How did Block
Island form?
How is it
changing now?
What will it look
like in the future?
Block Island:
a product of Erosion and Deposition
• weathering: the
change/breakdown of
rock
• erosion: movement of
weathered materials
• agents of erosion:
gravity, wind, glaciers,
and water (ocean
currents and waves,
streams, groundwater)
• deposition: the leaving
behind of materials carried
by erosion
• agents of deposition:
gravity, wind, glaciers and
water (same as for erosion)
Background
Information
Wind Erosion
• Wind: erodes land by
carrying away rock
particles
• mostly smaller
particles: sand and
rock dust (silt and
clay) are carried by
wind
Wind Deposition
• deposition by wind:
most common –
dunes (mounds of
wind-blown sand)
• gentler slope of a
dune – faces wind
Slumping: erosion from groundwater
and gravity
Large block falls off a cliff or bluff
forms- sea cliffs/bluffs
Erosion by ocean waves
• physical weathering:
force of the water
weathers and erodes
the shoreline affecting
beaches
• Big waves erode,
small waves deposit
(ex: Narragansett)
Ocean Wave Deposition
• beaches: deposits of sand or larger particles on ocean
(or lake) front
– beach changes seasonally
• berm: mini-bluff on a beach
• sand bar: long underwater ridge of sand carried away
from beach
Wave anatomy
• Wavelength:
distance between 2
crests or 2 troughs
• Wave height (2 X
amplitude): height
from trough to crest
• How could you
determine these??
Wave characteristics
• Wave period: the
time is takes for a
wave to pass a fixed
point
• Wave frequency: the
number of waves that
pass by in a given
time.
• How could you
determine these??
Wave characteristics 2
• Wave speed: the
distance traveled
by a given point
on the wave (such
as a crest) in a
given interval of
time. In equation
form:
Longshore Currents
• general movement
of sand along
beach: in same
direction that waves
hit shore
• longshore current:
movement of water
and sediment
parallel to, and near
shoreline
•
•
animation:
http://www.learnnc.org/lp/multimedia/14706
Real video:
http://www.youtube.com/watch?v=l70ioFTD6TY&saf
e=active&safety_mode=true
Features from longshore
current deposition:
• Headland: land
surrounded by water
on three sides
• spit: long narrow
deposit of sand
connected at one end
of shore
• tombolos: ridge of
sand deposits
connecting islands to
mainland
Glacial Erosion
• Glaciers are “rivers of
ice” slowly flowing,
moving sediments
and changing the
surface and shape of
the ground beneath it
• Last glacial ice age in
NE ended about
18,000 years ago
Glacial Deposits
• glacial till: 
unsorted glacial
materials
• stratified drift: 
sorted and deposited
in layers by meltwater
Glacial erratic
Features from glacial deposition:
• Glacial moraines:
landforms made
from glacial till
• terminal (end)
moraine: deposited at
the end
More features from glacial deposition:
• moraine
erratics: large boulders
transported by glaciers
•
More Glacial Deposition features
• drumlins: low tearshaped mounds of
till
• kettles:
depressions left
from melting ice
Stratigraphy
• Stratigraphy is the
study of strata, or
layers in the ground
• Stratigraphy can tell
us about how the
soil was deposited,
they are clues to the
land’s origin
Your tasks in
the field
(gathering evidence for your big
3 questions)
Task 1: create a topographic map
of Block Island
Another look
Converting an island into a
“topo” map
Hawaii topo
Creating a Contour/Elevation Map
• Use your elevation
observations to
determine the
higher/lower points on
BI
• Draw contour lines
onto the map
Try a sample map
Task 2: Sieving soil, to tell
particle size
• You will collect soil
samples to sift with
“sieves”
• These will separate the
soil into different sized
particles – gravel, fine
gravel, coarse sand, fine
sand and silt and clay
• Sieving Purpose: this
tells how the soil got
there
Soil sizes
Soil horizons, or strata
Sand, different sizes deposit by
different ways
• How is sand classified? If you
classify sand by size, you look
at the diameter of each sand
particle. Very coarse sand, like
you might see in a sand box,
has the largest particles. The
diagram below shows the
actual size of sand particles
from 0.5 to 2 mm in diameter.
• very fine 0.05 - 0.01mm
• fine 0.1 - 0.25mm
• medium 0.25 - 0.5 mm
• coarse 0.5 - 1 mm
• very coarse 1 - 2 mm
Conclusions from sieving and
soil profiling:
• The bigger the sand particle size, the
steeper the beach typically is:
– Steep, big particle beaches are formed from
strong deposition forces and erosion
• This makes them less stable
– Flat, small particle beaches are formed from
gentle deposition forces, and are more stable
Task 3: create a beach profile
Beach profiling provides clues as to:
• how the beach formed
• how is it changing now
General beach anatomy
How does the beach change over
time?
• The shape of the beach is
in continual change due
to the weather and tides.
• In calmer weather with
regular tides, sand is
gradually deposited onto
the beach from sand bars
out in the ocean, dunes
and berms may form.
• In stormy weather, the
beach erodes and sand is
brought from the beach to
sand bars, which run
parallel to the shore
(provides better surfing).
More of the same
Task 4: Measuring wind speed and
direction
• Wind speed: use an
anemometer
• Wind direction: use
the compass
Good luck
geologists!
Review: Fill in the characteristics
for each type of change
EROSION
DEPOSITION