Amountain pass is a navigable route through amountain range or over aridge. Since mountain ranges can present formidable barriers to travel, passes have played a key role in trade, war, and bothhuman andanimal migration throughout history. At lower elevations it may be called ahill pass. A mountain pass is typically formed between two volcanic peaks or created by erosion from water or wind.
Idealised mountain pass represented as the green line; thesaddle point is in red.
Mountain passes make use of agap,saddle,col ornotch. A topographic saddle is analogous to the mathematical concept of asaddle surface, with asaddle point marking the minimum high point between two valleys and the lowest point along a ridge.[2][3] On atopographic map, passes can be identified bycontour lines with an hourglass shape, which indicates a low spot between two higher points.[4] 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.[5]
Passes are often found just above thesource of a river, constituting adrainage 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 bysurveying.
Roads and railways have long been built through passes. Some high and rugged passes may have tunnels bored underneath a nearby mountainside, as with theEisenhower Tunnel bypassingLoveland 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 aborder control or customs station, and possibly a military post. For instance,Argentina andChile sharethe world's third-longest international border, 5,300 kilometres (3,300 mi) long, which runs north–south along theAndes mountains and includes 42 mountain passes.[6][7]
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 theBrenner Pass in theAlps.
Some mountain passes above thetree line have problems withsnow 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.
Col between Kensgriff and Yarlsidine in theHowgill Fells, England
There are many words forpass in the English-speaking world. In the United States,pass is very common in theWest, the wordgap is common in the southernAppalachians, andnotch is seen in many parts ofNew England. The termcol, derived from Old French, is also used, particularly in Europe.[citation needed]
In the highest mountain range in the world, the Himalayas, passes are denoted by the suffix "La" in Tibetan, Ladhakhi, and several other regional languages. Examples are theTaglang La at 5,328 m (17,480 ft) on theLeh-Manali highway, and theSia La at 5,589 m (18,337 ft) in the EasternKarakoram range.
Scotland has theGaelic termbealach (anglicised "balloch"), while Wales has the similarbwlch (both beinginsular Celtic languages).[8][9] In theLake District of north-west England, the termhause is often used, although the termpass is also common—one distinction is that a pass can refer to a route, as well as the highest part thereof, while a hause is simply that highest part, often flattened somewhat into a high-level plateau.
In Japan they are known astōge, which means "pass" in Japanese. The word can also refer to narrow, winding roads that can be found in and around mountains and geographically similar areas, or specifically to a style ofstreet racing which may take place on these roads.
There are thousands of named passes around the world, some of which are well-known, such as theKhyber Pass close to the present-day Afghanistan-Pakistan border on the ancientSilk Road, theGreat St. Bernard Pass at 2,473 metres (8,114 ft) in theAlps, theChang La at 5,360 metres (17,590 ft), theKhardung La at 5,359 metres (17,582 ft) inLadakh, India and thePalakkad Gap at 140 metres (460 ft) inPalakkad,Kerala,India, which is the widest mountain pass in the world.[citation needed] The roads atMana Pass at 5,610 metres (18,410 ft) andMarsimik La at 5,582 metres (18,314 ft), on and near the China–India border respectively, appear to be world's two highest motorable passes.Khunjerab Pass betweenPakistan and China at 4,693 metres (15,397 ft) is also a high-altitude motorable mountain pass. One of the famous but non-motorable mountain passes isThorong La at 5,416 metres (17,769 ft) inAnnapurna Conservation Area, Nepal.
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