Shale

Shale is a variety of mudstone. Mud—defined by geologists as any sediment with grains smaller than about 0.06 millimeters—is carried down rivers and deposited in the sea in thick layers. When this becomes rock, the result is mudstone. If the sediment particles are all clay size, that is, smaller than 0.004 mm, the resulting rock is claystone. Siltstone is more or less pure silt, with grains that are larger than clay and smaller than sand. Shale is at least two-thirds clay.

This shale specimen of lower Paleozoic age, from near Ancram, New York, also contains lenses of fine-grained limy sandstone. Before it was consolidated into rock, it was distorted—perhaps by slumping or other soft-sediment deformation early in the Paleozoic, perhaps during later tectonic movements. Notice, too, how much softer shale is than sandstone. This specimen shows in miniature how shale and sandstone behave in the landscape.

When shale undergoes greater heat and pressure, it becomes the metamorphic rock slate. With still more metamorphism, it becomes phyllite, then schist.

See more sedimentary rocks in the Sedimentary Rocks Gallery.

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