GEOLOGICAL WORK OF
NATURAL AGENCIES
INTRODUCTION
Rocks exposed at the surface of the earth are subject to direct or
indirect attacks of a number of natural agencies such as
atmospheric gases, heat, moisture, surface and subsurface water,
wind, sea water and ice.
These agencies are ceaselessly operating, at a places individually
and at places in close cooperation with one another on the surface
rocks, season after season and year after year.
They are thus responsible for modifying the physical features
existing on the surface.
The three major agents of change on the surface of the earth are
wind, rivers and glaciers.
GEOLOGICAL WORK OF WIND
Geological work of wind
Atmosphere is composed mainly of gases that are collectively known as air. Air
in motion is called wind.
Geological action by wind is also known as AEOLIAN processes.
In general it may be said that winds are born mainly due to non-uniform
heating of the surface of the earth at different places causing differences in
atmospheric pressure.
The pressure difference so created makes the atmospheric gases (the air) to
move from areas of high pressure to areas of low pressure in the form of winds.
During such a movement wind may create temporary or semi-permanent
changes on the land surface. Changes depending on wind volume, wind velocity,
nature of the surface, duration of time.
Erosional landforms formed by wind
WIND EROSION
Deflation, Abrasion Attrition
Wind erosion refers to the
progressive removal of
particles by the blowing action Blowouts Yardangs
of wind.
Wind performs work of erosion Pedestal
by at least three different Oasis rocks
methods:
Slack Ventifacts
Desert
Hammada pavements
Deflation (Latin: deflare to blow away)
The process of removal of particles of dust and sand by strong
winds is called deflation.
By itself, wind possesses not much erosive power over rocks or over
the ground covered with vegetation. But when moving with sufficient
velocity over dry and loose sands or bare ground covered with dust, it
can remove or sweep away huge quantity of the loose material from
the surface.
The regions which experience intense and sustained erosion are
called deflation zones.
BLOWOUTS
deflation may cause the
removal of sand from a
particular location to such
an extent that a big enough
depression is created,
sometimes with its base
touching the water table at
quite a depth. Such
depressions are variously
called blowouts.
OASIS
Much deeper and extensive depression where the water table is
intersected and it gets partially filled up with water is called an
OASIS
SLACK
Slack is another term used for
such depressions created by
deflation.
The Quattara depression of
western Egypt is one of the
biggest slacks. It is 300 km
long and 150 km wide; its
base is 130 m below sea level.
HAMMADA
Another feature produced due to
deflation is called a Hammada.
It is a bare rock surface in a
desert from over which thin
cover of sand has been blown
away by strong winds.
It is also called a desert
pavement and may extend for
considerable distance in a desert
region.
WIND ABRASION
Wind becomes a powerful agent for rubbing and abrading the rock
surfaces when naturally loaded with sand and dust particles.
This load is acquired by the strong winds quite easily when blowing
over sand dunes in deserts and over the dry ploughed fields.
This type of erosion involving rubbing, grinding, abrading and
polishing the rock surfaces by any natural agent (wind, water or
ice) with the help of its load while passing over the rocks is
termed as abrasion.
Wind abrasion is responsible for numerous features of erosion on
the land surface, especially in arid areas: yardangs, pedestal rocks,
ventifacts and desert pavements are few examples.
YARDANGS.
These are elongated, low-lying
ridges forming overhangs
above local depressions.
Their trend is parallel to the
direction of prevailing winds.
Yardangs commonly occur in
groups.
Yardangs are formed in areas
where rocks of alternate hard
and soft character are laying
one above another with a
general gentle slope.
PEDESTAL ROCKS
These are also often called mushroom
rocks because of their likeness to
mushrooms popping up closely in a
level land.
They are flat-topped rock masses that
are characterized with slender
supporting rock stems.
The top is commonly referred as
overhang and the slender stem as
pedestal.
Generally, the height of pedestal rocks
is about 2 meters from the surface
because of their density.
VENTIFACTS
These are small sized rock fragments showing
one, two or three or even more typically
wind-polished surfaces called faces.
