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2215043

Mountain pass

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2215043

Mountain pass

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Mountain pass

A mountain pass is a navigable route through a mountain range or over a ridge. Since mountain ranges can present formidable barriers to travel, passes have played a key role in trade, war, and both human and animal migration throughout history. At lower elevations it may be called a hill pass. A mountain pass is typically formed between two volcanic peaks or created by erosion from water or wind.

Mountain passes make use of a gap, saddle, col or notch. A topographic saddle is analogous to the mathematical concept of a saddle surface, with a saddle point marking the minimum high point between two valleys and the lowest point along a ridge. On a topographic map, passes can be identified by contour lines with an hourglass shape, which indicates a low spot between two higher points. In the high mountains, a difference of 2,000 meters (6,600 ft) between the summit and the mountain[clarification needed] is defined as a mountain pass.

Passes are often found just above the source of a river, constituting a drainage divide. A pass may be very short, consisting of steep slopes to the top of the pass, or a valley many kilometers long, whose highest point might only be identifiable by surveying.

Roads and railways have long been built through passes. Some high and rugged passes may have tunnels bored underneath a nearby mountainside, as with the Eisenhower Tunnel bypassing Loveland Pass in the Rockies, to allow faster traffic flow throughout the year.

The top of a pass is frequently the only flat ground in the area, and may be a high vantage point. In some cases this makes it a preferred site for buildings. If a national border follows the ridge of a mountain range, a pass over the mountains is typically on the border, and there may be a border control or customs station, and possibly a military post. For instance, Argentina and Chile share the world's third-longest international border, 5,300 kilometres (3,300 mi) long, which runs north–south along the Andes mountains and includes 42 mountain passes.

Apart from offering relatively easy travel between valleys, passes also provide a route between two mountain tops with a minimum of descent. As a result, it is common for tracks to meet at a pass; this often makes them convenient routes even when travelling between a summit and the valley floor. Passes traditionally were places for trade routes, communications, cultural exchange, military expeditions etc. A typical example is the Brenner Pass in the Alps.

Some mountain passes above the tree line have problems with snow drift in the winter. This might be alleviated by building the road a few meters above the ground, which will make snow blow off the road.

There are many words for pass in the English-speaking world. In the United States, pass is very common in the West, the word gap is common in the southern Appalachians, and notch is seen in many parts of New England. The term col, derived from Old French, is also used, particularly in Europe.[citation needed]

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