What type of sandstone is characterized by an impure or dirty composition?

• A. Quartz Arenite
• B. Arkose
• C. Graywacke
• D. Lithic Sandstone

Answer: (C)

Sandstones are distinguished by how much matrix material they contain and how mature the grains are. A sandstone with a substantial clay-rich matrix mixed with abundant fine material and rock fragments is graywacke. That clay-rich, fine-grained matrix gives it a dirty, impure look and often a darker color, reflecting rapid deposition with little time for grains to mature. In contrast, quartz arenite is a clean, quartz-dominated, well-sorted sandstone with very little matrix; arkose is immature but rich in feldspar, not defined by a heavy clay matrix; lithic sandstone contains many rock fragments but not typically the distinct clay-rich matrix that characterizes graywacke. So the impure or dirty composition points to graywacke.