It has a silt-sized grain size which is between this of sandstone and mudstone.
It contains quartz, feldspar, mica and coal and plant fossils.
If mica is present, the rock is flaggy.
It is often a durable and hard rock.
It can have internal layering, ripple marks, dessication cracks and cross bedding, but it does not break easily into layers.
It is most often grey but can be reddish in colour.
It is much less common than sandstones or shales, and it does not form thick deposits as do mudstones.
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