Geoscience Reference
In-Depth Information
Fig. 8.6
Depositional landforms: (a) Chesil Beach, a barrier (arrowed); (b) Dungeness, a cuspate foreland; (c) The Frisian Islands, a
barrier island chain; (d) Spurn Head Spit at the Humber Estuary mouth. (Images from NASA Earth Science Applications Directorate
(https://zulu.ssc.nasa.gov/mrsid/).)
(a) Foredune ridges
(b) Hummocky or transverse dunes
t
1
t
2
Fig. 8.7
Typical dune
morphologies. On gentle gradient
basement slopes (a), shore-parallel
beach ridges form. Foredune ridges
may develop on each beachridge to
form a sequence of dune-topped
ridges. If vegetation is patchy, the
dune topography is more irregular
(b) and if sparse or absent, a
transgressive sand sheet (c) may
form. On steep basement slopes
with good vegetation growth,
vertical accretion may give rise to a
high, single dune ridge (d).
Blowouts are associated with breaks
in vegetation cover and which
involve cannibalization of the dune
sand by ongoing wind action (e).
In time parabolic dunes may evolve
that transport sediment landward
through the dune system Temporal
evolution is denoted by t
1
-t
4
.
(After Woodroffe 2002.)
t
3
t
4
Sand supplied to beach
and dunes, incomplete
vegetation cover
(d) Single accreted foredune
(c) Transgressive sand sheet
t
4
Sand transport
t
3
t
2
t
1
Rapid sand
supply trapped
on vegetated
fore dune
Beach retreat.
Sand drift in transgressive
sand sheet
(e) Parabolic dunes and blowouts
t
2
t
1
Beach retreat.
Blowouts result in
parabolic dune migration
t
2
t
1
