Excerpts from Jim Conrad's
Naturalist Newsletter

siltstone

from the August 23, 2009 Newsletter, issued from the Siskiyou Mountains west of Grants Pass, Oregon:
SILTSTONE
Marble is metamorphosed limestone, as quartzite is metamorphosed sandstone. If you metamorphose shale, you get slate. Sedimentary rocks like limestone, sandstone and shale metamorphose when they're under lots of pressure for millions of years, or when they're heated, as when molten magma from deep within the Earth comes into contact with them.

Many rocks don't fall neatly into the sedimentary and metamorphic pigeonholes. Maybe they weren't under enough intense pressure for long enough, or got hot, but not hot enough.

Similarly, shale is rock composed of particles of a particular very small size -- the size of dust or the particles of smooth mud. Sandstone is the same thing, except that its particles are much larger -- the size of sand grains. Once again, many rocks of the shale/sandstone kind are composed of particles too large for the rock to be considered "classic shale," yet the particles are too small for the rock to be "classic sandstone." You can speak of "shaley sandstone" or "sandy shale."

On a ridge crest near here you can see what's shown above.

That rock is too hard and brittle to be either classic shale or sandstone, but too soft to be metamorphic quartzite or slate, plus its particle size is in-between that of shale and sandstone, and there might be a good bit of carbonate in it, too, giving it a touch of limestone/marble.

What do you call rock like that?

Until someone corrects me I'm calling it siltstone. It looks layered so I'm tempted to call it limy, shaley slate, but notice how it fractures in at least two directions, so the layering may be more a feature of partial metamorphosis than because originally it was deposited in layers. Also, it looks like much of the siltstone shown in Googled-up pictures. Does anyone have a different opinion?

The smooth, flat rocks below the outcrop are about the size of a hand.