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World Geography 3202/3200
Unit One: Land and Water Forms
Introductory Lesson
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Topography is a general term referring to the surface features of the earth like, hills,
mountains, valleys, plains, plateaus and other relief features. Your book defines it as " the
natural and human features of the earth's surface". The map to the right shows the
topographical features of South America.
Hills are typically defined as areas of upward relief less than 300 -600M.
Mountains are defined as areas of upward relief greater than 300-600M. It is important
to note that books differ on the actual relief required for a hill to be a mountain but the
difference is one of the degree of elevation. See if you can find a contradiction in your
own book!!
Plains are areas of flat land with very little relief. They are found at low elevations often
along coasts and in the center of continents.
Plateaus are areas of flat land with little relief. They occur at high elevations and are
associated with mountains. Like hills and mountains plains and plateaus differ based on
elevation.
Land Form Patterns
Have a look at a physical map of the world. Notice where the world's major mountain ranges lie.
Is there a pattern to these mountain ranges? You may be able to see other patterns. Later in this
section we will explain the reason for these patterns.
Can you identify the "Rockies in North America", the "Andes in South America", and the
"Himalayas in Asia" on the map?
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Most mountains and high elevations are located on coasts of continents.
Large mountain ranges occur along the west coast of North and South America. This is
quite evident in the relief image.
What is not evident is that major mountain ranges coincide with "tectonic plate boundaries". You
may or may not be familiar with tectonic plates. That concept will be covered Later in this
section. In any event by clicking on the map below you can see that the plate boundaries coincide
with the major world mountain ranges.
Earth's Interior
The earth's interior is neither all solid nor is it all molten. There are layers with a different
density, thickness and composition.
Furthermore the earth's crust is not one continuous layer. It is broken into many sections
known as plates. Some plates are quite small while others are quite large. They all float and move
on top of the molten asthenosphere. When they push together and when they pull apart they
create "tectonic forces". It is interesting to note; without this liquid layer called the asthenosphere
we would not experience earthquakes or volcanoes.
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Inner Core: The deepest part of the earth (1512 miles deep) is a solid that contains both
iron and nickel. It is because of this that the centre of the earth is a magnet, a compass. It
generates a magnetic field that protects the earth from flying out of orbit.
Outer Core: Outside of the inner core lays the outer core (1419 miles deep). This is
much like the inner core with the exception that it is a liquid that contains sulphur and
oxygen (which lowers the melting point).
Mantle: Occupying 1789 miles of the earth is the magma (iron and magnesium) that
makes up the mantle. It is mostly solid except the outer 200-300 Km which is extremely
hot and goopy; very plastic-like. This outermost liquid layer of the mantle is referred to
as the asthenosphere.
Mohorovicic Discontinuity: Boundary between the mantle (asthenosphere) and the
Lithosphere.
Lithosphere (crust): This is the top layer of the earth, which is basically hardened mantle
(magma). It contains two segments, the oceanic and continental crusts. Note: this is the
same crust, it just depends how thick it is to determine if it is part of the Oceanic or
Continental Crust. For instance, if you drained all of the water from earth, then there
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would be one, giant Continental Crust. Likewise, if you flooded everything, you'd have
one, giant Oceanic Crust.
Plate Tectonics
Explaining Compressional and Tensional tectonic Forces:
Tectonic plates move or float on top of the asthenosphere (orange in the figure). However they do
not float freely. The plates are forced in specific directions by the flow of magma beneath. Just
like a boat caught in a current plates move with the flow of magma. The magma forms
convectional currents. The magma closer to the core heats and then rises towards the surface as
its density decreases. Once the rising magma reaches the lithosphere it moves in opposite
directions.
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Compressional Forces occur where two tectonic plates come together. They compress
against each other.
Subduction Zones sometimes occur where compressional forces result from two plates
colliding.
Tensional Forces occur where two tectonic plates are pushed apart. The tension is
created as the plates move away from each other.
Ridge Zones sometimes occur where two plates move apart. The magma rises between
the plates and forms a ridge.
Reading a Tectonic Plates Map
Look at the map on p. 12 of your text.
Notice that subduction zones occur where the tectonic plates are moving towards each
other while Ridge zones occur where the plates are moving apart.
Notice that the Americas (North American Plate and South American Plate) are moving
away from Europe and Africa (Eurasian Plate and African Plate.
The Development of the "Theory of Continental Drift"
The Developer
Alfred Wegener was one of the first to propose the theory of continental drift.
