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• Landforms are continuously changing:
Tectonic and Volcanic activity builds up
the earth and gradation forces try to level
the earth
• If Gradational forces worked alone, the
earth would be leveled completely in 12
million years
Forms of Gradation Include:
Degradation – Any process that wears away
or rearranges landforms. Includes:
weathering, mass movements, erosion and
Aggradation – Includes the deposition of
eroded materials
Glaciation – Includes the movement of
materials through glacier ice
**Streams and Rivers are the primary
gradational forces in the shaping of our
Base Level
• The Base Level of a stream is the level
below which a stream can no longer erode.
The Ultimate Base Level corresponds to
sea level.
• A local base level can be a result of an
inland lake
• Artificial base levels result from the
construction of man made structures such
as dams that retard the natural flow of
water creating a reservoir
• Weathering encompasses a group of
processes by which surface and subsurface
rock disintegrates into mineral particles or
dissolves in water
• Bedrock is the Parent Material from
which weathered material or sediment is
• Two distinct types of weathering occur
Physical/Mechanical and Chemical
Physical Weathering
• The mechanical disintegration of rock
without any chemical alteration of the
parent material
Frost Shattering – when moisture in cracks
freezes the water expands, exerting
pressure on the rock
Physical Weathering
Crystallization – Similar mechanism to frost
shattering but pressures result from the
formation of salt crystals in cracks
Root Wedging – Plant roots can create
enough pressure to crack rock
Thermal Expansion – when rocks under go
rapid changes in temperature the related
stress can cause fracturing
Hydration – occurs where water is absorbed
by a mineral. This addition of water
initiates swelling and stress within the rock
Pressure-Release (sheeting/exfoliation) –
rocks that form under tremendous
pressures deep within the earth can slowly
fall apart once the pressure is relieved
Chemical Weathering
• The process by which rock is broken apart due
to chemical processes
• Chemical weathering attacks weaker minerals
within rock, changing them to a vulnerable
state and/or dissolving them in water
• Chemical weathering includes: Hydrolysis,
Oxidation, Carbonation and Solution
More Weathering Examples
• Material loosened by weathering is susceptible
to erosion and transportation. However if it
is to move down slope, friction, inertia and the
cohesion of particles must be overcome
• Weathered sediment is classified due to its
particle size
• Soil (mixture of sand and finer grain particles)
Sand, and Gravel will be the sediment size
classification used for this course
• The Angle of Repose is the steepness of a
slope when lose material comes to rest
• This angle is dependant on sediment size and
Course Sand
Fine Sand
Mass Movements
• Mass Movements are downward
movements of material propelled and
controlled by gravity
• Classes of mass movements include: Fall,
Slide, Flow, Creep