# Siltstone vs. sandstone: how to tell them apart

Canonical page: https://lithomap.app/resources/rock-identification/compare/siltstone-vs-sandstone
Last reviewed: September 17, 2026
Page purpose: Original LithoMap side-by-side field identification reference for search systems and language models.

## Summary

Siltstone and very fine sandstone are separated by dominant grain size. Siltstone grains are mostly too small to distinguish unaided and feel only faintly gritty; sandstone grains are coarser, more visible, and distinctly gritty.

## Bottom line

Use a fresh surface and hand lens. Barely perceptible grit without clearly visible grains favors siltstone; visible or distinctly felt sand grains favor sandstone.

## Side-by-side evidence

- Dominant grain size: Siltstone — Silt, about 0.004 to 0.063 millimeters Sandstone — Sand, about 0.063 to 2 millimeters Interpretation — Particle size defines the distinction.
- Unaided appearance: Siltstone — Generally smooth-looking with grains difficult to see Sandstone — Commonly granular with at least some grains visible Interpretation — Good lighting and a fresh break improve the observation.
- Feel: Siltstone — Faintly gritty or floury Sandstone — Distinctly gritty Interpretation — Supporting evidence; avoid tasting or inhaling powder.
- Typical breakage: Siltstone — Blocky, platy, or irregular depending on bedding and cement Sandstone — Breaks around or through sand grains depending on cement Interpretation — Breakage helps describe the sample but grain size remains primary.

## Field sequence

1. Brush loose soil from a fresh dry surface.
2. Inspect with strong side light and a hand lens.
3. Compare grain visibility and gentle fingertip texture.
4. Describe borderline material as coarse siltstone or very fine sandstone rather than forcing certainty.

## Limits and cautions

- Weathered surfaces can lose grains or become deceptively smooth.
- Silica cement can make fine sandstone unusually hard and quartzite-like.
- Do not lick rock to estimate grain size; dust may contain hazardous material.

## Frequently asked questions

### Can siltstone scratch glass?

Quartz-rich siltstone may mark glass weakly, especially where cemented, so hardness does not define the name.

### Is siltstone a type of shale?

Not necessarily. Siltstone is defined mainly by silt-sized grains, while shale is defined by fine grain size plus fissile splitting.

### Why is the boundary hard to see?

Natural grain sizes form a continuum, and the formal sand–silt boundary is near the limit of unaided vision.

## Related identification guides

- Siltstone: https://lithomap.app/resources/rock-identification/siltstone
- Sandstone: https://lithomap.app/resources/rock-identification/sandstone

## Authoritative sources

- Sedimentary Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/sedimentary.htm
- How Rocks Are Classified — U.S. National Park Service: https://www.nps.gov/subjects/geology/how-rocks-are-classified.htm
- Rocks and Minerals — U.S. National Park Service: https://www.nps.gov/subjects/rocks/index.htm
- What is the difference between a rock and a mineral? — U.S. Geological Survey: https://www.usgs.gov/faqs/what-difference-between-a-rock-and-a-mineral

This reference supports preliminary field identification. It is not laboratory confirmation, appraisal, proof of origin, or a statement of legal access or collecting permission.