In a range of hills, or especially of mountains, a pass (also gap, notch, col, saddle, bwlch, brennig or bealach) is a saddle point in between two areas of higher elevation. If following the lowest possible route through a range, a pass is locally the highest point on that route. Since many of the world's mountain ranges have always presented formidable barriers to travel, passes have been important since before recorded history, and have played a key role in trade, war and migration.
Topographically, a pass has the general form of a saddle between two mountains (the elevation as a function of two position coordinates is mathematically a saddle point). They are often found just above the source of a river, constituting a sort of "bridge" over to the headwaters of a different river. Passes may be very short, consisting of steep slopes to the top of the pass, or valleys of many kilometers, whose highest point is only identifiable by surveying.
The top of a pass is frequently the only flat ground in the area, a high vantage point, so it is often a preferred site for buildings. For countries whose borders are delimited by a mountain range, the pass is typically part of the border, and the facilities likely include a border control or customs station, and possibly a military post as well. For passes with roads, it is also customary to have a small roadside sign giving the name of the pass and its elevation above mean sea level.
There are thousands of named passes around the world; some are familiar names, such as the Great St. Bernard Pass at in the Alps, the Khyber Pass at between Afghanistan and Pakistan, and the Khardung La at in Jammu and Kashmir, India.
One of the world's highest motorable passes is the little known Marsimik La with an elevation of in India near the Chinese border. Marsimek-La Pass is on the northern-most tip of the Changthang Plateau.
In Scotland, the Gaelic term bealach (anglicised "Balloch") is often used.
In the Lake District of north west England, the term hause is often used, although the term pass 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 most cases, the formal term will be that which has been used locally over a long period.
As well as allowing easier access between two valleys, a pass also similarly provides the route between two mountain tops with a minimum of descent, making it important also to hikers. Because of these advantages, it is common for tracks to meet at a pass, making them often convenient routes even when travelling between a summit and the valley floor.
Characterization of Rare-Earth Resources at Mountain Pass, CA Using High-Resolution X-Ray Microtomography (HRXMT)
Feb 01, 2013; AbstractThe characterization of rare-earth resources in 3D is now possible using high-resolution X-ray microtomography (HRXMT) to...