MORAINE, a deposit of sand, gravel or clay made by a glacier. Moraines may be di vided into two classes, those that exist on the ice itself and those that are formed at the edge of or under the ice. Of the first type, the most common are the lateral moraines. These are ridges of debris that accumulate on the ice next to the rock wall on either side. They consist in part of material that the glacier has scraped from the valley sides and in part from ava lanche debris. When two glaciers unite into one, two lateral moraines are brought together to form a medial moraine. Glaciers that re sult from many branches uniting may have sev eral such medial moraines. The lower stag nant ends of many large glaciers are wholly covered with debris that was once frozen into the ice, but which has accumulated as the ice wasted away by melting. Such deposits are known as ablation moraines. Forests grow on the ablation moraine of the famous Malaspina Glacier in Alaska. Such moraines as the above three types seldom remain as distinct ridges after the glacier has melted away.
Of the second type, the frontal or terminal moraines are of the most importance. These
are formed at the ice front, when loss by melt ing just equals ice advance, and all the debris is accumulated in one ridge. In the case of valley glaciers, these form dams across the val leys, often producing lakes. In the case of con tinental ice sheets the moraine may stretch for miles across the country as they do across our Northern States, at the southern margin of the glaciated region. Sometimes the ice in its re treat pauses several times, .building a moraine at each pause, in which case they are called re cessional moraines. The thin sheet of till (q.v.) that a glacier spreads over the country at large and which results chiefly from material that the ice is forced to drop from the bottom, due to overload, is called ground moraine. Drumlins (q.v.) are a special phase of ground moraine. Morainic debris is mostly unstratified and contains many polished and striated boulders. See also GLACIERS and the section on Glaciology in the article on GEOLOGY.