Question 3
With the aid of diagrams, describe the characteristics and mode of formation of the following features.
- Stack;
- Sand bar.
Observation
Not many candidates attempted this question and most of those who attempted it performed poorly. The characteristics of stack include:
- an erosional feature
- found off the coast
- a pillar of hard resistant rock
- stands in water away from the coast
- varies in size depending on the resistance of the headland rock
- size continues to reduce
- further erosion of the stack produces another feature called stump
- examples include the Old Man of Hoy in the Orkneys; the Elephant Rock in the coast of Oregon in USA; the Needles in Isles of Wight; Old Harry Rocks in the Isles of Purbeck; Coast of Dakar; Coast of Ghana between Takoradi and Secordi; Coast of Scotland.
Formation of stack goes thus:
- formed along the coast
- formed by wave erosion
- a headland is needed for a stack to be formed
- wave erosion attacks both sides of the headland
- two caves are formed at both sides of the headland
- further wave abrasion and hydraulic action deepen the caves
- further erosion of the caves produces an arch when the two caves join together
- the roof of the arch collapses
- the seaward portion of the headland remains as a pillar called stack
Characteristics of sand bar are:
- a coastal landform
- made up of sand and shingles
- a wave depositional feature
- lies a few metres above sea level
- found in indented coasts and mouth of rivers
- lies parallel to the shore
- could be up to 20 – 26km in length
- could be covered and exposed at low tide
- could be a bay, bar or an offshore bar
- called tombolo if it joins two land masses
- examples are Escravos and Forcados bars (Nigeria), Senya Beraku near Winneba (Ghana), Lumley Beach (Sierra Leone), Chesil Beach in Dorset (England).
Mode of formation of sand bar goes thus:
- formed near the coast
- formed by wave deposition
- takes place in shallow sea waters
- sea waves push sand towards the shore
- the backwash and under currents drag materials down the beach
- rivers bring alluvium toward the shore
- further deposition under the water builds up the bar
- coasts should be sheltered from strong currents to allow accumulation of deposits
- bars grow or extend in the direction of long shore drift