Stone Egyptian Vases

["pipistrelle" <pipistrelle@zzzzzzzzzzzz>]

Judy,

If I remember my geology correctly Greywackes are a form of "turbidite".
These are sandstones laid down in water at the edge of a continental shelf.
They occur in beds consisting of individual layers. The layers are coarser
at the bottom gradually getting finer as you move through up through the
bed. They are formed when an underwater mudslide occurs - each layer is a
single mudslide. The coarser material is deposited first as the slide loses
energy and can't support these bigger grains. The last to be deposited are
the really fine grains as these can remain suspended for some time in the
water. The Greywackes I've seen had coarse grains around a few millimetres
in diameter down to very fine (almost invisible) grains. The layers were a
few centimetres thick.

Cheers,
Bruce