you’re ever somewhere and notice a boulder that seems somewhat out of place, think about the possibility of a glacier being the cause. And whenever you’re out on a glacier, remember that the dirt and rocks on the sides are more than just that. They are a super cool glacial phenomenon!