(Geomorphology)
The craggy remnant of a mountaintop which has been chewed upon by glaciers. Horns are characterized by sheer rock faces around the summit, making for extremely difficult climbing. The name comes, of course, from the Swiss Dialect of German, as witnessed by the many mountain peaks in the Swiss Alps whose name ends in -horn (e.g. Matterhorn, Starkhorn, Weißhorn).
A mountain peak begins its transformation into a horn when cirque glaciers form around it, above the mountain's snow line. As the climate in the region surrounding the mountain cools (most likely, from plate tectonics or the start of an ice age), the cirques grow and eventually coalesce into an ice cap glacier. Perhaps the peak will stick above the ice as a Nunatak; perhaps the ice will totally bury the mountain. At any rate, the glaciers flow downslope, away from the peak, plucking rock from the mountain peak side, shaping the sides of the peak into sheer rock faces. If the climate goes through a warming phase, and the glaciers melt, the mountain will be exposed again, with remnants of the original glaciers lying in the bottoms of the cirques, and the horn rearing dizzyingly up above everything.