Then, under the background of the relatively slow uplift of mountains and the influence of periodic climate change, the main valleys in this area experienced at least seven relatively short valley development periods characterized by erosion-accumulation cycles, and six river terraces, a set of floodplain deposits and related cave deposits were formed in the middle and lower reaches of Zifang River (Figure 5- 16).
According to the geological period model, the Quaternary evolution process of Zifang River Basin can be divided into the following stages:
1) Lower Nihewan layer accumulation period-"Hongshixia period" erosion (which may be equivalent to cutting in the Yellow River period): from the middle and late Pliocene to the early Pleistocene, the climate was relatively dry and cold, and rivers were mainly widened and accumulated in the form of lateral erosion, forming a river valley that cut the planation plane in Tangxian period, and a set of thickness equivalent to "Early Nihewan period" was accumulated in its interior or on the surface of Tangxian county. There are steep limestone cliffs on both sides of the valley in the middle reaches of Zifang River, and the bottom is hard and thick timely sandstone, so the valley widening is extremely limited, and only a shallow V-shaped valley with a width of about 100 ~ 300 m and its gravel layer are formed. In the lower reaches of the river, the lateral erosion of the river forms a wide U-shaped valley with a width of 1 ~ 2 km, which is filled with calcareous cemented gravel layer.
At about1.6mab.p., with the arrival of warm and humid climate, the river erosion was strengthened, and the tug-of-war t 6 of 80 ~ 100 m was the product of this stage, which was formed by the river cutting alluvial deposits in the early Pleistocene. Because the cutting effect is most obvious in Hongshixia section, it is called Hongshixia erosion here. The surface observation shows that in Hongshixia section, the river at this stage was cut in hard Mesoproterozoic medium-thick purple timely sandstone; However, in the reach below Hongshixia, the river is incised in the relatively soft medium-thick limestone stratum of the Middle Ordovician. Because of the different resistance of lithology to river erosion, another important breakpoint between the middle and lower reaches of Zifang River is formed with the hard and thick timely sandstone in Hongshixia area as the boundary. In Hongshixia section on the north side of the breakpoint (Figure 5- 17), due to the large river bed gradient caused by the breakpoint and the hard lithology of the thick sandstone, the river valley in this section has been in the process of continuous and intense undercut and traceable erosion in the middle and late Quaternary, thus forming a local expression with a width ranging from several meters to tens of meters to 80 ~ 100 meters in sandstone. At the same time, the first-class bedrock formed by lithologic differences and related waterfalls, ponds and water droplets develop at the bottom of the valley. In the wide valley in the lower reaches of Zifang River, the tug-of-war terrace with the highest U-shaped wide valley of about 80m was excavated in Hongshixia period. After that, it experienced many river accumulation-erosion cycles.
2) The middle accumulation period in Nihewan (or loess accumulation period in lower Sok Li)-Tongchuan erosion: In the late early Pleistocene (about 1.6 or 1. 1 ~ 0.8 Ma ago), the climate in North China entered a dry and cold stage, and valley accretion began to dominate again. Almost at the same time that the lower part of Lishi Loess and the middle part of Nihewan River developed, the Zifang River, Qingtian River and Danhe River in Yuntai Mountain all experienced a stage of river valley lateral erosion filling, and the curvature of the river channel further increased. A set of calcareous cemented gravel layer and red soil layer with the thickness equivalent to several meters to several tens of meters in the middle part of Nihewan River were filled in the river valley. Subsequently, in the early and middle period of the Middle Pleistocene (about 0.8 ~ 0.4 Ma ago), the climate began to change to relatively warm and humid, and the river erosion in Taihang Mountain area resumed and entered Tongchuan erosion stage. During this period, the river cut and formed the T5 terrace of about 60m tug-of-war of Zifang River and the T4 terrace of about 100m tug-of-war of Qingtian River. At the same time of river erosion, karst caves or karst springs are in the accumulation period due to the strengthening of karst water, forming layered travertine deposits, such as Wangyao Cave in Yuntai Mountain, Qinglongxia Cave and river valley.
