Siltstone

Silt is a size term used for material that's smaller than sand (generally 0.1 millimeter) but larger than clay (around 0.004 mm). The silt in this siltstone is unusually pure, containing very little sand or clay. The absence of clay matrix makes siltstone soft and crumbly, even though this specimen is many millions of years old. Siltstone is defined as having twice as much silt as clay.

The field test for siltstone is that you can't see the individual grains, but you can feel them. Many geologists rub their teeth against the stone to detect the fine grit of silt. Siltstone is much less common than sandstone or shale.

Siltstone usually forms offshore, in quieter environments than the places that make sandstone. Yet there are still currents that carry off the finest clay-size particles. This rock is laminated. It's tempting to suppose that the fine lamination represents daily tidal surges. If so, this stone might represent about a year of accumulation.

Like sandstone, siltstone changes under heat and pressure into the metamorphic rocks gneiss or schist.

For more photos see the Sedimentary Rocks Gallery.

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