The narrow Puerto Rico Insular Shelf has a very diverse and patchy sediment cover reflecting the wide variation in physical and biological factors affecting the shelf environment. The north shelf is subjected to the highest wave energies, the largest influx of river sediment and has few coral reefs. A significant portion of the north insular shelf has a sediment cover that is probably in textural equilibrium with present processes. On the lower wave energy shelf around the remainder, coral reefs and mangrove forests play an important role in the distribution of sediment. The dominant sediment type on all portions of the Puerto Rico shelf is calcareous skeletal sand. (Coral, molluscs, calcareous algae, and foraminifera predominate.) Relict skeletal components, some of which appear to have been subaerially recrystallized, are common sediment constituents.