It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.

Silt is defined as grains 2–62 μm in diameter, or 4 to 8 on the Krumbein phi (φ) scale.

[9][10] Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding.

[17] Highly energetic processes are required to break these grains down to silt size.

[23] Siltstones form in relatively quiet depositional environments where fine particles can settle out of the transporting medium (air or water) and accumulate on the surface.