During the Devonian transgression, coastal deposits formed as which of the following?

• A. Thick deep-water limestone sequences
• B. Volcanic ash beds
• C. Eolian dunes
• D. Glacial till

Answer: (A)

The main idea is how a rise in sea level changes where sediments are deposited and what those sediments look like in a carbonate-dominated Devonian sea. When sea level rises during a transgression, the coastline retreats landward and deeper water covers previously exposed areas. In warm Devonian seas, carbonate producers—shells and skeletons of coral, coralline algae, crinoids, and other organisms—continue to build up limestone on the subsiding shelf. Over time this creates thick sequences of limestone that record continued carbonate deposition as the environment shifts from nearshore toward deeper water. The other options don’t fit the pattern of a transgression in a carbonate-rich Devonian setting: volcanic ash beds come from eruptions, not sea-level change; eolian dunes form in desert, wind-blown settings; and glacial till forms from ice, not marine transgression.