In the regressive sequence described, which unit would be the youngest?

• A. Limestone
• B. Shale
• C. March deposits
• D. Sandstone

Answer: (B)

When a sequence forms during a regression, sea level drops and the shoreline shifts toward the sea, causing sedimentation to advance in time toward environments that are progressively closer to land and often quieter and less energetic. The uppermost layer of such a stack records the most recent conditions as this regression continues. Shale represents fine-grained, low-energy deposition that can accumulate when the water is quiet and currents are weak, often forming late in a regressive sequence. This makes it the youngest unit in the described stack. By contrast, limestone tends to form in relatively shallow, clearer-water settings, sandstone forms in higher-energy shoreface to delta environments, and marsh deposits indicate a coastal wetland setting that can appear earlier or later depending on local conditions, but in this scenario the shale sits at the top as the most recently deposited material.