![]() This boundary, instead of cutting across layers, indicates a break in time in an otherwise continuous rock record. Wikipedia's Fault page does a good job showing and explaining various fault types.Īt the right the most conspicuous boundary separates the lower rock from newer rock above. In the picture, strata on the left appear to have been ground downward as the block of rock on the right pushed beneath the block on the left. Faults are fracture zones between two blocks of rock, typically with one side being repositioned higher or lower than the other, or to the right or left, as during an earthquake. However, on the left side, the strata are curved. On the fracture's right side, layers, or strata, dip downward toward the left. Notice that a fracture zone cuts across the picture from top-right, to bottom-center. That's siltstone which has undergone some violent treatment. FEATURES TO LOOK FORĮspecially because sedimentary rocks often are layered, frequently - especially in roadcuts - you see features disrupting the layering, as shown at the right. The entire process of forming rock from loose matter is referred to as lithification. The sedimentary-rock-forming process typically is accompanied both by cementation and compaction of the sediment - squeezing the particles together. In geology, cementation occurs when mineral matter is precipitated in the spaces between lose particles, "gluing" them together. At the left you see rock salt precipitated along a salt pond's edge, where seawater is allowed to evaporate so the salt can be sold commercially.Ībove we've spoken of particles being cemented together. Gypsum may be the best known chemical sedimentary rock, but also there's rock salt (halite), certain kinds of limestone, baryte and sylvite. With magnification you'd see that even the stone between the obvious shells consists of more shells of tiny, even microscopic organisms who lived in the water along with larger mollusks.Ĭhemical sedimentary rock forms when minerals in solution become so concentrated (supersaturated) that they precipitate (form solid matter). The accompanying photo shows seashells compacted into soft limestone. Biological rocks include commonly encountered limestone, chert and coal. With such doubts, our picture shows good old ambiguous "mudrock."īiological sedimentary rock forms when dead organisms - notably organisms with shells in the oceans - pile up and become cemented together. What's shown in the picture is like shale with a lot of carbonate in it, though one thinks of shale as being more layered. Still, "mudrock" is a handy concept because all the mentioned mudrock types grade into one another, plus there's disagreement on how to define the various mudrock kinds. However, the term "mudrock" is a general one, embracing shale, slate, siltstone, claystone, mudstone and more. Mudrock accounts for over half of all known sedimentary rocks. The mudrock shown at the left is clastic, mostly consisting of silt- and clay-size fragments. Three types of sedimentary rock are recognized, and the most abundantly occurring type, despite its unfamiliar name, is "clastic" in origin.Ĭlastic rock is composed of pre-existing rock fragments. "CLASTIC," BIOLOGICAL & CHEMICAL SEDIMENT Sedimentary rocks in northern Alabama, photo by Bruce J. Morganīecause sedimentation often takes occurs erratically - as during floods, dust storms, changes in ocean current - the resulting accumulation often is layered, as shown above.
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