crust

n. 外壳,坚硬的外壳,面包皮
v. 盖以硬皮,结硬皮

The snow crusted the lake last night.

昨夜的雪使湖面结了冰。

The crust on the snow was thick enough for us to walk on it.

外层的雪已经冻硬，可以在上面行走。