Geology of the National Parks

Main Topics, Unit 9


Overview of the main topics you will encounter in Unit 9.

Nice Bryce: Stories in Sediment

  • Weathering changes large rocks into small pieces and salts
  • After being transported, these small pieces and salts are deposited as sediment
  • Sediment is slowly changed to sedimentary rock
  • Transformation is NOT magic, but happens because:
    • Hard-water deposits cement grains together
    • Squeezing compacts grains
    • Recrystallization as new minerals grow creates interlocking grains

Classy classification

  • Clasts (another name for pieces) make clastic rocks; dissolved salts make precipitates (rock salt, Death Valley borax)
  • Limestone is both--precipitated as shells, which are clasts
  • Subclassify clastic rocks by size:
    • Clay (tiny) makes claystone, also called shale
    • Silt (small) makes siltstone, sand (bigger) makes sandstone
    • Cobbles (still bigger) make cobblestone, boulders (even bigger) make boulderstone, both often called conglomerate

Environment is Evident

  • Clues in the rock tell the environment in which the sediment was deposited. For example:
    • Sand dunes, lizard tracks? Desert
    • Quiet-water muds, fish fossils? Lake
    • Corals and shells? Coastal Reef
  • Takes lots of study to know the rocks that different environments produce, but now 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
  • Sediment layers initially are nearly horizontal (mass wasting flattens steep clastic layers)
  • Layers on top are younger than those below (Principle of Superposition)
  • After being hardened by hard-water deposits, etc., layers may be stood 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, raindrop imprints go down into 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
  • Gives geologic time scale:
    • Cenozoic=New Life, Age of Mammals
    • Mesozoic=Middle Life, Age of Dinosaurs
    • Paleozoic=Old Life, Age of Shellfish
    • Precambrian=really old, Age of Algae