1. Look for Clues in Texture:
* Grain Size:
* Clastic Rocks (made of fragments):
* Conglomerate: Large, rounded pebbles, often in a matrix of finer material.
* Breccia: Large, angular fragments, indicating minimal transport.
* Sandstone: Grains the size of sand. Look for different types:
* Quartz sandstone (shiny grains)
* Arkose sandstone (pinkish, with feldspar)
* Greywacke sandstone (dark, with rock fragments)
* Siltstone: Grains too small to see individually, feels like fine sandpaper.
* Mudstone/Shale: Extremely fine grains, often fissile (splits into thin layers).
* Fossils: The presence of fossils is a strong indicator of sedimentary origin.
* Layering (Stratification): Sedimentary rocks typically form in layers (strata).
* Graded Bedding: Larger grains at the bottom, finer grains at the top, indicating a decreasing current.
* Cross Bedding: Layers at an angle to the main bedding, indicating movement of sand dunes or currents.
* Color:
* Red: Often indicates iron oxide (hematite)
* Gray: Often indicates clay minerals
* Black: Can indicate organic matter or carbon
2. Consider the Environment of Formation:
* Continental:
* Fluvial (river): Conglomerate, sandstone, siltstone, mudstone.
* Glacial: Till (unsorted mix of sizes), glacial outwash (sorted sands and gravels).
* Desert: Sandstone, evaporites (salt, gypsum).
* Marine:
* Shallow: Limestone, sandstone, conglomerate.
* Deep: Shale, mudstone, chalk.
3. Feel the Rock:
* Hardness: Generally, sedimentary rocks are softer than igneous or metamorphic rocks.
* Texture: Some feel rough (sandstone), others smooth (mudstone).
4. Use a Hand Lens:
* Magnify the grain size and look for individual minerals.
* Observe the shape and texture of grains.
5. Additional Tests:
* Acid Test: Some carbonates (limestones) fizz when a weak acid (like vinegar) is applied.
* Streak Test: Rub the rock on a streak plate (unglazed porcelain) to see its color in powder form.
Important Note: It's always helpful to have a field guide or online resource for sedimentary rocks to help with identification. The best way to learn is by observing, comparing, and practicing!
