A valley step (German: Talstufe or Talschwelle) is a prominent change in the longitudinal slope of a valley, mainly in trough valleys formed by glaciers.
Typically, a valley formed by glaciers has a series of basins with intervening steps formed by the locally varying erosion depths of valley glaciers. After the ice melts, this initially becomes a sequence of lakes with intermediate rapids or waterfalls. The transportation of gravels, erosion and sedimentation processes in the streams lead to the formation of sequences of flat valley bottoms and gorges.
Typical valley step in a trough valley (Vilsalptal, Allgäu Alps). In the foreground its U-shape is almost filled in; behind is a gentle ravine
Such basin forms, with an abrupt beginning, often appear as marked steps, which may separate the flatter sections of the valley. Large steps were often formed where the erosion forces of the glacier that once filled the valley were suddenly increased, for instance, when large glaciers merged. They are then referred to as confluence steps. Similar steps are formed when moving sheets of ice merge into glacial streams, which can intensify surface scouring of the rocks (exaration). If these valley steps are close to the upper end of a valley, they are also referred to as a valley head or trough head; otherwise, they subdivide the course of the valley into often very distinct individual valley sections, with gentler and broader hollows alternating, chain-like, with and narrow intermediate valleys or gorges. In the interior of mountain ranges, these valleys dictate the structure of settlements, and may result in isolated communities that are difficult to access.
Side valleys, which rise high above the valley floor in a trough valley, also join the main valley at a marked step in the terrain, the stream descending over the valley wall of the main valley. This is referred to as a hanging valley, and the step is seen geomorphologically as the valley wall of the main valley, so it is not a valley stage in the narrow sense described above. Borderline cases are, for example, cirques, which typically form basins in which there is often a lake which either drains periodically over the valley threshold or seeps entirely into the subsoil.
Valley steps may also have existed before the glaciation of mountainous terrain or may be formed without the aid of glacial processes, typically due to varying erosion resistance of the rock, or due to active faults. These are then only accentuated by glacial and / or fluvial erosion. Examples of this are the tobel valleys in sandstones or, often only underground, draining troughs in the limestone, where the valley step is formed by downward erosion.
Valleys that were formed without the aid of glaciers are often found in the area of cuestas. These can then also create a very characteristic structure of a river course in its lower reaches on the broad plains and form a significant obstacle to navigation.
A glacier is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. Glaciers slowly deform and flow under stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques, moraines, or fjords. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that forms on the surface of bodies of water.
A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over a very long period of time. Some valleys are formed through erosion by glacial ice. These glaciers may remain present in valleys in high mountain or polar areas. At lower latitudes and altitudes, these glacially formed valleys may have been created or enlarged during ice ages but now are ice-free and occupied by streams or rivers. In desert areas, valleys may be entirely dry or carry a watercourse only rarely. In areas of limestone bedrock, dry valleys may also result from drainage taking place underground rather than at the surface. Rift valleys arise principally from earth movements, rather than erosion. Many different types of valley are described by geographers, using terms that may be global in use or else applied only locally.
Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.
A jökulhlaup is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst floods from Vatnajökull, Iceland, which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption, but it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir.
A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The lines along the crest formed by the highest points, with the terrain dropping down on either side, are called the ridgelines. Ridges are usually termed hills or mountains as well, depending on size.
A cirque is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie and cwm. A cirque may also be a similarly shaped landform arising from fluvial erosion.
Glacial landforms are landforms created by the action of glaciers. Most of today's glacial landforms were created by the movement of large ice sheets during the Quaternary glaciations. Some areas, like Fennoscandia and the southern Andes, have extensive occurrences of glacial landforms; other areas, such as the Sahara, display rare and very old fossil glacial landforms.
In geomorphology a river is said to be rejuvenated when it is eroding the landscape in response to a lowering of its base level. The process is often a result of a sudden fall in sea level or the rise of land. The disturbance enables a rise in the river's potential energy, increasing its riverbed erosion rate. The erosion occurs as a result of the river adjusting to its new base level.
In geology, a depression is a landform sunken or depressed below the surrounding area. Depressions form by various mechanisms.
Lake Algonquin was a prehistoric proglacial lake that existed in east-central North America at the time of the last ice age. Parts of the former lake are now Lake Huron, Georgian Bay, Lake Superior, Lake Michigan, Lake Nipigon, and Lake Nipissing.
