Geomorphology has the aim at studying and understanding the Earth’s landform. So it studies the forms that determine the appearance of a landscape and the underlying causes of those forms. The appearance of what surrounds us is the result of a series of phenomena which can be grouped into two broad categories:
- endogenous processes (tectonic, seismic, volcanic)
- exogenous processes (phenomena linked to the atmosphere, the hydrosphere, the biosphere)
The terrestrial forms derive usually from the overlap of more of these processes. If we think big the landform features, they appear primarily as the result of endogenous processes to which we owe the structure of the Earth’s crust: continental masses; ponds, ditches and ocean ridges; great mountain ranges. If we look more closely at a limited area of the Earth’s surface we would see, in all their variety, the forms of modeling due to exogenous processes: valleys erosion, plains and alluvial terraces, karst forms. In nature there are different processes and agents that determine over the time a change of the landscape forms. Structural geomorphology studies the forms related to the “geological structure” in a specific area; these can be affected by:
- tectonic (seam of the layers, presence of faults)
- strata (presence of different rocks in the sedimentary sequence)
- type of rock (by its characteristics depending on how much it is moldable by erosion)
The human geomorphology instead is interested in the forms caused or influenced directly and indirectly by human activities. Being now the man a modifying agent of the landform, much more of many natural processes.
The territory that collects rainfalls and water gathered in the same river is called the river basin; this basin is bounded by the watershed lines running on the ridges, thus separating one basin from another. The water catchment areas have very different sizes depending on the importance of the waterways to which they belong. The basin of a river is divided into many sub-basins as the number of its tributaries, and each of them, in turn, into many smaller ponds (depending on how many its sub-tributaries are).
Morphology of some types of riverbed
Riverbed dug in the rock, within a mountain valley. The shape is irregular and depends on the mode of erosion, by the resistance of the rock, from its fracturing, etc … riverbed excavated in rock, but covered in large part by pebble accumulations left at the end of each overflow. It takes shape a gravel bed in the form of temporary islands. The relationship between depth and width is much less than in the previous case. Riverbed characterized by an expanse of pebble floods, crossed by a network of little etched channels. It is typical of the streams, with plenty of material transport on the bottom. The abundance of debris forces the current to deflect and to divide frequently. Wide riverbed with sandy islands. It is similar to the above but with well-defined islands consist of mainly sandy floods. Bed with meanders, with well-defined banks, within a floodplain. One can see the traces of the old abandoned meanders. The relationship between depth and width returns to be greater than in the previous cases.
The alluvial fans are accumulations of fan-shaped debris(cone area) that are deposited in the main valleys and plains at the mouth of the river valleys. Generally the alluvial cones that form the piedmont plains are large and nearly flat, while are smaller and steep those of small streams that are frequent in the valleys, on the edge of the alluvial valley floor. The formation of the cones is derived from the distribution and deposit of alluvial material on the entire surface of the cone, when it is active. This occurs because of the frequent movements of the stream riverbed, following the rays of the “fan”.
The pyramids or erosion pillars are very characteristic forms, created by the run-off rain flows on rocks or not coherent and heterogeneous deposits, containing large blocks, such as some moraine and river- glacial deposits of the Alps. The erosion of the run-off flows makes stand out thin pinnacles, topped by boulders as a protection. Instead of the rock, at the top of the erosion pillar you can find other protective elements:
- a residue of a harder rock bank
- a clump of plants
In some cases the pinnacles resist or they also take shape without any protective element.