How are stacks, needles and stumps formed by the coast?
Picture by David Lewis from Wiki Commons
First of all a bay is formed exposing headlands.
Waves then attack a line of weakness in the headlands by a process called abrasion.
Abrasion is where rocks carried by the waves are thrown against the headland wearing it down.
Continuous erosion in this spot will open up the crack and develop it into a sea cave. The waves will then attack the back wall of the cave until it cuts through the rock forming an arch. Further erosion by solution will widen the base of the arch.
Solution is when acids in the sea water dissolve rocks wearing down a cliff or headland. This erosion will widen the base of the arch to a point where it can no longer support the wait of the rocks above and so it will collapse.
This leaves a largely intact section of headland and a stranded column of rock. This stranded column of rock is called a stack.
Over time the stack erodes by weather and wind into a pointed shape. This is called a needle!
And sometimes the stack or needle gets too tired and decides to lie down e.g. fall over. The leftover part usually looks like a stump above the water. Hence the name stump!
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