Charlotte heard the scrunch of boots on gravel.

夏洛特听到了长筒靴走在砾路上嘎吱嘎吱的声响。

He had been so absorbed with watching the building that he hadn't heard footsteps on the gravel path.

他留神观察那栋建筑，竟没有听见砾路上的脚步声。

Sand is finer than gravel.

沙子比细。