While pondering the similarities between the coastlines of South America and Africa, Wegener
came up with an idea: What if the continents were once all connected and just drifted over the
years? It is this German man to whom we credit with the proposal of the theory of Continental
Drift.
The Evidence
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Ferns and Reptiles in many different continents
Glaciers that were then tropical forests
Same types of rocks on continents close to each other (yet separated by an ocean)
The coastlines of Africa and South America
The Flaw in Wegener's Theory
However, there was one major flaw in Wegener's theory of continental plates. He thought that
each of the continents were a separate plate--they were just drifting on a never-changing ocean.
The Canadian Correction
J. Tuzo Wilson was the 1960’s Canadian scientist who resurrected Wegner’s theory after years of
disbelief by the science community.
Today, we know that that's false, thanks to the discovery of crustal plates. The plates of the earth
are not composed of just land; they're composed of ocean, too. In some cases, the plates are just
land, in others they're just ocean, and, in still other cases, they consist of land and ocean. They
each have different boundaries and move in all different directions.
Continental Drift
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It refers to the movement of the more than 20 plates (9 major) due to convergent, divergent, and
transform boundaries. The continents drift at a rate of 2 inches a year, or, as fast as a fingernail
grows. It is believed to have started 200 million years ago. The original single continent was
named Pangaea and the original ocean that surrounded the land was called Panthalasa. Figure1.8
on p. 10 of your book shows the process of the continents moving apart. View the animation
above to see an approximation of how continental drift might have occurred.
Plate Tectonics Lab
Major Mountain ranges tend to coincide with subduction zones at the boundaries between
two converging plates. The tremendous compressional forces literally fold the thin crust in to
mountains. Remember this occurs over millions of years.
Volcanoes can occur at subduction zones or at ridge zones. At subduction zones the
compressional forces sometimes leave a crack in the crust that allows the magma to reach the
surface. At ridge zones the plates are moving apart which allows magma to reach the surface
from the asthenosphere. Because of the excessive number of active volcanoes on the coast around
the pacific ocean it is known as the Pacific Ring of Fire.
Earthquakes often occur at transform zones where the plates are moving beside each other instead
of toward or away from each other. Look Closely at figure 1.10 on page 12 and you can see
transform zones where the plates are moving side by side.
Notice that subduction zones occur where the tectonic plates are moving towards each other while
Ridge zones occur where the plates are moving apart and transform zones occur where plates are
moving side by side.
Notice that the Americas (North American Plate and South American Plate) are moving
away from Europe and Africa (Eurasian Plate and African Plate.
Folded Mountains
Most Major Mountain ranges were formed by folding.
The collision of continental plates causes the thin crust (lithosphere) to bend.
For example the Appalachian mountains and the atlas mountains were formed by folding when
North America and Africa collided 400 million years ago.
All rock that is put under extreme pressure for long periods of time (thousands or
millions of years) will fold like clay. Folding is a process in which the Earth's plates are pushed
together in a roller coaster like series of high points and low points. Folding bends many layers of
rocks without breaking them. The peaks are known as anticlines while the valleys are known as
synclines.
In this familiar figure we can see where the plates come together and that folded
mountains have formed.
Tectonic Faults
Fault lines are great cracks in the crust.
Mountains Formed by Normal Faults
Normal Faults are caused by tensional forces. When the land moves apart at a fault line one plate
drops down lower that the other.
Mountains Formed by Reverse Faults
Reverse faults are caused by compressional forces where plates move together at the fault. The
land that rises above can form a mountain.
Fault Block Mountains
Fault Block Mountains sometimes form when many layers of the Earth's crust are moved
vertically upward between two parallel fault lines. Vertical force is caused by the earth's internal
pressure. The mountains that are formed in this way are called fault-block mountains. The Sierra
Nevada mountains in California and Nevada, and the Grand Teton range of Wyoming are
examples of fault-block mountains.
Overthrust Faults
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Overthrust Faults are caused by compressional forces where plates move together at the fault.
They are a class of reverse faults that have had folding occur before the faultformed.
Volcanic Mountains
Volcano is an opening in the earth's crust through which magma, gasses, and ash erupt. The shape
of the volcano depends on the thickness of lava. Most volcanoes occur on plate boundaries.
