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题目材料：
The oldest layer of the Grand Canyon's Palaeozoic rocks, the Tapeats Sandstone, formed on an ancient seashore, and this layer is immediately below the Bright Angel Shale, a hardened marine mudstone containing invertebrate fossils known as trilobites, animals related to modern crayfish. Since trilobites changed fairly rapidly with time, changes in their shape and form can reveal the relative age of the rocks they are found in. Examination of the trilobites found in the Bright Angel Shale showed that the rock in the west of the canyon is older than the same layer to the east. Thus the sea that deposited the Bright Angel Shale encroached from west to east in a marine transgression, laying down sand along beaches and mud in deeper water.
以上解析由 考满分老师提供。
