页岩层顶部是由远古时候掩埋森林的沉积物构成。由于埋藏其下的煤已经被挖走，所以这些页岩有开裂和剥落现象。
The shale roof-made of the sediment that destroyed the forest so long ago-is cracking and flaking off now that the coal below it has been removed.
当顶部的圈套击中头部震动对底部产生开裂音头。
When the top head is struck the snares vibrate against the bottom head producing a cracking sound.
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