Three Types of Volcanoes
Ash and Cinder cones are the simplest type of volcano. They are built from particles and blobs
of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the
air, it breaks into small fragments that solidify and fall
as cinders around the vent to form a circular or oval
cone. Most cinder cones have a bowl-shaped crater at
the summit and rarely rise more than a thousand feet
or so above their surroundings. Cinder cones are
numerous in western North America as well as
throughout other volcanic terrains of the world. They
are very explosive because the lava is very thick and
slow flowing. It blocks the vent causing pressure to
build up to high levels before it finally blows.
Shield volcanoes built almost entirely of fluid lava flows. Flow after flow pours out in all
directions from a central summit vent, or group of vents, building a broad, gently sloping cone of
flat, domical shape, with a profile much like that of a warrior's shield. They are built up slowly by
the accretion of thousands of highly fluid lava flows called
basalt lava that spread widely over great distances, and then
cool as thin, gently dipping sheets. These are the least
explosive because the lava is so thin and fluid that it does not
block the vent and prevents pressure build up.
Composite volcanoes have a conduit system through which
magma, from a reservoir deep in the earth's crust, rises to the
surface. The volcano is built up by the accumulation of material erupted through the conduit and
increases in size as lava, cinders, ash, etc., are added to its
slopes. The lava varies causing it to be explosive one time
while the next time it can be gentle flowing.
Physical Weathering
Weathering vs. Erosion
Weathering is the breakdown of rock and minerals.
Erosion is a two-part process that starts with (1) breakdown of land (weathering) and also
includes the movement (transportation) of these weathered materials and is followed by (2)
Deposition of the eroded material occurs when it is dropped in a new location.
Denudation is a term that refers to the wearing down or smoothing off of land features. The
processes of weathering and erosion are denudational. They are also gradational because they
grade the earth’s surface.
Physical Weathering vs. Chemical Weathering
Physical weathering is the breakdown of rock and minerals by mechanical stress whereas
chemical weathering breaks down rock with chemical reactions often including water.
Types of Physical Weathering
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Frost fracture: the expansion of freezing water that causes rocks to crack.
Heat expansion: rocks can expand and subsequently fracture.
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Plant growth: expansion due to root growth as shown in the picture below.
Burrowing animals: tunnelling animals can increase the size of existing cracks
Exfoliation: as internal pressure is released from certain rocks, it can cause layers to split
and fall off.
Environment's Affect on Physical Weathering
Fast temperature changes like occur in the desert increases the amount of physical weathering due
to heat expansion. Conversely, regions like the tropics where there is little temperature change the
amount of physical wreathing due to heat expansion is minimal.
Abundant precipitation combined with alternating freezing/thawing temperatures increases the
amount of frost fracture. Conversely, the absence of those climatic conditions reduces the amount
of frost fracture.
Running water increases physical erosion as friction occurs between water and rock.
Ocean waves cause hydraulic pressure and abrasion on the shore leading to physical weathering.
Chemical Weathering
Types of Chemical Weathering
Chemical weathering is the breakdown of rocks and minerals by chemical reactions and usually
involves the action of rainwater.
There are three different types of chemical weathering described below.
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The formation of solutions as rainwater absorbs CO2 , SO2, and other chemicals from the
atmosphere along with organic acids from the soil which then reacts with rock and
minerals causing some to dissolve and move away.
Hydrolysis, like the first process, involves the minerals in solution. In this case, carbonic
acid reacts with silicates in some rocks leaving a soft clay from which potassium, sodium
and magnesium are subsequently leached.
Oxidation is the reaction of metallic minerals to oxygen (mainly in water). This results in
the formation of oxides, which tend to be softer than the original mineral. For example,
rust on iron.
How Environmental Conditions Affect Chemical Weathering
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Heavy rain, running water and abundance of water increases the amount of dissolving
that occurs. Conversely less abundance of water leads to less dissolving.
High temperatures will increase the rate of chemical reactions. It is a fact of chemistry
that heat increases the speed of many reactions like oxidation.
Ocean water contains salt which can increase the rate of many reactions like oxidation.
River Life Cycle
Drainage Basins
Drainage Basin is the area of land drained by a river and its tributaries.
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The Amazon Basin, is the planet's largest body of fresh water with 1100 tributaries 17 of
which are more than 1000 miles long. Many of the tributaries begin in Colombia and
Peru.
The Nile river, in Africa is the longest river in the world.
