The Needle's shape is a result of the chalk being heavily folded during the Alpine Orogeny, an orgenic phase during the late mesozonic era, resulting in the chalk being near verticle. The layers of chalk continue under the sea, rising again in Dorset and the Isle of Purbeck.
The stacks have been formed by wave erosion around the headland. The pressure of wind and water are known as the processes of coastal geomorphology. The force of water crashing against the rock weakens cracks in the headland, causing them to later collapse, forming free-standing stack.
The medium hardness of the chalk means medium resistance to abrasive and attritive erosion. The formation process usually begins when the sea attacks small cracks in a headland and opens them. The cracks then gradually get larger and turn into a small cave. When the cave wears through the headland, an arch forms. Further erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast—the stacks that we seen today.
To log this cache please answer the following:-
1. Estimate the height of the tallest Needle.
2. Briefly describe what effects further erosion will have on the stacks.
3. Is there any evidence in the nearby cliffs around the headland that further stacks may one day develop?
4. Post a selfie or picture of your GPS with the Needles in the background (optional).