They abound in many rock deserts.
prolonged wind abrasion causes the different
sides of initially rough pieces to be polished,
face after face. Once one face is partially or
fully polished, the fragment may overturn
during strong guests of wind, exposing a second
face of the wind. This process may repeat
repeatedly creating multifaceted fragments.
These winds polished and facetted rock
fragments are called ventifacts.
DESERT PAVEMENTS
These are flat rock surfaces covered by
rounded or sub rounded pebbles spread all
over them, and are typical features of rock
deserts.
Initially these rock surfaces are covered with
fine particles of clay, silt and sand that have
been removed by wind action.
These pebbles may get further polished with
time and even acquire thin film of oxides of
iron and manganese. This oxide film is
particularly referred as desert varnish.
Such coated pebbles give a characteristic
brown, dark red or even black appearance
to the desert pavements.
ATTRITION BY WIND
The sand grains and other particles lifted by the winds from different places
are carried away to considerable distances.
During this journey, the particles are not moved in straight lines for the simple
reason that all the particles are not of the same weight and the wind velocity
also varies from base to the top of the current.
The grains are, therefore, moved in a zigzag fashion, colliding with one
another again and again. These repeated collusions cause lot of mutual
rubbing due to impacts.
The wear and tear of load sediments suffered by them due to mutual
impacts during the transport process is observed in wind as well as rivers.
It is termed attrition and is primarily responsible for reduction in size of the
load particles during their transport.
SEDIMENT TRANSPORT BY WIND
Sediment transport by wind is an important aspect of geological work of wind
and particles movement by wind occurs in 3 ways; they are;
Suspension – dust and silt particles caught up by the blowing wind remain
suspended to few hundred or a few thousand meters.
Saltation – the downwind movement of particles in a series of jumps or
skips.
Creeping – the heaviest materials are rolled along the ground surface. The
transportation through rolling and creeping is also known as traction.
WIND DEPOSITION
Wind formed deposits are called Aeolian deposits.
The rock particles in the Aeolian deposits are generally well
rounded and are sorted according to their size and weight.
Wind deposits are mainly two types
1. Accumulations of sand, called "Sand dunes"
2. Deposits of silt, called "Loess"
SAND DUNES
A dune is a hill of sand built either by wind.
Dunes occur in different shapes and sizes.
Most dunes have a gentle slope towards the
wind-ward side and a steep face towards the
lee-side.
The valley or trough between the dune is
called SLACK.
A Dune field is an area covered by
extensive sand dunes.
1. CRESCENT DUNE
Crescent shaped dunes with a steep slip face on the inward or concave side.
Commonly known as BARCHAN
2. TRANSVERSE DUNE
A relatively straight elongate dune oriented perpendicular to the wind
direction.
They are formed in areas with strong winds where more sand is available.
3.LONGITUDINAL DUNES
Longitudinal dunes are long ridges of sand
that form parallel to the prevailing wind.
These dunes occur where sand supplies are
moderate and the prevailing wind direction
varies slightly.
In sections of North Africa, Arabia, and
central Australia, longitudinal dunes can
reach nearly 100 meters high and extend for
more than 100 kilometers.
4.PARABOLIC DUNES
U shaped mounds of sand with convex noses
trailed by elongated arms.
These are formed around blowouts where the
erosion of vegetated sand leads to a U shaped
depression
LOESS
Is a deposit of wind blown silt
and clay particles, typically in
the 20-25 micrometer size
range.
These deposits are typically
nonstratified and have a
grayish yellow colour.
It composed of many minerals
including quartz, feldspar,
hornblende and calcite.
Other Depositional Features by Wind
Sand Sheets
They are flat, gently undulating
plots of sand surfaced by grains
that are too large for saltation.
Sand Seas or Erg
An erg is a broad, flat area of desert
covered with wind swept sand with
little or no vegetative cover.
MESA - A big landmass with
steep sides and flat top
resembling a table.
PLATEAU - An elevated upland
with one or more steep sides
spread over a very large area.
Ripple mark - Wave like
undulations developed on the
rock surface by wind blow