Life Cycle of a River
Rivers change over time and seem to go through three stages:
1. Youth
2. Maturity
3. Old Age
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Youth (figure 2.7 p.28)
o Are usually found in highland or mountain regions.
o They tend to have a steep slope (high gradient)
o Usually have a small volume of water
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They have a rapid flow of water
There is usually very rapid erosion especially vertically
A narrow “V” shaped valley is characteristic
Water falls and rapids are common
Maturity (figure 2.7 p.28)
o Most high relief is eroded
o Gentler slope
o Many well developed tributaries
o Broad flat river valley
o Well developed flood plain
o More lateral erosion than vertical
o Meandering results
Late Maturity (figure 2.7 p.28)
o Remember the stages are not distinct.
o These changes occur over long periods of time.
Old Age (figure 2.7 p.28)
o Almost no slope
o Very little relief
o Elaborate meandering
o Oxbow lakes develop
o Often swampy areas around river
o Very muddy due to slow speed
o Most susceptible to flooding because of large flood plain.
Evaluating Evidence to Determine the Age of Rivers
There are six common pieces of evidence you can look for to determine the stage of a river.
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Slope of the river (steeper = younger)
Relief of the banks (steeper = younger)
Width of the valley (wider = older)
Meandering (more = older)
Size of flood plain (wider = older)
Rapids or water falls (more = younger)
Two Directions of River Erosion
1. Vertical erosion makes rivers deeper as is the case in young rivers
2. Lateral erosion makes rivers wider leading to the meandering of mature rivers.
River Deltas
Delta is a low lying area at mouth of river formed by deposition of silt deposition occurs because
river slows as it enters ocean or lake. There are three types, arcuate, digitate and estuarine.
Arcuate Delta:
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Named from Latin word for curved in the shape of a bow.
Fan shaped
The Nile Delta depicted on p. 32 of your text is a good example
To the right is a satellite photo of the Nile Delta, notice the fan shape on the right-hand
side of the mount of the river.
Digitate Delta:
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From Latin for finger
Delta with long fingers of sediment reaching into the sea
The Mississippi Delta is a good example depicted on p. 32.
To the right is a satellite photo of the Mississippi Delta, notice that it runs out into the
Gulf of Mexico like a finger - unlike the Nile Delta above.
Estuarine Delta:
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Formed when river runs into a bay or estuary
Tidal mud flats form which can be seen at low tide
Sediment deposited from river outflow and from Tidal inflow
The Seine River Delta depicted in figure 2.12 on p. 32 of your book is a good example.
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To the right is a satellite photo of the Amazon Delta, notice the tidal flats or islands that
cover and uncover with the tide.
This picture below shows a small estuarine delta in Notre Dame Bay.
Continental Glaciers
Continental Glaciers vs. Alpine glaciers
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Continental glaciers cover parts of continental land masses like Greenland;
Alpine glaciers are found high in mountain valleys, above the snow-line.
Differences:
o Location; Alpine glaciers are only found on mountain tops whereas continental
glaciers are only found at the earth's poles regardless of elevation.
o Size; Alpine glaciers are smaller compared to Continental glaciers.
Similarities:
o Both move and cause erosion
o Both change the landscape
o Both developed in constantly cold temperatures below freezing.
o Figure 2.13 on page 34 of your text shows a continental glacier and the the land
forms it creates through weathering and deposition.
Features of Continental Glaciation
Outwash plain:
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like a river Delta;
meltwater flowing from a glacier deposits silt like river deltas;
silt is deposited in layers;
small particles are carried further away;
larger particles drop closer to the glacier.
Terminal Moraine:
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heap or ridge of bulldozed gravel that marks the end of the forward motion of a glacier;
as a glacier retreats it deposits debris/gravel.
Erratics:
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large boulders that were transported long distances and dropped;
they now sit in a region and look very much out-of-place.
Drumlins:
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egg shaped hill;
formed under glaciers;
sloped or pointy end points in direction of ice flow;
formation
o ice melts under glacier
o deposits of gravel are made
o glacier moves forward
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o
deposits are bull-dozed along and catch up in rough areas forming piles or
drumlins.
Eskers:
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long deposits of eroded glacial material;
formed by sub-glacial streams that deposit material like all rivers;
sometimes known as Highways of the North because they are good for traveling on with
ATV's.
Evidence for Direction of Glacier Movement
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The gently-sloped end of drumlins point in the direction of glacier movement.
The terminal moraine marks the furthest extent of the glacier.
The layers of silt in an outwash plain can indicate direction of glacier movement. fine
particles would be at the leading edge while larger particles would have been closer to the
glacier.
