What is the landform of Ya Dan, and what are the reasons for its formation?

There are two key factors in the formation of Ya Dan. First, the geological basis for the development of this landform, that is, lacustrine sedimentary strata; The second is external force erosion, that is, strong directional wind erosion and flowing water erosion in the desert.

Lakes in arid areas often include repeated water inflow and water withdrawal in their formation history, thus developing overlying mudstone layers and sandy soil layers. Wind and running water can take away loose sandy soil layer, but it has limited effect on hard mudstone layer and gypsum cementation layer. However, the dense mudstone layer is not indestructible, and the expansion and contraction effect caused by severe temperature difference in desert area will eventually lead to the disintegration of mudstone layer, and the exposed sandy soil layer will be carried away by wind and running water and evolved into a groove shape; The part still covered by mudstone is relatively stable, forming large or small strip mounds, and the landform of Ya Dan gradually appears.

The external factors that form Ya Dan are usually considered as the dominant factors of strong prevailing winds, but this is not a single dominant factor. For example, in the east of Acic Valley, the direction of Sanlongshayadan is southeast, perpendicular to the prevailing northwest wind direction and consistent with the flow direction of mountain torrents, indicating that floods play a leading role in this Ya Dan; In addition, some Ya Dan are formed by the interaction of wind and flowing water, such as Ya Dan in Longcheng.