A rock containing large, rounded cobbles and boulders:

Question

BlackTom AI · Accepted Answer

Rounded, coarse clasts indicate a conglomerate: clastic sedimentary rocks are classified by grain size and by the shape (roundness) of their clasts.

1) Relevant knowledge points and concepts
- Clastic sedimentary rocks are named by grain size and clast roundness. The Wentworth scale: sand = 0.0625–2 mm, granules = 2–4 mm, pebbles = 4–64 mm, cobbles = 64–256 mm, boulders >256 mm.  
- “Conglomerate” = a coarse-grained sedimentary rock with rounded gravel- to boulder-sized clasts cemented in a finer matrix.  
- “Breccia” = similar size range but with angular, not rounded, clasts.  
- High-energy transport (rivers, beaches, alluvial fans) rounds clasts by abrasion; limited transport or mass-wasting preserves angular clasts.

2) Step-by-step reasoning
- The rock contains cobbles and boulders (64 mm and larger), so the clast sizes are well above sand size. That places the rock in the gravel-to-boulder class of coarse clastic rocks.  
- Next classify by clast shape: the clasts are described as large and rounded. Rounded clasts are diagnostic of abrasion during transport.  
- Combining size (cobble/boulder) with rounded shape matches the definition of a conglomerate.

3) Intermediate “calculations” / size matching
- Cobble: 64–256 mm; boulder: >256 mm → Both are >> 2 mm (sand upper limit).  
- Rounded + gravel/boulder size → Conglomerate (not sandstone, which is 0.0625–2 mm).

4) Why the correct answer is correct
- Conglomerate explicitly names a rock made of rounded, coarse clasts cemented together. The rounded nature indicates transport and abrasion consistent with conglomerate formation.

5) Why other options are incorrect
- Breccia: clasts are angular, not rounded, so it doesn’t fit.  
- Boulderstone: not a standard sedimentary rock name in geological classification; it’s not the formal term.  
- Sandstone: grain sizes are much smaller (0.0625–2 mm), so cannot contain cobbles and boulders.

6) Conclusion
- Therefore the correct identification is Conglomerate, which matches the provided answer.

A rock containing large, rounded cobbles and boulders:单项选择题

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