Overview of the main topics you will encounter in Module
Nice Bryce: Stories in Sediment
We saw earlier that weathering changes large rocks into small pieces and salts.
And we saw that after being transported, these small pieces and salts are deposited as sediment.
Sediment, then, is slowly changed to sedimentary rock.
Transformation is NOT magic, but happens because:
Hard water deposits cement grains together;
Squeezing from the weight of more sediment compacts, shoving grains closer together;
Recrystallization, as new minerals grow, creates interlocking grains.
Classy classification
We generally classify sedimentary rocks as either clastic (made of clasts- another name for pieces) or precipitates (made of materials precipitated from dissolved salts, such as rock salt or Death Valley borax).
Limestone might be called both clastic and precipitated because it is often made of shells that are clasts, but shells are made of materials that creatures precipitated from water.
We generally subclassify non-limestone clastic rocks by the size of the clasts:
Clay (tiny) makes claystone, also called shale;
Silt (small) makes siltstone;
Sand (bigger) makes sandstone;
Cobbles (still bigger) make cobblestones, and boulders (even bigger) make boulderstones; both are often called conglomerates.
Environment is Evident
Clues in the rock tell the environment in which the sediment was deposited. For example:
Sand dunes and lizard tracks indicate a desert
Quiet-water muds and fish fossils indicate a lake
Corals and shells indicate a reef in the ocean
Many studies were required to learn the rock types that different environments produce, but now this is well-known.
May I Take Your Order?
Something must exist before it can be moved or cut; a clastic rock is younger (that is, formed more recently) than the clasts it is made of, and a fault is younger than the rocks that it cuts.
Layers on top are younger than those below (called the Principle of Superposition).
After being hardened by hard-water deposits, etc., layers may be tipped up or turned over; however, the rocks contain many "up" indicators that tell us which way was right-side up when the sediment was deposited, so we can learn whether it was turned over.
Getting Into "Up" Indicators
Mud cracks, footprints, and raindrop imprints go down into the mud.
Tops of slightly slanting sand-dune layers are eroded by wind.
Shells on a beach are typically flipped into the stable hollow-side-down position.
Bubbles rise toward the tops of lava flows.
Nothing Succeeds Like Succession
Using these rules, we can put rocks in order from oldest to youngest.
Remarkably, this puts fossils in order, so the more similar in age, the more similar in type—we call this the "Law" of Faunal Succession.