On the east edge of the Nine Turns Tunnel, rainwater has eroded the marble, creating a long and narrow cave. Gneiss, another type of metamorphic rock, is scattered along the sides of the creek. In particular, the stretch between Paisha Bridge and Swallows Grotto is an important location for observing and distinguishing between Taiwan's two oldest metamorphic rock types, gneiss and marble.
九曲洞東緣的「幽谷煙聲」景致，是雨水順著岩石節理侵蝕而成的狹長大理岩洞，溪畔分佈著片麻岩，在白沙橋與燕子口之間，是觀察辨認片麻岩與大理岩兩種古老岩石的最佳地點。