在河流形成的初期和河流的上游，流水的侵蚀有下切和侧蚀两种，下切变深，侧蚀变宽，上部侵蚀早，下部侵蚀晚，所以上大下小，形成的V型谷。金鞭溪在河流的下游以沉积为主，水流平缓侧蚀为主，基本没有下切，形成宽广的U型谷。

At the upstream in the early stage a river is formed, the fluvial erosion process includes undercutting and lateral erosion. Undercutting makes the river deeper and lateral erosion makes it wider. Because the upper part of the rock stratum is eroded earlier than the bottom part, a V-shaped valley is formed. Because the Golden Whip Brook is at the downstream of the river, sedimentation process prevails, and the gentle flow causes mainly lateral erosion and barely any undercutting, so a wide U-shaped valley was formed.