《徐霞客游记·游颜洞记》
但水从洞中流出流进，一直逼近朝外的洞壁，所以没有桥就不能行走。
但水所出入，直逼外壁，故非桥莫能行。
However, because the water flows in and out of the cave and goes straight up to the exterior cave wall, a bridge is needed to walk across it