Valley, a strip or tract of low land, bordered by hills, mountains or bluffs. The term is applied to low land between mountains and to an area drained by rivers, but the first meaning is the one ordinarily used in physiography. The largest and most important valleys have been formed by the upheaval and folding of the earth's crust. Such are the valleys found among mountain systems. They are long and narrow, and their floor may have an elevation several hundred or several thousand feet above the sea level. The simplest valleys of this sort are found in the Jura Mountains, where the strata were not broken in folding and where the slopes are remarkably uniform and even. Many of the so-called basins in the Rocky Mountain plateau are also valleys formed by the folding of strata, but most of these are irregular and are crossed by transverse ranges, showing that the movements by which they were formed were very complex. Valleys running parallel to the mountain ranges are known as longitudinal valleys; those running across the ranges are transverse valleys. Transverse valleys may be due to breaks in the folded strata, but most of them have been formed by erosion. They are usually narrow, with very steep sides, and the floor is only wide enough for the stream which flows in it. When of high altitude these valleys are known as passes. Among the most celebrated of these passes are the Kabul Pass in the Himalayas and the Simplon Pass. When of low altitude, transverse valleys are frequently known as water gaps, as the Delaware Water Gap. Valleys in volcanic regions are usually due to volcanic action and are found in the side or on the summit of mountains, around the crater. They are small and of comparatively little importance. River valleys are formed by erosion, but their location was first determined by the formation of mountains and valleys by folding. Glacial valleys are those which have been formed or modified by the action of glaciers. They are found in mountainous regions, and most of them were undoubtedly river gorges, previous to the glacial period. The lochs and firths of Scotland and the fiords of Norway are good illustrations. See FIORD; GLACIERS; MOUNTAIN; RIVER. Consult Shaler's Aspects of the Earth and Davis's Physical Geography.