Alpine Glaciers
Alpine glaciers are like very slow moving rivers of ice flowing down high mountain valleys. Like
continental glaciers, alpine glaciers create land forms by weathering and deposition. They
typically erode the mountain beneath them into a u-shaped valley with steep sides. Some alpine or
valley glaciers are 1000m thick and up to 160 km long, though most are only a few km in length.
Features of Alpine Glaciation
Cirque
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a circular hollow cut into bedrock during glaciation;
side and back walls are steep but front wall opens downward.
Cirque Formation
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alpine glacier freezes onto mountain valley and as is proceeds it plucks/gouges rock from
the mountain top leaving the cirque shape.
Arête
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steep knife edged ridge between two cirques in a mountainous region.
Hanging Valley
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a high level tributary valley from which the ground falls sharply to the level of the lower,
main valley. The depth of the lower valley is due to more severe glaciation.
Lateral Moraines
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land-form deposited at the side of a glacier.
Terminal Moraines
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deposits that mark the farthest extent of the alpine glacier the same as with continental
glaciers.
Fjords
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alpine Glaciers erode troughs and valleys in the mountain;
glacier valley reaches the coast;
glacier melts and sea water floods the valley;
fjords are very common in Norway and a quick search on web can find you some
amazing pictures.
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Erosion of Submerging Coastlines
Terms Related to Water/ Wave Erosion
Three Weathering Terms
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Hydraulic pressure: The pounding force of water/waves
Corrosion: Minerals such as calcium carbonate and limestone dissolve in the water
Abrasion: rock and sand particles suspended in the water bump, grind, scrape and gouge
surfaces the water hits.
Longshore drift terms
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Headlands: the protrusions of land that extend the farthest out into wave action.
Longshore drift: refers to the fact that dominant waves have enough energy to carry
silt/sand from headlands along the shore where it is later deposited.
Wave Refraction: waves bending around headlands as they hit the shallow water by
shore
Spit: A ridge of sand running away from the coast, usually with a curved seaward end.
Spit grows in the prevailing direction of longshore drift. Ends are curved by the action of
waves in different directions.
Bay Bar: A ridge of mud sand or silt extending across a bay. Formed when spits stretch
across the mouth of the bay.
Bay Beach: An accumulation of sediment deposited by waves and longshore drift along
the shore of a bay.
Straightening of an Irregular Submerging Coastline
Irregular submerging coastlines have headlands that protrude out from the shore line.
The erosion of the headland can deposit silt in the bay which forms a bay beach as it tends to
reduce the irregularity of the coastline. The headland is reduced due to erosion and the bay is
being filled by deposition.
Longshore drift results in some sand being deposited parallel to the shore but connected to the
headland. These silt deposits are known as spits.
Longshore drift and deposition can continue to the point that the spit closes off the mouth of the
bay. This extensive deposit is known as a bay bar. As you can see it tremendously reduces the
irregularity in the coastline.
Continued erosion and deposition can straighten a coastline over a long period of time.
Erosion of Emerging Coastlines
The Evolution of Sea Stacks
Sea stacks are common in eastern Canada. Many have plant growth on top. Sea stacks are
columns of land standing in the ocean just off shore. They are created over a long period of time
after a series of other land structures have eroded away. First sea caves are formed in a headland.
Continued erosion turns sea caves in to the second land feature, searches. sea arches are an
interesting sight too and have drawn many tourists to the northern Peninsula of Newfoundland.
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Erosion and eventual collapse of the arch top leaves a sea stack standing in the ocean. Figure
3.11 on pages 49-49 shows this evolution from three different points of reference. You must look
at this figure it will help cement the concepts in your mind.
Sea Cave Formation: "a" in figure 3.11 on page 48
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waves strike the headland first;
waves refract around the headland and put hydraulic pressure on both sides of the
headland;
erosion of the weak portions create caves and blow holes in the sides of the headland.
Sea Arch Formation : "b" in figure 3.11 on page 48
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eventually sea caves, on alternate sides of the headland get deeper until they connect
inside the headland forming a complete passage way or tunnel or “arch” through the head
land.
Sea Stack Formation: "c" in figure 3.11 on page 48
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continuous erosion, of sea arches, causes the collapse of the ground over the arch;
this leaves a pillar or column or “STACK” of land standing alone where the headland
was.
Straightening of Emerging Coastlines
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emerging coastlines straighten in much the same fashion as submerging coastlines;
erosion of headlands creates sea caves, sea arches and sea stacks instead of bay beaches,
spits and bay bars;
however the result is similar in that headlands are reduced in size which straightens the
coastline. Compare the headland length in figure 3.11 before and after erosion.