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Science / Geology / Grade 8 / sg8r1

Rebuild a Truncated Rock Sequence

Combine layer order and cutting evidence without inventing dates.

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Start with the stated conditions: these sedimentary layers are upright and have not been overturned. Lower layers were deposited first. An intrusion is younger than the rocks it cuts. An erosion surface is younger than the material it removes. A later deposit can cover that surface. These relations give order, not exact ages or elapsed times.

Worked example

In another upright model, bottom layer A is followed by B. An intrusion C cuts both. If erosion D explicitly removes part of C and a new layer E then covers D, the order is A, B, C, D, E. A line merely ending in a drawing is not enough: use the stated cutting evidence.

Upright layers R, S and T are cut by intrusion J. Erosion surface E cuts off T and J. Layer U covers that surface. The labels match the written evidence; the sketch gives no numerical dates.
Upright model, not to scale. E is a stated erosion surface, not an unexplained gap.

An Invented Outcrop

Sedimentary layers lie from bottom to top as R, S and T. They are upright and were not overturned. Intrusion J cuts all three. Erosion E later removes the top of T and cuts off the top of J. Sedimentary layer U is then deposited on surface E. No further deformation occurs. The sketch is not to scale and provides no numerical ages.

Question 1 Which of R, S and T was deposited first?
Question 2 How does the age of intrusion J compare with T?
Question 3 Which named event is latest in the supplied history?
Question 4 What evidence places J before erosion E?
Question 5 Can the drawn thickness of U establish that it formed exactly 12 million years ago?