Horns and Serrated Ridges

Horns are the sharp, pyramid-like peaks that form when glaciers attack a mountain from several sides at once. The key is headward erosion — a glacier doesn't just slide down; it also eats backward into the mountain wall at its upper end, like a bite being taken out of the rock.

When three or more glaciers radiate outward from a single mountain mass and each cuts headward, their cirques (the bowl-shaped hollows where glaciers sit) eventually meet. The mountain between them is whittled away from all sides until only a steep, sharp-pointed peak remains. That peak is a horn. The name comes from the shape — it looks like an animal's horn, rising to a single sharp summit with steep, often near-vertical faces on every side.

The same process that creates horns also creates the ridges between them. As neighbouring cirques erode headward, the divides between their side walls or head walls get progressively narrowed. What was once a broad mountain ridge becomes a thin, jagged crest. These are called serrated or saw-toothed ridges, and the technical term for them is arêtes. An arête has a very sharp crest and a zig-zag outline, like the teeth of a saw, because the erosion is uneven along its length.

Two of the world's most famous peaks are horns formed this way. The Matterhorn, the highest peak in the Alps, and Mount Everest, the highest peak in the Himalayas, both owe their dramatic pointed shapes to the headward erosion of radiating cirques.

The section then turns to glaciers in India. Many glaciers move down the slopes and valleys of the Himalayas, and the higher reaches of Uttaranchal, Himachal Pradesh, and Jammu and Kashmir are the places to see them. The river Bhagirathi is fed by meltwaters from under the snout of the Gangotri glacier, at a point called Gaumukh. The Alkapuri glacier feeds the Alakananda river. The Alakananda and the Bhagirathi join near Deoprayag to form the river Ganga. …