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Sandstone vs Siltstone: Grain Size and Texture

Sandstone and siltstone are clastic sedimentary rocks separated primarily by the dominant detrital grain-size fraction. Sandstone is dominated by sand-sized particles, approximately 0.0625 to 2 millimetres, whereas siltstone is dominated by silt-sized particles, approximately 0.004 to 0.0625 millimetres in a commonly used Wentworth-style scale. Cement, matrix, weathering, polish, and mixed grain populations can hide that boundary, so evaluate a representative surface and report transitional sandy siltstone or silty sandstone when appropriate. Use a hand lens on an existing clean surface. Repeated individually resolvable sand grains and a distinctly granular fabric support sandstone. A much finer, comparatively smooth surface whose dominant grains remain unresolved or only faintly gritty supports siltstone. Bedding, colour, and hardness do not replace a grain-size estimate. Photograph the bed or specimen with scale and record stratigraphic position, grading, laminations, contacts, sedimentary structures, and weathering. Inspect an existing clean natural or broken surface with a hand lens; do not create dust or a fresh break solely for classification. Look across several areas for repeated individual detrital grains. A matrix or cement should not be mistaken for the dominant grain population. Estimate whether sand-sized or silt-sized material dominates, and record obvious mixtures as silty sandstone, sandy siltstone, or uncertain fine clastic rock. Treat colour, hardness, bedding thickness, and one weathered face as supporting observations rather than grain-size substitutes. For mapping, reservoir, engineering, provenance, or research decisions, use a qualified geologist and petrography, point counting, or particle-size methods. The sandstone–siltstone boundary is compositional and gradational. A mixed rock can change name across one bed or under a different classification convention. Cement, matrix, silicification, metamorphism, polish, and weathering can mask grains or make a fine rock feel deceptively rough. A tactile grit impression is subjective and can expose skin or lungs to hazardous dust. Do not lick, bite, grind, dry-brush, or rub unknown material. Colour alone cannot distinguish these rocks, and a photograph without scale or a representative clean surface may not resolve the dominant grain size.