Landforms of Coasts
Erosional and Depositional
Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter.
Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin.
Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities.
Other copyrighted material is used under the fair use clause of the copyright law of the United States.
Depositional (Barrier) Coasts
Cape Henlopen, DE
St Joseph Spit, FL
Hatteras Is, NC
Assateague Is, MD
St Simons Sound
Jekyll Island, GA is a good example of a barrier
island. It stands several miles off the mainland,
separated by a n area of primarily tidal marsh
with some tidal channels or “rivers”.
Image from USGS 7.5’ Jekyll Island, GA map, 1993.
The island itself is a little over 8 miles long and
two miles wide, and made of sand. The muddy
marsh makes it wider at some points, but are
nowhere above sea level at spring high tide.
St Simons and St Andrews Sounds are tidal
inlets or simply inlets. The first separates Jekyll
from St Simons Island and the second from
Cumberland Island. Tidal channels branch off
from them and move water onto and off of the
marshes.
Marsh to
Mainland
~1 mile
As with most of the islands on the Georgia
coast, the north and south ends have very
different characters, as the next slides show.
St Andrews Sound
North Jekyll Island
Much of the northern end of the island is
undergoing erosion. this is the up-drift
end of the longshore transport system and
the waves do not bring sand across St
Simons Sound with then, so they get it off
of this end of the island.
The beach is littered with dead trees (and
their stumps) that usde to grow where
they now lie (above), and the waves are
undercutting the live forest as well (left).
South Jekyll Island
The south end of the island is a different story. Here
the beach is very wide, and growing wider. The line
of vegetation in the upper picture was at the shore in
the 1950’s. A boat that sank well offshore in the
middle 1990’s has now been engulfed by the
prograding sand, and is now onshore even at normal
high tide (left).
All in all the island has grown by over ½ mile since
1953.
This rate is much higher than the erosional rate on
the north end of the island.
The southern end of Jekyll
Island, like the southern
ends of most of the Atlantic
barrier islands, has a hooked
shape. This occus because
the waves that transport
sand southward along the
front of the island refract into
the inlet. This drives
longshore transport around
the end of the island and
onto the back side for a
short distance, as the arrow
indicates.
This feature is called a
recurved spit or just a spit.
St George Island on the Gulf Coast of Florida is different from Jekyll Island. It is much longer (about 30 miles) and much
narrower (the sand island is nowhere wider than about 1/3 mile). The inlets separating it from the adjacent Islands (St
Vincent Island to the west and Dog Island to the east) are narrower. Finally, between the island and the mainland is an open
body of water (Apalachicola Bay) rather than tidal marsh.
Notice also that there are spits on both ends, though the western one is more pronounced than the eastern.
*
Image from Google Maps
2 miles
The conventional wisdom for why these differences occur has to do with
the relative strengths of the waves and tides as sediment movers. On the
Georgia coast the spring tide rangwe is upwards of 3m (10’) and so this
coast is called a mesotidal coast or tide-dominated barrier system. The
longshore transport is trumped, if you will, by the need to move that amount
of water into and out of the inlets twice a day.
The Florida Gulf coast, in contrast, has a normal spring tide range of less
than a meter (1-2’) and in general there is only one tide cycle per day. This
type of coast is called microtidal or a wave-dominated barrier system.
Less water movement by tides requires fewer, narrower inlets, and little
sediment is transported into the back-barrier bay for constructing marsh.
Just west of St Vincent Island (visible
at the east edge of the map) is the St
Joseph Peninsula or St Joe Spit. It is
a recurved spit, but instead of being
on the down-drift end of an island it is
a peninsula built out from, and still
connected to, the mainland. Like the
south end of Jekyll Island is
continues to prograde.
Also like Jekyll Island it would be an
island if a huge boulder seawall were
not maintained just north of Cape
San Blas (arrow). That point would
long ago have eroded through
without human interference.
Image from Google Maps
2 miles
There is another interesting
depositional feature of
coasts that can occur even
on what are generally
erosional coasts like this one
in New England.
Nahant is a northern suburb
of Boston and occupies what
are generally called “Nahant
Island” and “Little Nahant
Island” (arrow) though, in
fact, the entire landform is a
peninsula.
The narrow connectors
between the high, rocky
“islands” and the mainland
were constructed from sand
brought into place by a
northern longshore transport
and a southern one created
by waves refracting around
the islands.
2000’
Image from Google Maps
This is a tombolo.
2 miles
It should be clear from
earlier slides that the
longshore drift system
tends to prograde the
down-drift ends of islands
and spits pretty
aggressively.
Image from Google Maps
On microtidal coasts the
mouths of some bays can
be entirely closed off, like
this one in southern
Delaware. There is an
artificial cut for navigation
(just under the “k” in
“Park”), but the bays
would be cut off from the
sea if the cut were not
actively maintained.
This is called a baymouth bar.
This slide schematically summarizes all the landforms of depositional (barrier) coasts.
Erosional Coasts
Other coasts, such as this
one in southern Maine, are
very much different from
the barrier island coasts of
the southeast. They are
very rugged and rocky and
are rarely straight for any
significant distance.
There are projections of
rocy land into the sea
called headlands.
Between them are coves of
various sizes, often (but
not always) with a small
pocket beach at their
heads.
There are also offshore
rocks of various sizes,
some connected to the
mainland by tombolos
(below).
1000’
200’
Images from Google Maps
The ground-level view
reinforces the rugged
terrain of these coasts.
View northward across a pocket
beach from one headland to another
near Biarritz, France, on the Atlantic.
People for scale.
View northward across a pocket
beach toward a headland in Olympic
National Park, WA on the Pacific.
People for scale. (Photograph by Dr.
Sam Peavy, used by permission.)
Various of the landforms characteristic of erosional coasts are obvious in Dr. Peavy’s photograph. Waves, lacking sediment
supplied to them otherwise, wear away at the base of a seacliff, creating a wave-cut notch. The cliff is then maintained by
mass-wasting as the notch saps its support.
Relatively easily erodable parts of the cliff may be eroded right through, leaving a sea arch. When the arch collapses its
outer support will remain as an offshore sea stack.
Sea Stack
Sea Cliff
Sea Arch
(Photograph by Dr. Sam Peavy, used by permission.)
Wave-cut notch
The retreat of the seacliffs leaves behind a relatively level surface at about wave-base depth (or the
depth at which they can effectively erode) called a wave-cut platform, which may be visible at
spring low tide. Deep erosion into a broad headland can create a seacave.
Wave-cut platform
Sea cave
(Photograph at Point Arena N.S. by Thomas H. Johnson, Jr. Used by permission.)
View from inside a seacave at Biarritz, France
The western coast of the U.S. is erosional because it is also emergent. The dashed lines in
Rev. Johnson’s photograph indicate three different levels of raise wave-cut platforms. These
are called marine terraces or wave-cut terraces.
This slide schematically summarizes the landforms of erosional coasts.
Unfortunate Pirate
of the Caribbean
Black Pearl