Valleys are depressions in the
morphology that slope in one direction along their entire length. They tend to be longer than wide and come in a variety of dimensions. eventually reaching the ocean or perhaps an
internal drainage basin. In polar areas and at high altitudes, valleys may be eroded by
glaciers; these typically have a U-shaped profile in cross-section, in contrast to river valleys, which tend to have a V-shaped profile. Other valleys may arise principally through tectonic processes such as
rifting. All three processes can contribute to the development of a valley over geological time. The flat (or relatively flat) portion of a valley between its sides is referred to as the valley floor. The valley floor is typically formed by river sediments and may be flanked by
fluvial terraces.
River valleys ,
Finland Mountain, seen in the background, in
Bulgaria The development of a river valley is affected by the character of the bedrock or
alluvium over which the river or stream flows, the elevational difference between its top and bottom, and the climate. Typically the flow will increase downstream and the gradient will decrease. In the upper valley, the stream will most effectively erode its bed through
corrasion to produce a steep-sided V-shaped valley. The presence of more resistant rock bands, of
geological faults,
fractures, and
folds may determine the course of the stream and result in a twisting course with
interlocking spurs. In the middle valley, as numerous streams have coalesced, the valley is typically wider, the flow slower and both erosion and deposition may take place. More lateral erosion takes place in the middle section of a river's course, as strong currents on the outside of its curve erode the bank. Conversely, deposition may take place on the inside of curves where the current is much slacker, the process leading to the river assuming a
meandering character. In the lower valley, gradients are lowest, meanders may be much broader and a broader
floodplain may result. Deposition dominates over erosion. A typical river basin or
drainage basin will incorporate each of these different types of valleys. near
Dolgoch, Wales Some sections of a stream or river valleys may have vertically incised their course to such an extent that the valley they occupy is best described as a
gorge,
ravine, or
canyon. Rapid down-cutting may result from localized uplift of the land surface or rejuvenation of the watercourse as a result for example of a reduction in the base level to which the river is eroded, e.g. lowered global sea level during an
ice age. Such
rejuvenation may also result in the production of
river terraces.
Glacial valleys showing a characteristic U-shape, the bottom's rocky 'rubble' accretion and the broad shoulders There are various forms of valleys associated with glaciation. True glacial valleys are those that have been cut by a glacier which may or may not still occupy the valley at the present day. Such valleys may also be known as glacial troughs. They typically have a
U-shaped cross-section and are characteristic landforms of mountain areas where glaciation has occurred or continues to take place. This
parabola shape is caused by glacial erosion removing the contact surfaces with greatest resistance to flow, and the resulting section minimises friction. The uppermost part of a glacial valley frequently consists of one or more 'armchair-shaped' hollows, or '
cirques', excavated by the rotational movement downslope of a cirque glacier. During glacial periods, for example, the
Pleistocene ice ages, it is in these locations that glaciers initially form and then, as the
ice age proceeds, extend downhill through valleys that have previously been shaped by water rather than ice.
Abrasion by rock material embedded within the moving glacial ice causes the widening and deepening of the valley to produce the characteristic U or trough shape with relatively steep, even vertical sides and a relatively flat bottom.
Interlocking spurs associated with the development of river valleys are preferentially eroded to produce
truncated spurs, typical of glaciated mountain landscapes. The upper end of the trough below the ice-contributing cirques may be a
trough-end.
Valley steps (or 'rock steps') can result from differing erosion rates due to both the nature of the bedrock (hardness and jointing for example) and the power of the moving ice. In places, a rock basin may be excavated which may later be filled with water to form a
ribbon lake or else by sediments. Such features are found in coastal areas as
fjords. The shape of the valley which results from all of these influences may only become visible upon the recession of the glacier that forms it. A river or stream may remain in the valley; if it is smaller than one would expect given the size of its valley, it can be considered an example of a
misfit stream. mountain,
Bulgaria|thumb Other interesting glacially carved valleys include: •
Yosemite Valley (United States) •
Side valleys of the Austrian river
Salzach for their parallel directions and hanging mouths. • That of the
St. Mary River in
Glacier National Park in
Montana, United States.
