The lithology of Lalashan is mainly comprised of sandstone and shale. Shale is easily eroded and weathered, which causes many ridge line and valleys to form. The water is fast flowing and erosive at the upstream of Dahan River, when encountering hard rock or ridges, the river meanders through the mountains.
Many river sections even form 180 degree back bar and resemble in shape the switchbacks like a mountain road. Looking down at the valley from the road, the meandered mountains appear in the valleys, stretching into the river, like three turtles playing in the water! The twisting beauty is the origin of the image of “three Swimming Turtles”.
拉拉山岩性以砂、頁岩互層為主，頁岩容易被侵蝕、風化，產生了許多河谷和山脊稜線。大漢溪上游，水流湍急，侵蝕強盛，遇到堅硬的岩層或山脊，河道只得曲折繞行於群山之間，形成曲流，許多段河道甚至形成180度的回頭灣，形成曲流山腳。由高處往河谷看，數個曲流山腳在山谷間交錯出現，伸入河中，彷彿三隻烏龜探頭出來戲水！極富曲折之美，這就是三龜戲水的由來。