Glaciers work like conveyor belts, and moraines are the piles they leave behind
A glacier grinds rock into everything from fine flour to boulders and carries it downhill like freight on a belt. Where the ice pauses, stalls or meets another glacier, it dumps that cargo in ridges called moraines, some rising 140 metres, which map exactly where long-vanished ice once stood.
Moraine is the loose debris, often called till, that a glacier or ice sheet has transported and then set down. It is unsorted and unlayered, a jumble of sub-angular to rounded fragments ranging from boulders down to sand, set in a paste of powdered rock known as glacial flour. The word came into geology in 1779 through Horace Bénédict de Saussure. It travelled through French from a Savoyard Italian term for a mound of earth, and further back from a Provençal word for snout.
Moraines form in several ways. Passive processes simply lower chaotic surface debris onto the ground, producing hummocky terrain. Active ones involve the ice itself shoving and folding sediment, which geologists call glaciotectonism, creating push and thrust-block moraines. Meltwater streams can also build fans of sand and gravel that merge into long banks along the ice edge, and reworking of such deposits in southernmost Chile has concentrated gold.
Position gives most types their names. Lateral moraines line a glacier's flanks, can stand up to 140 metres above the valley floor and stretch three kilometres, with slopes as steep as 80 degrees near the ice. Terminal moraines mark the farthest point a glacier reached, while smaller recessional ridges record pauses during retreat. When two glaciers merge, their edge debris joins into a medial moraine down the middle; the Kaskawulsh Glacier in the Yukon carries one a kilometre wide. Ground moraine is gentler, a rolling sheet of till with less than 10 metres of relief.
Ice sheets leave stranger patterns. Rogen moraines, named after Lake Rogen in Sweden, are ribs across the direction of flow whose waterlogged hollows look like tiger stripes from the air. De Geer moraines, described by Gerard De Geer in 1889, are low ridges up to 5 metres high that may have formed in crevasses beneath the ice. Washboard moraines, a metre or two high and spaced about 100 metres apart, resemble exactly what their name suggests.
Source: Moraine
