Geography- Formations

• Formation of a Levee

  1. Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.

  2. Sediment that has been eroded further upstream is transported downstream.

  3. When the river floods, the sediment spreads out across the floodplain
• Formation of an Ox-Bow lake


  1. At a river meander, water is pushed towards the outside of the bend causing erosion, forming a river cliff.

  2. The slower flow on the inside of the bend causes deposition.

  3. Over time erosion by hydraulic action (the sheer force of water eroding the bank away) and abrasion (material firing at the banks causing them to be eroded away) narrows the meander neck.

  4. In time, usually in flood, the river will cut right through the river neck taking the most direct route.

  5. Now the fastest flow is in the middle of the river and deposition will occur on the river banks

  6. This eventually completely cutting off the river meander leaving an ox-bow lake.

  
• Formation of a Cave or Cavern


  As rain falls the water absorbs C02 forming a weak carbonic acid. When this rain falls onto limestone it enters through a swallow hole and makes its way down joints and along bedding planes dissolving the lime stone as it goes. Caves or caverns are formed when the limestone dissolves more quickly than the rock surrounding it. This maybe when joints and bedding planes are closer together. These cracks allow for water to pass more freely with allows the rock toe dissolved more fully and eroded by the passage of water (hydraulic action and corrosion) forming the cave or cavern.
• Formation of limestone pavement


  Thousands of years ago sea shells, coral and micro-skeletons compressed to form limestone. The limestone consisted of vertical layers called joints and horizontal layers called bedding planes. During glaciation, the soil was removed exposing the limestone to chemical weathering. Rainwater forms a weak carbonic acid as it absorbs C02 when falling. Over time the rain dissolves the joints, widening them. These widened joints are called grykes. The surface is broken into blocks called clints.

  
• formation of a gorge (limestone)

  As a cavern becomes larger over time, joints and bedding planes also become wider. The rock becomes weaker and more unstable. The roof of the cave becomes less supported causing it to crack further until eventually it collapses. This forms a deep, steep sided valley known as a gorge.

  
• Formation of a swallow hole or pothole


  Limestone is a permeable rock. Streams that flow over it quickly disappear into enlarged joints. The water (which is acidic) dissolves the limestone along the joints and the erosive power of water can also play a part in making the gryke larger (hydraulic action and corrosion) where the river goes underground is called a swallow hole. Often this can be simply a hollow or depression on the surface below which there is a deep, wide, vertical crack. This future can sometimes be seen at the surface and is then called a pot hole.

  
• Formation of a Meander

  Faster flow of water on the outside of the bend (meander) which causes lateral erosion, undercutting the bank. Hydraulic action (the sheer force of the water eroding away the river bank) creates a river cliff. Due to the slower flowing water on the inside of the meander, the river loses the energy used to transport its lad and deposits it, forming a river beach.
• Formation of stalactites and stalagmites


  The water that dissolves limestone carries calcium carbonate. This water drips from the roof of the caves or caverns very slowly. As it drips, some of the water evaporates leaving behind some deposits of calcium carbonate. Overtime this builds up, forming a finger of drip-stone or stalactites. Not all the water evaporates here so drips onto the cave floor below. This dripping onto the cave floor creates a stalagmite which has a rounded top and wide base. Over a long period of time the stalactite and stalagmite fuse together forming a pillar or column.

  
• Formation of a ribbon lake

  1. Found where the glacier meets softer rock

  2. The glacier erodes more deeply at these points, because the softer rock is less resistant to erosion such as abrasion

  3. After the glacier has melted, the deeper parts fill with meltwater, forming ribbon lakes.
• What is the formation of a waterfall

  1. River erosion wears away less resistant rock faster than hard rock to form a step

  2. The falling water erodes a deep lake called a plunge pool by hydraulic action

  3. The swirling water causes corrosion and abrasion and undercuts the hard rock above

  4. There is nothing left to support the hard rock as it is overhanging so the hard rock collapses

  5. This process is repeated over time so the waterfall retreats upstream creating a gorge (a deep valley with very steep sides and a narrow valley floor.)
• Formation of a v-shaped valley

  - In the upper course, a river flows downhill eroding the landscape vertically

  - Hydraulic action is when the sheer force of the water causes erosion and cuts down vertically

  - This exposes the sides of the valley to weathering

  - Freeze thaw weathering looses rocks and steepens sides

  - Rocks fall into the river and help abrasion, leading to more erosion

  - River transports rock downstream making channel deeper creating V-shaped valley between interlocking spurs
• What is the formation of a limestone pavement?

  1. Limestone was being formed on the sea floor during the carboniferous period from calcium carbonate from the shells of dead marine creautures

  2. The limestone was laid down in horizontal layers called beds seperated by bedding planes

  3. earth movements caused the limestone to be raised up to form mountains

  4. The limestone cracked to form vertical joints

  5. The limestone is permeable, allowing water to pass through it

  6. During glaciation the glaciers removed the soil on top of the limestone

  7. Rainwater absorbs CO2 from the atmosphere and turns it into a weak carbonic acid

  8. The acid rainwater lands on the limestone and dissolves it.

  9. The acidic rain enlarges the joints

  10. This makes a flat weathered surface of large blocks called clints and gaps in between them called grykes. This is called a limestone pavement
• How do swallow holes/potholes form

  1. Surface streams do not flow far over exposed limestone as they rapidly seep into the rock

  2. When the water flows over the limestone it enlarges the joints in the rock through chemical weathering

  3. Water can easily seep through the rock layers

  4. Eventually, a surface stream will disappear down a hole and flow along underground channels

  5. The resulting hole in the surface of the landscape is called a swallow hole. Sometimes referred to as potholes
• Formation of a cave

  1. Limeston is eroded by the process of carbonation

  2. Water enters limestone via a swallow hole or pot hole and makes its way through the rocks = percolation

  3. As it makes its way through bedding planes and down joints it dissolves the rock

  4. Some of the rock is heavily jointed so it dissolves quicker to create an underground gap called a cave

  5. If the roof of the cave collapses due to gravity a cavern is formed
• What is the formation of stalactites and stalagmites

  1.When water drops from the roof of the cavern a tiny amount evaporates which means solid calcite is deposited on the cavern roof.

  2. These deposits build up over a very long period of time and form features called stalactites which hang down from the ceiling of a cavern

  3. Some of the water may also drop onto the floor where it splashes and evaporates, depositing calcite

  4. As more and more calcite builds up, short and stumpy features grow upwards from the ground. These are called stalagmites.

  5. Occasionally stalagmites and stalactites meet to form a pillar