External Geological Agents
1. WIND: can cause erosion, mass wasting and sedimentation
The main erosion processes cause by wind are:
DEFLATION forms a layer of rocks that is called a desert
pavement (stony field - reg).
ABRASION forms mushroom-shaped rocks
Aeolic sedimentation when wind finds an obstacle in the landscape
Sand dunes are formed (in sandy deserts (ergs))
Differents types of dunes: Barchan dunes
2. GLACIERS: are divided into an accumulation zone and an ablation (wastage)
zone along an equilibrium line. Glaciers are made up ice and snow, water, rocks
and sediments. A glacier moves very slowly (few centimeters to a few meters
• There are different types of glaciers:
Alpine glaciers: cirques, horns, arete , U-valley, moraines (lateral, medial,
Continental glaciers: which are continuous masses of ice that are much larger than
alpine glaciers. Small continental glaciers are called ice fields. Big continental glaciers
are called ice sheets.
3. Wild water:
GREAT AMOUNT OF
STREAMS ARE NON DEFINED NOR
PERMANENT COURSES OF WATER
RAINSTORM MODIFY THE RELIEF
some parts are
harder to erode
Intermittent , short
very steep slopes
ice melts or a period
of heavy rains.
• 4. RIVERS:
• 0.0001% of the Earth's water .
• Different sources: lakes , springs (from underground), rain or melting snow
and ice (high mountains)
To narrow river consider its drainage basin and its floodplain:
drainage basin flood plain
1.Early maturity V-shape
2. Late maturity.
plain. River begins
to meander. Flood
3. Old age. Very
Rivers (erosion) three valley shape.
• 5. Ground water
• Groundwater : water in the ground is stored in the spaces between rock particles.
• A groundwater aquifer is an underground layer that holds water within soil,
porous rock, or similar material.
• A groundwater well may be used to extract water from a groundwater aquifer.
• Karst relieves are landscapes shaped by the dissolution of a soluble bedrock layer,
(usually carbonate rock such as limestone or dolomite) by a mildly acidic water.
• These landscapes display different:
• surface landforms (sinkholes, dolines, disappearing and reappearing streams,
• underground features (extensive caves and cavern systems may be formed. In
caves, the deposition of calicium carbonate may form stalactites, stalagmites,
is constantly moving
has a lot of energy
•6. Sea water in coastal areas
Salty sea reacts in
chemical ways with
rocks and makes
Stones, other sediments
are carried by waves
and they crash into
the coastal rocks.
sea caves, arches,
The sediments (sand, mud
accumulate in sedimentary
basins , forming layers or
These layers of sediment are sinking
deeper and deeper into the basins.
Under conditions of
The sediments become rock
(lithified) through a process called
DIAGENESIS (it may take millions
Clastic sedimentary rocks such
as conglomerate, sandstones,
Chemical sedimentary rocks,
such us halite, gypsum
rocks such us limestone, coal.
Types of sedimentary rocks