Valley
A valley (from the Latin vallis) is a plain between mountains or heights, a depression of the earth's surface between two slopes, with an inclined and elongated shape, which forms a hydrographic basin at the bottom of which a fluvial course is lodged.
V-shaped valleys predominate in a young relief, characteristic of river valleys: the slopes, little shaped by erosion, converge on a very narrow bottom. On the contrary, an advanced state of erosion gives rise to alluvial valleys, with a flat and wide bottom, made up of alluvial deposits between which the water course can ramble. The U-shaped valleys, characteristic of the valleys or "troughs" glacial, have very abrupt walls and a concave bottom. In certain cases, when an old glacier recedes, the bed of one of its tributaries remains at a great height above that of the former and flows into its slope, often forming waterfalls. A third type of valley is cradle-shaped or raft-shaped: they are wide, gently sloping, and shallow.
When a river is captured by another or when its bed is closed by moraines or other types of deposits, a dead valley or decapitated river is left below, which no longer has a water course. In other cases, a valley has no natural outlet, because it is closed off by a counterslope, and the waters that flow through it penetrate the ground and continue their course through an underground network. These blind valleys are typical of karstic terrain. Likewise, in many arid regions, rivers cannot leave their hydrographic basin, flowing through endorheic valleys. A valley may have been entirely carved out of a sedimentary terrain by its water course, but generally, it makes its way through depressions of tectonic origin. Depending on whether they are, there is a fracture valley, trench , failure angle, etc. A longitudinal valley is oriented parallel to the folds of a mountain range, while a transverse valley is perpendicular to them.
Types of valleys
Geomorphologically, there are differences between narrow valleys and wide valleys, the main ones are described below.
Characteristics of narrow valleys
The water currents that generally occupy the lower part of the valley appear completely confined and strongly controlled for lateral migration, in this way the channel adjustment processes occur directly at the bottom of the channel, modifying the slope and even incising the bed; associated with these processes, bank instability and landslides may occur. The proximity of the mountains to the riverbed make these valleys generally unattractive for urban developments.
The development of canyon valleys is closely related to the intensity of the geomorphological processes that form the valley and with the geology, specifically with the lithological composition or with the faults. Valley materials vary from bedrock to residual soils in the form of colluvium, debris flow, and other depositional materials.
The location of canyoned valleys is more frequent in the upper parts of the hydrographic basin where the rivers have little flow, the slopes are high and the walls of the valley frequently show bedrock without cover. When the mountains that surround the valley are very resistant to weathering and erosion, the valley presents a canyon configuration even in the middle of the basin. waterfalls and rapids are frequently found in the watercourses that drain these valleys.
Although it is generally believed that V valleys are perfectly symmetrical, the truth is that they also present a certain degree of asymmetry, with the wall or slope on the left being steeper or steeper than the one on the left. the right (in the case of the Northern hemisphere).
Examples of a V shaped valley in Svaneti, Caucasus Mountains.
Characteristics of wide valleys
Wide valleys are associated with the famous plain rivers (mature and old) where the channel occupies a reduced part of the valley since the alluvial plain is wide. In this you can see features of the landscape that are not found in the canyoned valleys such as: alluvial terraces, dikes, natural, madreviejas, abandoned riverbeds, border complexes. Like the plain, the riverbed is also wide presenting width relationships depth greater than 10. The plain is subject to recurrent flooding, so it is not static or stable.
The plain is generally composed of unconsolidated sediments that erode rapidly during floods and river rises. A river channel may change position in the broad floodplain, and the floodplain, in turn, is periodically modified by flooding as the channel moves from one place to another.
During periods of normal or low water, the river that runs through the valley is confined to its course and does not spill onto the floodplain. The floodplain created by lateral erosion and the gradual retreat of the valley walls is called an erosive floodplain and is characterized by a thin cover of gravel, sand, and silt a few decimeters or a few meters thick. On the other hand, under the bottom of many wide valleys are deposits of gravel, sand and silt that reach 100 or more meters in thickness. These thick deposits form when changing conditions force the river to drop its load across the valley floor; This floodplain formed by the construction of the valley floor, or gradation, is called the gradation floodplain.
Floodplains of this type are much more common than erosional ones and usually tend to be found in the lower reaches of rivers. Both floodplains, the erosional and the aggradation, show forms such as meanders, braids, natural ridges, sediment deposits, and meandering channels.
Examples of a U-shaped valley in Acatlán (Hidalgo), Valle de Acatlán.
Incision and widening of the valleys
Both in the past and in the present, various processes have acted and are acting in the sense of deepening and widening the valleys, although the evidence of said action may be lost or weakened over time. If a current were left free to reach its base-level by itself, it would erode the bed directly downward, forming a vertical-walled chasm in the process.
But because the current is not the only agent at work in the formation of the valley, the walls of most valleys slope up and away from the valley floor. Over time, even the steepest gorges' walls will slope outward in relation to the axis of their valleys. As a stream cuts down and deepens its channel into the ground surface, weathering, runoff, and landslides come into play, constantly wearing down the valley walls, pushing them back, away from each other.
Material, under the influence of gravity, is pulled from the valley walls below and discharged into the current, to be moved onward eventually to the oceans.
The result is a valley whose walls widen outward and upstream from the stream to form a typical cross section.
The speed with which the valley walls are reduced and the angles they adopt depend on several factors, always thinking in geological times, that is, millions of years. If the walls are made of unconsolidated material (which is vulnerable to erosion and mass movement), the speed will be fast; but if the walls are made of resistant rock, the rate of erosion will be very slow, and the walls will be able to rise almost vertically from the bottom of the valley. In addition to cutting down its channel, a stream also cuts from side to side, or laterally on its banks.
In the early stages of valley widening, when the stream is still above its base-level, downward erosion predominates. Subsequently, as the current approaches its base level, downward erosion becomes less and less important; at this stage a larger proportion of the energy of the current is devoted to the erosion of its banks. As it oscillates from one side to the other, it forms a floodplain on the bottom of the valley that tends to ever widen and the valley becomes ever wider.
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