## 2 Geology
### Stratum  
According to regional geological data, the strata exposed on the surface of the research area are mainly pre-Quaternary bedrock (including Qingbaikou Qb-Paleogene E), Quaternary lower Pleistocene series Changde formation, Quaternary middle Pleistocene series Mawangdui formation, and Quaternary Holocene series. The main stratum is briefly described as follows:  
(1) Pre-Quaternary bedrock (Qb-E)  
Changde city and its periphery are covered by Quaternary bedrock, therefore the Paleogene systems of Proterozoic, Paleozoic, Mesozoic, and Cenozoic are considered as pre-Quaternary bedrock in this study. The Qingbaikou system is the oldest stratum exposed in the survey area, and this includes the Lengiiaxi and Banxi groups distributed in the north of the investigation area located in the Taiyangshan area of Dingcheng District, Changdedding City. The Nanhua and Sinian system are only distributed in Taiyangshan. Cambrian, distributed in the northwest, is the most widely distributed stratum in the survey area. Cambrian stratum is concentrated to the east of Shibatan Town, such as Liantai-Huaiishugang-Teijiazui-Guanshiyan, and in the west of Guanxi Town, in places such as Matousi-Fengjiagiao-Zhangshuwan, Taiping-Gongjiawan-Lijia-Nanyan. There is a small amount of Cambrian distributed in the Liujiapo area of Chou Town. The Ordovician is distributed in the northwest corner of the survey area, mainly in the NW-trending band in Jinjishan-Zhaojiawan of Shibatan Town, and the Zhoujia-Zhaizishan-Changlinggang reservoir of Guanxi Town. Cretaceous is concentrated in the northwest of Guanxi Town, in places such as Shuomudang-Fujiashan and Wujiawan-Baozhengwan, and there are scattered outcrops in the Quaternary distribution area. Paleogene is concentrated in the N-NE of Liuye Lake in Changde City, namely in the Huangjiachong-Shiwan-Xiaowupu area. (2) Quaternary lower Pleistocene series Changde formation (Qp\({}_{1}\)_cd_)  
It is mainly distributed in the hilly-to-hilly land morphologic province, which spreads in planar or banded form. It is specifically distributed in Hefu-Guanxi town and the Dingjiagang-Doum Lake-Caoping area. The stratum elevation is generally 60 \(\sim\) 120 m, and the sediment is generally 10 \(\sim\) 30 m thick, with a maximum thickness of up to 70 m. The lithology is mainly a gravel layer with a small amount of lentioid sand and clay layer. The thickness of the sediment varies frequently from bottom to top, and the phenomenon of internal scour and filling, erosion, and cutting is very common. It is the alluvial-diluvial fan body mainly composed of braided river deposits, with a small amount of debris flow deposits inside. (3) Quaternary middle Pleistocene series Mawangdui formation (Qp\({}_{2}\)_mv_)  
It is distributed in hilly, hilly land, undulating plains and other areas. The elevation is 40-150 m, and the thickness of the stratum varies greatly from 3 to 20 m. Generally, the thickness increases at the place where the geomorphic change occurs. The lithology is mainly brownish red, brownish yellow reticulate clay, containing (weak) reticulate iron-manganese clay; reticulate gravel clay is commonly distributed at the bottom. It is in unconformable contact with the lower strata (Changde formation, Baishajing formation, etc. ), which is usually covered at a small angle or cut into the lower sand and gravel layer at a large angle. (4) Quaternary Holocene series (Qh)  
It is developed in modern river gullies in relatively low topography areas, and the stratum is distributed in relatively limited areas with a small thickness of 2-9 m. Many ofthem have a river dual structure with a gravel layer at the lower level and a silty clay layer at the upper.