The Beach, A River of Sand
River of Sand Video
Berm: the nearly horizontal
portion of the beach on the
backshore formed by the
deposition of sand
Where does sand come from?
From where do beaches come?
• Sand, along with gravel, silt and clay are
collectively known as sediment, and are
produced by the mechanical and chemical
breakdown of rocks.
• Once disaggregated from the original source
rock, this material is then eroded and
transported by either wind, water, or ice, often
ending up at the deposits of rivers or lakes, as
sand dunes, or ultimately as sediment in the
sea.
What about when there are no
mountains?
• In areas where there is no good source of
sedimentary material from mountains or
volcanoes, sand is often entirely composed of
organic material i.e. shell fragments, coral,
and the tests (skeletons) of small planktonic
organisms. The sand is said to be “biogenic.”
Sand deposition
If the sediments
are…
very fine sand
sand
cobbles
boulders
then they were
deposited by
wind
small waves
big waves
glaciers
La Jolla, California
Summer Beach
Gentle waves
pile sand on
the beach
La Jolla, California
Winter Beach
Strong
waves carry
sand off the
beach
depositing it
temporarily
on off-shore
sand bars
• The shape of the beach is determined by how
it formed…
– Why is it better to surf in the winter?
it is all about sand movement
Sand created features
Longshore Drift
consists of the transportation of sediment along a coast at an angle to
the shoreline. It is dependent on
1. direction of the prevailing wind
2. swash (turbulent water that washes up on the beach)
3. backwash (offshore flow of water)
Natural Sand Deposition Features
What’s happening to Block Island?
a local tombolo
a local spit
Local barrier beaches on south
shore
Some features are
directly created by
man
Rip Currents
• Bands of fast moving water
moving off shore
• Look for a channel of
choppy water with a
noticeable color difference
DRAMATIC SEDIMENT
FEATURES
Sea Arch
Sea Stack
Blow Hole
blow hole Hawaii
Common constituents of sand:
• Minerals:
– Quartz : clear (doesn’t break down easily)
– Feldspar : pinkish-tan
– Mica: black and flaky
– Olivine : olive
– Hornblende : black/grey, dull
– Garnet: reddish
• Biogenic:
– Coral
– Shells
– Foraminifera
– Coralline algae
• Rock:
– Volcanic basalt (black islandic rock)
– Granite
How oceanographers
analyze sand…
•
•
•
•
•
Color  composition
Rounding  distance traveled or age
Size  strength of waves, wind, glacier
Pits  directly from volcano
Sorting  distance traveled, # of
sources of material
… so scientists also know about the
beach’s location by…
•
•
•
•
•
it’s steepness (from size)
what the parent material is (color)
distance the sand traveled to get there
the power of the beach’s waves (shape)
biogenic material…continental or island,
tropical, temperate or polar
• The influence of man (?)
Wentworth sediment size scale, and
resultant beach shape, for example
Boulder
Cobble
Pebble
Granule
Very coarse sand
Coarse sand
Medium sand
Fine sand
Silt
Clay
Sediment size (mm)
>265
65-265
4-64
2-4
1-2
0.5-1
0.25-0.5
0.07-0.25
0.25-1/256
<1/256
Average beach slope
irregular
19°-25°
13°-19°
11°
9°
7°
5°
5°
<5°
<5°
What can you tell?
Beaches are important habitats
Headlands – Point Reyes, California
Look!
Elephant Seals
“Haul Out” to molt
beachmaster battle
FUN with sand!!
THE END