在十七世纪和十八世纪，整个遗址被一层坚硬的灰色粘土覆盖，这极大减少了进一步的侵蚀。

During the seventeenth and eighteenth centuries, the entire site became covered with a layer of hard grey clay, which minimised further erosion.

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牙齿有一层叫做釉质的坚硬表层。

Teeth have a hard surface layer called enamel.

《牛津词典》

在工业中，钻石可以制成工具上的一层坚硬且耐腐蚀的涂层。

In industry, diamond can form a tough, non-corrosive coating for tools.

《柯林斯英汉双解大词典》