How to identify sedimentary rocks by grain size and layering

Hold the rock at reading distance in daylight. If separate grains show, grain size names the rock: larger than a pea is gravel (conglomerate, breccia), down to the finest grit a fingernail can feel is sand (sandstone), and finer is silt or mud. If the surface is smooth and even with no grains, the rock is a mudstone, a limestone or chalk, or a chert, and the next clues are splitting, fossils and luster.

Rub a broken face with a fingertip: sandstone feels like sandpaper and sheds a grain, siltstone is faintly gritty, mudstone and shale feel like dried clay, chert feels waxy.

Both are cemented gravel. Conglomerate clasts are rounded and usually of several rock types; breccia clasts are angular and often one rock type, because they broke in place and never traveled. The cement may be white calcite, rusty iron or gray silica, and deserves its own close shot.

Shoot straight down on a face with at least ten clasts beside a coin so roundness can be judged. Concrete is the lookalike: rounded aggregate, but in a uniform gray paste with air pockets and usually a flat formed face.

Sandstone is sand grains, usually quartz, glued together. A fresh face is sugary, each grain catches a tiny highlight, and a fingernail can dislodge one. Cement sets the color: red and brown mean iron oxide, white means calcite, gray and glassy means silica. Bedding shows as parallel lines of changing grain size or color; cross-bedding shows those lines meeting at low angles.

Photograph the broken face at a slant so the grains sparkle, then the edge so bedding shows. If the rock breaks through the grains with a glassy surface and sheds nothing, it is quartzite and belongs on the metamorphic page.

These are the fine end and the hardest for a camera. Shale is fissile: it splits along bedding into thin flat sheets, and a weathered piece looks like a stack of stiff paper. Mudstone has the same grain size but breaks into blocks. Siltstone is slightly gritty with faint laminations. All three run gray, black, red-brown or olive.

The photo that works is an edge view in side light, where fissility shows as fine parallel lines like a closed book; a top-down shot of a smooth gray surface identifies nothing. Slate is the lookalike: harder, with a dull sheen, and it rings when tapped.

Limestone is usually gray, sometimes tan or near white, dull and fine, and often holds visible fossils: crinoid stems like stacked discs, brachiopods, coral, shell hash. It weathers to a pitted surface with solution grooves. A gray fine rock with a fossil is a strong limestone case; without one it could equally be mudstone, dolomite or hornfels. Limestone fizzes in acid, but a photo cannot show that; fossils, pitting and the weathered rind are the evidence.

Coquina is limestone made of loosely cemented shell fragments, cream to tan, full of voids like granola. Chalk is very fine, soft, white and marks your fingers. Shoot both with a coin in side light so fragments throw shadows.

Chert is silica, not lime, and sits in limestone as gray, tan or black nodules and bands. It is hard, breaks with a waxy conchoidal fracture and sharp edges, and often shows a white weathered rind over a darker core. Shoot a broken nodule so rind and core are both in frame.

Coal is black, light and blocky. Bituminous coal shows alternating dull and bright bands; anthracite is glossy with conchoidal fracture and passes for obsidian until you feel the weight.

Evaporites, mainly gypsum and rock salt, are rare as loose finds: gypsum is white, pink or clear with satin faces, rock salt is clear to orange with cubic cleavage, and both are soft enough for a fingernail to mark.

Photographable clues are bedding planes, clast roundness, cement color, fossils, fissility on an edge, the weathering rind against a fresh break, and shell voids. A photo cannot show fizz, hardness, weight, grit or smell.

Three shots cover nearly every sedimentary rock: a face, an edge and a close-up.

Site reference photos of the material, not finds from any particular place.

Two questions that arrive with nearly every sedimentary upload.