rimstone

例句：

1. 1. The river flows over smooth rimstone.
   河流流过光滑的砾石。
2. 2. The path is paved with smooth rimstone.
   小径铺着平滑的砾石。
3. 3.The rimstone was formed by the slow movement of water.
   砾石是由缓慢流动的水形成的。