Chishan Fault is the easternmost thrust fault in the southwestern foothills, and it might be the first thrust fault within this area. It was possibly formed in the early Pleistocene. W.S. Chen et al. (2005) believed that the Wushan Formation at the southern hanging wall of Chishan Fault thrusts upon the Kutingkeng Formation. The vertical difference between the two sides of the fault is at least 2,000 m.

The hanging wall of Shenshueikeng outcrop is an early Pliocene Wushan Formation. The foot wall belongs to Pleistocene Kutingkeng Formation. A shear band few meters in width is exposed and shears the colluvium of Holocene. It reveals that the Chishan fault is still active during Holocene. The profile of Shenshueikeng river no. 1 (Figure 1) is located on the main stream of the Shenshueikeng river. Four beds are exposed: Wushan Formation, Kutingkeng Formation, Holocene Colluvium, and the Recent Terrace gravel bed. The profile shows that a 3-meter thick gravel bed is unconformity on the Wushan Formation, Kutingkeng Formaion and Holocene Colluvium. Chishan Fault has not cut through the terrace gravel bed. It shows that Chishan Fault has not been active after the deposition of the gravel bed. The 14C dating in the Holocene Colluvium is 7730-8330 yr BP. The Colluvium is unconformity on the Wushan Formation. After the activity of Chishan Fault, the Wushan Formation thrusts upon the Holocene Colluvium. The strike and dip of the fault is N72ºE, S76º. The mudstone of the Kutingkeng Formation thrusts the west side of the Holocene Colluvium. The strike and dip of the fault is N38ºE, N48º.

Figure 1. Profilw of Shenshueikeng No. 1.

The profile of Shenshueikeng no. 2 (Figure 2) is located on the branch of the Shenshueikeng river. It is about 80 m to the south of Shenshueikeng no. 1. The beds of the profile are Wushan Formation, Kutingkeng Formation, and the Recent Terrace gravel bed. The gravel bed is also unconformity on the Wushan Formation and Kutingkeng Formation. The Fault does not cut through the gravel bed. The Kutingkeng Formation thrusts upon Wushan Formation. The strike and dip of the fault is N80ºE, 50ºN. The terrace sediment is unconformity on the Wushan Formation. Although the dip of the unconformable surface is 12ºS, it has not shown any relations to the fault, thus whether its tilt was affected by the fault or the hill creep is unidentifiable.

Figure 4-2. Profile of Shenshueikeng No. 2.