Wrap Up

Review of the main topics and ideas you encountered in Unit 9.

Nice Bryce: Stories in Sediment

• Weathering makes small pieces and salts.
• After transport, they are deposited as sediment.
• Sediment is slowly changed to sedimentary rock.
• Transformation is NOT magic:
  • Hard-water deposits cement grains together;
  • Squeezing compacts grains;
  • Recrystallization as new minerals grow creates interlocking grains.

Classy classification

• Clasts 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 makes sandstone;
  • Cobbles make cobblestone, boulders make boulderstone, usually call both conglomerate.

Environment is Evident

• Can tell environment in which sediment or sedimentary rock was deposited:
  • 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; clastic rocks are younger than the clasts, and faults younger than faulted rocks.
• Layers initially nearly horizontal (mass wasting flattens steep clastic layers).
• Layers on top are younger than those below (Principle of Superposition).
• After hardening, layers may be stood up or turned over, but many “up” indicators tell us this happened.

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 beach are flipped into stable hollow-side-down position;
• Bubbles rise toward tops of lava flows.

Nothing Succeeds Like Succession

• Can put rocks in order;
• Puts fossils in order--“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