In a sequence of sedimentary rocks, you find a layer of poorly sorted sedimentary rock that includes many scratched cobbles. Below this layer is a body of granite that has a polished upper surface bearing many striations

How would you interpret such a rock sequence?
What will be an ideal response?


Answer: These characteristics are signatures of glaciation. Evidently the granite existed before the glacial episode and was scratched and polished by the glacier. When the glacier's budget shifted to wastage being greater than accumulation, it melted away and left behind a deposit of till, which is poorly sorted and contains scratched cobbles. Later, this till may have been lithified to produce a solid sedimentary rock (tillite).

Environmental & Atmospheric Sciences

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