COARSE vs FINE-GRAINED SANDSTONE : What It Tells a Geologist The size of sand grains in sandstone reveals a lot about how and where it formed. 1. Coarse-Grained Sandstone: Formed in high-energy environments like river channels, beaches, or alluvial fans. The grains are larger because water or wind was strong enough to carry and drop them quickly. It usually means the rock is closer to the source, poorly sorted, and often a better reservoir for fluids like oil or groundwater. 2. Fine-Grained Sand