A valley step ( or ''Talschwelle'') is a prominent change in the longitudinal slope of a

valley A valley is an elongated low area often running between hills or mountains and typically containing 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 ...

, mainly in trough valleys formed by

glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

s. Typically, a valley formed by glaciers has a series of basins with intervening steps formed by the locally varying

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

depths of

valley glacier Glacier morphology, or the form a glacier takes, is influenced by temperature, precipitation, topography, and other factors. The goal of glacial morphology is to gain a better understanding of glaciated landscapes and the way they are shaped. T ...

s. After the ice melts, this initially becomes a sequence of lakes with intermediate

rapids Rapids are sections of a river where the river bed has a relatively steep stream gradient, gradient, causing an increase in water velocity and turbulence. Flow, gradient, constriction, and obstacles are four factors that are needed for a rapid t ...

waterfall A waterfall is any point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls can be formed in seve ...

s. The transportation of

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

s, erosion and

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to th ...

processes in the streams lead to the formation of sequences of flat valley bottoms and

gorge A canyon (; archaic British English spelling: ''cañon''), gorge or chasm, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tend ...

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 A valley is an elongated low area often running between hills or mountains and typically containing 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 ve ...

, and the step is seen

geomorphologically Geomorphology () is the scientific study of the origin and evolution of topographic and bathymetric features generated by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand why ...

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,

cirque A (; from the Latin word ) is an amphitheatre-like valley formed by Glacier#Erosion, glacial erosion. Alternative names for this landform are corrie (from , meaning a pot or cauldron) and ; ). A cirque may also be a similarly shaped landform a ...

s, 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 Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus. The su ...

. 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 In geography and geology, fluvial sediment processes or fluvial sediment transport are associated with rivers and streams and the deposits and landforms created by sediments. It can result in the formation of ripples and dunes, in fractal-shaped ...

. 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

cuesta A cuesta () is a hill or ridge with a gentle slope on one side, and a steep slope on the other. In geology, the term is more specifically applied to a ridge where a harder sedimentary rock overlies a softer layer, the whole being tilted somew ...

s. 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 Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...

Literature

Stefan Rasemann: ''Geomorphometrische Struktur eines mesoskaligen alpinen Geosystems. Kapitel 2.3: Reliefformen des Hochgebirges.''
Dissertation Rheinische Friedrich–Wilhelms–Universität Bonn, 2003, p. 77 (pdf data; 3,95 MB) {{Glaciers Glacial landforms