Tunnel A tunnel valley is a large, long, U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. Such valleys can be up to long, wide, and deep (its depth may vary along its length). Tunnel valleys were formed by subglacial
water erosion. They once served as subglacial drainage pathways carrying large volumes of meltwater. Their cross-sections exhibit steep-sided flanks similar to fjord walls, and their flat bottoms are typical of subglacial glacial erosion.
Meltwater In northern Central Europe, the Scandinavian ice sheet during the various ice ages advanced slightly uphill against the lie of the land. As a result, its meltwaters flowed parallel to the ice margin to reach the North Sea basin, forming huge, flat valleys known as
Urstromtäler. Unlike the other forms of glacial valleys, these were formed by glacial meltwaters.
Transition forms and shoulders ,
Utah, with very striking shoulders Depending on the
topography, the
rock types, and the
climate, a variety of transitional forms between V-, U- and plain valleys can form. The floor or bottom of these valleys can be broad or narrow, but all valleys have a shoulder. The broader a mountain valley, the lower its shoulders are located in most cases. An important exception is canyons where the shoulder almost is near the top of the valley's slope. In the Alps – e.g. the
Tyrolean Inn valley – the shoulders are quite low (100–200 meters above the bottom). Many villages are located here (esp. on the sunny side) because the climate is very mild: even in winter when the valley's floor is filled with fog, these villages are in
sunshine. In some stress-tectonic regions of the
Rocky Mountains or the Alps (e.g.
Salzburg), the
side valleys are parallel to each other, and are
hanging. Smaller streams flow into rivers as deep canyons or
waterfalls.
Hanging tributary flowing from a hanging valley valley,
Rila Mountain,
Bulgaria A hanging valley is a
tributary valley that is higher than the main valley. They are most commonly associated with U-shaped valleys, where a tributary glacier flows into a glacier of larger volume. The main glacier erodes a deep U-shaped valley with nearly vertical sides, while the tributary glacier, with a smaller volume of ice, makes a shallower U-shaped valley. Since the surfaces of the glaciers were originally at the same
elevation, the shallower valley appears to be 'hanging' above the main valley. Often, waterfalls form at or near the outlet of the upper valley. Hanging valleys also occur in
fjord systems underwater. The branches of
Sognefjord are much shallower than the main fjord. The mouth of
Fjærlandsfjord is about deep while the main fjord nearby is deep. The mouth of
Ikjefjord is only deep while the main fjord is around at the same point. Glaciated terrain is not the only site of hanging streams and valleys. Hanging valleys are also simply the product of varying rates of erosion of the main valley and the tributary valleys. The varying rates of erosion are associated with the composition of the adjacent rocks in the different valley locations. The tributary valleys are eroded and deepened by glaciers or erosion at a slower rate than that of the main valley floor; thus the difference in the two valleys' depth increases over time. The tributary valley, composed of more resistant rock, then hangs over the main valley.
Trough-shaped Trough-shaped valleys also form in regions of heavy topographic
denudation. By contrast with glacial U-shaped valleys, there is less downward and sideways erosion. The severe downslope denudation results in gently sloping valley sides; their transition to the actual valley bottom is unclear. Trough-shaped valleys occur mainly in
periglacial regions and in
tropical regions of variable wetness. Both climates are dominated by heavy denudation.
Box Box valleys have wide, relatively level floors and steep sides. They are common in periglacial areas and occur in mid-latitudes, but also occur in tropical and arid regions.
Rift Rift valleys, such as the
Albertine Rift and
Gregory Rift are formed by the expansion of the
Earth's crust due to
tectonic activity beneath the Earth's surface. ==Terms for valleys==