A tunnel valley is a 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. They can be as long as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep.
U-shaped valleys, also called trough valleys or glacial troughs, are formed by the process of glaciation. They are characteristic of mountain glaciation in particular. They have a characteristic U shape in cross-section, with steep, straight sides and a flat or rounded bottom. Glaciated valleys are formed when a glacier travels across and down a slope, carving the valley by the action of scouring. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice, called glacial till or glacial erratic.
The glacial history of Minnesota is most defined since the onset of the last glacial period, which ended some 10,000 years ago. Within the last million years, most of the Midwestern United States and much of Canada were covered at one time or another with an ice sheet. This continental glacier had a profound effect on the surface features of the area over which it moved. Vast quantities of rock and soil were scraped from the glacial centers to its margins by slowly moving ice and redeposited as drift or till. Much of this drift was dumped into old preglacial river valleys, while some of it was heaped into belts of hills at the margin of the glacier. The chief result of glaciation has been the modification of the preglacial topography by the deposition of drift over the countryside. However, continental glaciers possess great power of erosion and may actually modify the preglacial land surface by scouring and abrading rather than by the deposition of the drift.
Fluvioglacial landforms are those that result from the associated erosion and deposition of sediments caused by glacial meltwater. These landforms may also be referred to as glaciofluvial in nature. Glaciers contain suspended sediment loads, much of which is initially picked up from the underlying landmass. Landforms are shaped by glacial erosion through processes such as glacial quarrying, abrasion, and meltwater. Glacial meltwater contributes to the erosion of bedrock through both mechanical and chemical processes.
The Geology of Yorkshire in northern England shows a very close relationship between the major topographical areas and the geological period in which their rocks were formed. The rocks of the Pennine chain of hills in the west are of Carboniferous origin whilst those of the central vale are Permo-Triassic. The North York Moors in the north-east of the county are Jurassic in age while the Yorkshire Wolds to the south east are Cretaceous chalk uplands. The plain of Holderness and the Humberhead levels both owe their present form to the Quaternary ice ages. The strata become gradually younger from west to east.
This glossary of geography terms is a list of definitions of terms and concepts used in geography and related fields, which describe and identify spatial dimension, geographic locations, topographical features, natural resources, and the collection, analysis, and visualization of geographic data. Related terms may be found in glossary of geology, glossary of astronomy, and glossary of environmental science.
Overdeepening is a characteristic of basins and valleys eroded by glaciers. An overdeepened valley profile is often eroded to depths which are hundreds of metres below the deepest continuous line along a valley or watercourse. This phenomenon is observed under modern day glaciers, in salt-water fjords and fresh-water lakes remaining after glaciers melt, as well as in tunnel valleys which are partially or totally filled with sediment. When the channel produced by a glacier is filled with debris, the subsurface geomorphic structure is found to be erosionally cut into bedrock and subsequently filled by sediments. These overdeepened cuts into bedrock structures can reach a depth of several hundred metres below the valley floor.
The Rotbach is the 19.9-kilometre-long, left-hand and southeasterly headstream of the Dreisam in the Southern Black Forest in the south German state of Baden-Württemberg.
During the Vashon Glaciation a series of lakes formed along the southern margin of the Cordilleran Ice Cap. In the Puget Sound depression, a series of lakes developed, of which Lake Russell was the largest and the longest lasting. Early Lake Russell’s surface was at 160 ft (49 m) above sea level, draining across the divide at Shelton, Washington into early Glacial Lake Russell. When the ice margin receded northward, the lake expanded. When it reached the Clifton channel outlet, the water levels dropped to 120 ft (37 m) above sea level. The new longer and lower level lake is referred to as Lake Hood. The glacier continued to retreat until the northern outlet of the Hood Canal was reached as the water level equalized with Glacial Lake Russell becoming part of that body of water.
Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers. They are transported, sorted and deposited by streams of water. The deposits are formed beside, below or downstream from the ice. They include kames, kame terraces and eskers formed in ice contact and outwash fans and outwash plains below the ice margin. Typically the outwash sediment is carried by fast and turbulent fluvio-glacial meltwater streams, but occasionally it is carried by catastrophic outburst floods. Larger elements such as boulders and gravel are deposited nearer to the ice margin, while finer elements are carried farther, sometimes into lakes or the ocean. The sediments are sorted by fluvial processes. They differ from glacial till, which is moved and deposited by the ice of the glacier, and is unsorted.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
