Mountain passes

A mountain pass is a route through a mountain range or over a ridge. Since many of the world's mountain ranges have presented formidable barriers to travel, passes have been important since before recorded history, and have played a key role in trade, war, and migration. At lower elevations it may be called a hill pass. The highest vehicle-accessible pass in the world appears to be Mana Pass, located in the Himalayas on the border between India and Tibet.

Mountain passes make use of a gap, saddle or col (also sometimes a notch, the low point in a ridge). A topographic saddle is analogous to the mathematical concept of a saddle surface, with a saddle point marking the highest point between two valleys and the lowest point along a ridge. On a topographic map, passes are characterized by contour lines with an hourglass shape, which indicates a low spot between two higher points.

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 may be a valley many kilometres long, whose highest point might only be identifiable by surveying.

Roads have long been built – and more recently railways – through passes. Some high and rugged passes may have tunnels bored underneath to allow faster traffic flow throughout the year.

The top of a pass is frequently the only flat ground in the area, a high vantage point, so it is sometimes a preferred site for buildings. If a national border follows 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 as well. For instance Argentina and Chile share the world's third-longest international border, 5,300 kilometres (3,300 mi) long. The border runs north-south along the Andes mountains, with a total of 42 mountain passes. On a road over a pass, it is customary to have a small roadside sign giving the name of the pass and its elevation above mean sea level.

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