What is the origin and significance of river terraces?

The vertical upward and downward movement of the earth's crust is affected. The formation of terraces is mainly formed by the downcut erosion of rivers under the influence of the vertical upward and downward movement of the earth's crust, which is the result of the combined action of internal and external dynamic geological processes of the earth. There are several terraces, and there have been several movements; The higher the terrace location, the older the formation age.

Classification:

1. Erosion terraces. Constructed of bedrock, there is often little alluvium left on the terraces.

2. Stacked terraces. Consists of alluvial deposits. According to the different degrees of river downcutting, the cutting and superposition relationships of the terraces can be divided into: the upper terraces, which are the new terraces stacked on top of the old terraces; Terraces are stacked inside, and new terraces are stacked on top of old terraces.

3. Pedestal terrace. When terraces are formed, the river undercuts beyond the valley floor of the old river valley to reach and expose the bedrock.

4. Buried terraces. That is, the early terraces were buried by the new terraces. It is of great scientific and economic significance to use the methods of physics, chemistry, biology and chronology to study the progression, structure, age, genesis and distribution of terraces.

Genesis: River terraces are formed in a relatively stable accumulation and rapid downcutting process. Due to the different conditions of downcut erosion, the causes of terraces are also different.

The main causes are: climate change, the formation of climatic terraces; tectonic movements, forming tectonic terraces; changes in the erosion datum; The impact of human activities, such as the construction of water conservancy projects, has changed the datum.

Research implications: Research terraces are of scientific and economic importance. The river terraces reflect the history of the development of the river valley, as well as the nature and magnitude of the neotectonic movements of climate change in the region at different times, and the study of the transport chain base helps to restore the paleogeography of the basin.

The alluvium of river terraces is often rich in a variety of heavy metal placers, such as gold, tin, monazite and other amyx. Roads and railways are often built on terraces; Cities, factories, and settlements are often distributed on terraces; In mountainous or hilly areas, terraces are the main fertile fields, and in plain areas, terraces are also good farming areas.

River terraces are formed under the combined action of tectonic movement of the earth's crust and erosion and accumulation of rivers. When the floodplain and valley are formed, due to the rise of the earth's crust or the relative decline of the erosion datum, the original riverbed or floodplain is undercuted, and the part that is not undercut is higher than the flood water level and becomes a terrace, so the river cuts the riverbed up and down on a new plane. After that, when the crustal tectonic movement is in a relatively stable period or a declining period, the slope of the longitudinal profile of the river becomes smaller, the kinetic energy of the flowing water weakens, the downward erosion of the river weakens or stops, and the lateral erosion and sedimentation increase, so the river valley is re-widened and a new floodplain is shaped.

In the course of long-term geological history, if the earth's crust undergoes multiple up-and-down movements, it will cause river erosion and accumulation to alternate, thus forming multi-level terraces in river valleys.

In general, it can be divided into the following three main types: erosion terraces, erosion-stacking terraces, and stacked terraces (which are further divided into upper and inner terraces).