*2.2. Topography and Geomorphology*

Geomorphologically, the Yinchuan region is higher in the west and lower in the east, and the southern part is higher than the northern one. The western part is occupied by the Helan Mountains, reaching an altitude of 1500~3200 m. The central part is characterized by a flat plain tilted from southwest to northeast, which is formed by flood, alluvial flood, river, and lake deposits. The terrain of the Taoling salt platform in the eastern part is undulating. There are several loess-like clay sand, silty sand, sand gravel, and other strata accumulated due to flood and aeolian deposits, with a small amount of Quaternary gravel residue between them. The topography and geomorphology of the Yinchuan area today were formed through the comprehensive influence of physical, chemical, and biological effects across a long geological period. Based on the sedimentary origin, the terrain of the study area can be divided into erosion, accumulation, and aeolian terrain. Based on the morphology, it can be divided into the piedmont alluvial plain and alluvial plain. The accumulation terrain includes first-order terrace, second-order terrace, low-level alkaline beach, and the Yellow River floodplain. The aeolian terrain includes fixed, semi-fixed, and active dunes.

The study area is located in the Yinchuan fault basin. To the east, the Yellow River fault is connected with the Ordos block. To the west, the eastern foot of the Helan Mountains is connected with the mountain transition. A series of faults developed in the plain, some controlling the boundary of the plain, and some hiding inside the plain. The fault dips steeply towards the center of the plain, resulting in a "ladder" fault depression. The eastwest and northwest faults mainly control the north-south boundary of the plain, and some are hidden in the plain [29–32]. The distribution and properties of faults with different strikes vary greatly.

The exposed strata in the study area are mainly Quaternary strata, which can be divided into four series according to age: early, middle, late Pleistocene, and Holocene. On the cross-section, Yinchuan pool sinks towards the center. The middle part of the Quaternary stratum is the deepest, and the East and west gradually become shallower
