Checkerboard Mesa, Zion NP, UT. Checkerboard Mesa gets its name from the intersection of sub-vertical fractures/joints with more gently tilted layers. Knowing that these are lithified sand dunes. What sedimentary structure forms these tilted layers? a. cross-beds b. ripple marks c. mudcracks d. graded beds

a. cross-beds

Explanation:

Lithified sand dunes are formed when marine, or in this case, aeolian sand dunes become compacted or hardened. Deposition of minerals on these hardened surfaces creates layers on inclined surfaces. These layers get stacked over time, which creates interesting patterns and, as this stacking occurs on tilted surfaces, they form cross patterns. This entire process is called cross-bedding.

Ripple marks are almost uniform patterns usually forming in one direction and plane, which cannot form cross-patterns.

Mud cracks form when a muddy sediment dries up, leaving almost hexagonal cracks behind, a very different type of pattern.

Graded beds are the systematic change in grain size, usually arranging with coarses sediments at the base, and progressively finer grains at the surface. This also cannot form cross patterns.