In fact, the available sand is insufficient to form a continuous blanket overlying the unconformity.

事实上，所补入的砂质物不足以在不整合面上形成连续的盖层。

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The overlying strata of the coal bed roof are dominated by sedimentary rocks such as shale, sand stone and limestone, whose physical and mechanic properties differ greatly.

煤层顶板覆岩一般以页岩、砂岩、灰岩等沉积岩为主，其物理力学性质差异很大。

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