Chapter 18 – Erosion by Wind and Waves
Wind can erode the land to form desert pavement and ventifacts. When wind removes the top
layer of the surface (a process called deflation), it leaves behind larger rocks which are packed
together to form desert pavement. The sand particles the wind carries abrade, or polish some
rocks to produce a ventifact. Wind carries sand similar to a stream moving its load. Depending
on the velocity of the wind, the wind can carry smaller particles aloft, but heavier particles
bounce and jump across the surface in a process called saltation.
Wind can deposit material to form dunes and loess. There are four classifications of sand dunes
which are determined by the amount of sand present, their shape, and orientation relative to
the wind direction. The two crescent shaped dunes are barchan (with the tips pointing away
from the wind), and parabolic (with tips pointing into the wind). Barchan dunes form in areas
with limited amounts of sand and strong winds. Parabolic dunes form when plants anchor the
edges and sand collects around the rim of the deflation hollow. Transverse and longitudinal
dunes are found in areas with large amounts of sand and are formed in a pattern of linear
ridges and valleys. The wind blows across the ridges and valleys in a transverse dune. The wind
blows down the ridges and valleys in a longitudinal dune.
The side of the dune facing into the wind has a more gentle slope and is called the windward
side. The side away from the wind is more steep and called the leeward side. The leeward side
is also called the slipface as the sand tumble down that side.
If the wind is fairly constant in the direction it is blowing, the dunes will move in that direction.
This is called dune migration.
A windblown deposit of smaller silt and clay particles is called loess.
As waves erode rocky coastlines, a sea cliff is formed. The softer rocks along the cliff are
removed first, forming a sea cave. If the cave entrance extends all the way through to the
opposite side of a peninsula, a sea arch is formed. If the top of the arch falls, a sea stack is
formed. The nearly level platform at the base of the sea cliff is called a wave-cut terrace.
The energy of waves will be more focused on the parts of land that jut out into the ocean.
These are called headlands. Over time, the effect of wave erosion is to wear down headlands
and deposit materials in bays or lagoons. This has the end result of the coastline becoming
straighter.
Waves deposit material to form a beach along the coast. Beaches are all different colors
depending on the material being eroded. They could be black from cinder and ash, red from
scoria, white from coral, or yellow and tan from quartz.
Waves come into the shoreline at an angle and move back out to sea perpendicular to the
shoreline. This has the effect of generating a current that moves parallel to the shoreline and is
called the longshore current. The longshore current moves sand down the beach unless it
meets an obstacle such as large rock formation or a manmade jetty.
The raised part near the back part of a beach, which is deposited during large storms, is called a
berm. When the incoming water is slowed down as it approaches shore, sand can be deposited
in a mound offshore called a sand bar. As vegetation grows on the sand bar, more sand can be
deposited until a barrier island is produced. Barrier islands are important coastline protectors
during major storms. A narrow deposit of sand connected at one end to the shore is called a
spit. A sand deposit that connects the mainland to an island is called a tombolo.
When sea level is rising, as it is now, a submergent coastline is formed. When U-shaped glacial
valleys are submerged, narrow deep bays are formed called fiords. When the mouth of a river
is submerged, an estuary is formed. Here the saltwater and freshwater move up and down
stream due to tidal effects, and a brackish water environment is created. When sea level is
dropping, an emergent coastline forms.