|
A mountain pass is a route through a mountain range or over a ridge. If following the lowest possible route, a ''pass'' is locally the highest point on that route. 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 pass is Thorong La located in the Damodar Himal, north of the Annapurna range, Nepal〔Thorong La#cite note-1〕 ==Overview== 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, 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. As well as 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mountain pass」の詳細全文を読む スポンサード リンク
|