GEOSC 10
Geology of the National Parks

Main Topics, Unit 8

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Overview of the main topics you will encounter in Unit 8.

Cape Cod girls don't have any combs
Look away, look away
They comb their hair with codfish bones
We're bound for Australia.
Cape Cod boys don't have any sleds
Look away, look away,
They slide down dunes on codfish heads
We're bound for Australia.
Cape Cod brides don't have any veils
Look away, look away
They shield their eyes with codfish tails
We're bound for Australia.
—Traditional sea shanty

Getting the Most from the Coast: Cape Cod, Acadia, and Friends

  • There are many "types" of coasts (beaches, reefs, mud flats, cliffs, deltas, etc.; we’ll especially focus on beaches)
  • Waves move LOTS of sand, mostly in, out, in, out…, which sorts by size to give sandy beaches
  • Waves move a little extra sand out during winter storms (breaking waves come in through air without sand, go out along surface with sand)
  • And waves move a little extra sand in during summers (a wave surges up beach a bit faster than the water flows back out, and thus moves a bit more sand in than out)

The Coast Is Friendly—the Ocean Waves

  • Waves go slower in shallower water
  • The first part of a wave slows as it nears the coast and waits for the rest of the wave to catch up, so the wave "piles up" to break, and turns to come almost straight in
  • But "almost" is not "completely" straight in, and this slight angle drives longshore drift of sand and water
  • Eventually, some sand is lost to deep water below the reach of waves
  • A beach thus needs sediment supply to balance this loss

The Coast Is Friendly—Buoy Meets Gull

  • New beach sand is supplied by longshore drift from river-fed deltas
  • Or by erosion of the coast behind the beach
  • We can lose the longshore drift from river-fed deltas, and thus promote erosion of land behind the beach with houses, roads, and such, by
    • Dams on rivers (Elwha River dams in Olympic caused beach loss)
    • “Dams” along coast (jetties or groins that stick out, block drift to trap sand, and let clean water pass to erode beyond)
    • Past sea-level rise, which flooded river valleys so river sediment fills bays and doesn’t reach outer beaches (e.g., Chesapeake)
    • Past deposition (especially by glaciers) forming coastal land (e.g., Cape Cod) where there are no big rivers to supply sand

But many coastal residents are crabby

  • Most U.S. coasts are retreating (about 75%)
  • From past sea-level rise and glaciation, dams and coastal modifications (see previous point, "The Coast Is Friendly—Bouy Meets Gull")
  • From sea-level rise now (from warming expanding water and melting ice, plus groundwater mining, with worries about future of huge ice sheets)
  • From pumping of groundwater, oil and gas allowing compaction and sinking
  • Many regions pushed down by ice-age ice are still rising, while those bulged up before the ice-age ice are sinking; mountain-building also can matter