Lamination
(c) 2005 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Lamination is a kind of sedimentary rock structure in which thin layers, or laminae, are recognizable. A lamina is generally less than a centimeter in thickness; in this piece of laminated siltstone, 5 centimeters on a side, the laminae are less than a millimeter thick.
Lamination may be due to differences in mineral composition, or grain size, or merely color. In this specimen the mineral grains appear to have been sorted in gentle currents. In the lighter laminae, carbonate minerals cohere and form a harder stone than in the darker laminae, thus they stand out after surface weathering.
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