3) Upper Nihewan layer accumulation period (or upper Sok Li loess accumulation period)-clear water erosion: in the late Middle Pleistocene (about 0.4 ~ 0. 15 Ma ago), the climate in North China changed to dry and cold stage again, and valley accretion began to dominate again. In the valley downstream of Zifang River in Yuntai Mountain, due to the lateral erosion and filling stage, the curvature of the river is further increased, and a meandering river is gradually formed near the exit pass. A set of local calcareous cemented gravel layer is filled in the valley, with a thickness of several meters to more than ten meters, which is roughly the same as that of the upper part of Shangnihewan layer or Lishi loess. Subsequently, in the early and middle period of the Late Pleistocene (about 0. 1.5 ~ 0.08 Ma ago), the climate turned relatively warm and humid, and the river erosion resumed, and the mountainous areas in North China entered the cutting stage in the clear water period. This river cutting formed the T4 terrace of tug-of-war about 27m in Zifang River and the T3 terrace of tug-of-war about 50m in Qingtian River. At the same time, layered karst travertine accumulation also developed in Qinglong Gorge Scenic Area.
4) Erosion in Lower Malan Loess Accumulation Period-Wujiatai Period: In the middle of Late Pleistocene (about 80 ~ 50ka ago), the climate in North China entered the last glacial period of dry and cold and the early glacial period, with valley accretion dominating. The lower reaches of Zifang River, Yu He River, Xianshen River, Dongdahe River and Danhe River in Yuntai Mountain have gone through a stage of lateral erosion filling, and the curvature of the river channel has been further increased. A set of gravel layers and yellow soil layers with the same thickness as the lower part of Malan Loess are filled in the valley. Subsequently, in the middle and late Late Pleistocene (about 50 ~ 30 ka ago), with the change of climate to relatively warm and humid, river erosion resumed. The downward cut of the river forms a 27m T3 tug-of-war terrace between Zifang River and Dan River. Stone curtains, stone curtains, stone bells, stalagmites, stalactites, etc. developed in caves in scenic spots such as Qinglong Gorge, Qingtianhe Gorge and Fenglin Gorge. Most of them began to develop from this stage. In view of the tug-of-war terrace of about 27m formed in Wujiatai area near the downstream outlet of Zifang River, this section of erosion is called Wujiatai stage erosion here.
5) Upper Malan Loess Accumulation Period-Banqiao Erosion: In the late Late Pleistocene (about 30 ~ 12ka ago), the climate in North China entered a dry and cold climate stage equivalent to the peak of the last ice age, with valley accretion as the main factor. The lower reaches of Zifang River, Danhe River, Yu He River and Qingtian River in Yuntai Mountain are filled with a set of loess gravel layers which are several meters to nearly ten meters thick. Subsequently, from the end of late Pleistocene to the beginning of Holocene (about 12 ~ 8ka ago), the climate gradually changed to a relatively warm and humid direction, the river erosion resumed, and the mountains and rivers entered the Banqiao period. Downcut of the river forms a tug-of-war T2 of about 10m in the valleys of Zifang River, Yu He River, Danhe River and Qingtian River.
6) Gaolan period accumulation-modern cutting accumulation: After Banqiao period erosion, in the early and middle Holocene (about 8-4ka ago), the mountainous area of North China entered the Gaolan period accumulation stage, and a set of loose gravel layers with a thickness of 2-4m was filled in the lower reaches of Zifang River, Yu He River, Danhe River, Dongdahe River, Xianshen River and Qingtian River in Yuntai Mountain and its adjacent areas. Subsequently, in the middle and late Holocene (about 4 ~ 3 ka ago), after the Neoglacial, rivers in mountainous areas of North China experienced a relatively common stage of erosion and undercutting, and a tug-of-war T 1 terrace of about 2 ~ 5 m was formed in the valleys of Fangzi River, Danhe River, Qingtian River, Xianshen River and Yu He River. Subsequently, the alluvial gravel layer with a thickness of tens of centimeters to nearly one meter was accumulated, that is, the floodplain accumulation in the valley, and most of the floodplain experienced short-term and small-scale cutting. However, in the middle and upper reaches of the river valley where the river gradient is relatively large, the rivers are mostly in the stage of continuous erosion. Due to the differences in lithology and erosion resistance, rock ridges and landforms with different heights are formed in the longitudinal direction of the river bed.