Geoscience Reference
In-Depth Information
and are parallel to the shore (Figure 19.25a). These dunes
develop due to shore vegetation trapping sand blown by the
wind off the beach and gradually forming a linear (along-
shore) ridge or dune. Other kinds of dunes commonly seen in
coastal settings are blowouts and parabolic dunes, discussed
in Chapter 18 (Figure 18.18). In places where the supply of
sand is especially large and the winds are consistently strong,
massive
transgressive dune fields
form. These dune fields are
Drift of
deposited
sand
Tombolo
Former
island
Land
Beach
Refraction of waves
around island
(a)
0°
U.K.
BELGIUM
Figure 19.23 Cape Cod, Massachusetts.
This satellite image
shows a beautiful example of a curved spit, one that formed be-
cause the prevailing longshore drift flows to the north.
50° N
Paris
Mont-Saint-Michel
Bay of
Biscay
FRANCE
ITALY
45° N
baymouth bar
. If and when this closing occurs, the bay will no
longer be directly connected to the ocean and will then be called
a
lagoon
. The water in a lagoon is typically brackish relative to
the open ocean and thus is an important ecosystem. Sometimes
longshore sediments converge from two different directions,
causing a spit to grow perpendicular to the shore. If this kind
of spit grows so that it reaches a large sea stack or island, it is
called a
tombolo
(Figure 19.24).
5° E
SPAIN
Coastal Dunes
Another type of depositional landform
associated with sandy coasts is sand dunes. In contrast to
desert environments where dunes form in poorly vegetated,
sandy areas, coastal dunes develop because the supply of
sand along beaches can be very high. A common type of
dune form associated with beaches is a
foredune
. Foredunes
range in height from 1 m to perhaps 10 m (3.3 ft to 33 ft)
(b)
Figure 19.24 Tombolos.
(a) Tombolos form when longshore
drift extends to a preexisting island. (b) A beautiful example of a
tombolo is Mont-Saint-Michel in France.
A spit that entirely encloses a bay.
A spit or sandbar that connects an island to the
Baymouth bar
Tombolo
mainland.
A brackish body of water that lies behind a baymouth
Lagoon
bar.
Foredune
A dune that forms parallel to the shore when sand
blows inland from the beach.
