Which one of the following is not a sedimentary structure? a. Outwash; b. Ripple mark; c. Mud crack; d. Cross-bed; e. Lamination.
The Correct Answer is Explanation is :
Correct Answer: a. Outwash
Explanation:
Sedimentary structures are physical and geometric features formed at the time of sediment deposition or shortly after. These structures provide important clues about the depositional environment and processes. Let’s examine each option to determine which one does not qualify as a sedimentary structure.
a. Outwash – Not a sedimentary structure
Outwash refers to sediments deposited by meltwater from glaciers. It typically consists of well-sorted sand and gravel deposited in outwash plains. While it describes a depositional environment and sediment type, it is not a sedimentary structure. Instead, it is classified as a glacial depositional landform, not a structure within sedimentary rocks.
b. Ripple mark – Sedimentary structure
Ripple marks are small-scale ridges formed on the surface of sediment layers due to the movement of water or wind. They are common in shallow marine, fluvial, and aeolian environments. Ripple marks can be symmetrical (from wave action) or asymmetrical (from current flow) and provide valuable information about paleocurrents.
c. Mud crack – Sedimentary structure
Mud cracks (also called desiccation cracks) form when muddy sediment dries and contracts, creating polygonal cracks. They are indicators of subaerial exposure and drying conditions, commonly found in floodplains, tidal flats, or lake beds.
d. Cross-bed – Sedimentary structure
Cross-bedding consists of inclined layers deposited by wind or water currents. It forms in environments such as rivers, deserts, and beaches. Cross-beds indicate the direction of current flow and are significant in interpreting past sedimentary environments.
e. Lamination – Sedimentary structure
Lamination refers to very thin layers (less than 1 cm thick) within sedimentary rocks. They result from subtle changes in sediment supply, often in calm water environments like lakes or deep marine settings.
Conclusion:
While ripple marks, mud cracks, cross-beds, and laminations are true sedimentary structures, outwash is a depositional feature associated with glacial activity, not a structure within sedimentary rock layers. Therefore, the correct answer is a. Outwash.