This EarthCache invites geocachers to look more closely at the sea stacks in Freshwater Bay. The learning point of this EarthCache is to familiarise the geocacher with the processes of hydraulic action that have formed Stag and Mermaid Rocks.
Everything you need to answer the questions is available by attending the co-ordinates and reading this lesson. There's no need to research anything extra online, although you’re welcome to do so if you want to.
Stag and Mermaid Rocks are two sea stacks in Freshwater Bay, at the western edge of the Isle of Wight. This is one of the most picturesque beaches in the county and lies just south of the town of Freshwater.
From the beach or suggested coordinates, look out to the eastern side of the bay and you will see two sea stacks. Mermaid Rock is the one closer to the shore, and Stag Rock is the one beyond that. There used to be another rock, Arch Rock, between the Mermaid and Stag Rocks, but it collapsed in 1992, and it's 'feet' can now only be seen at low tide.
A stack or sea stack is a geological landform consisting of a tall column of rock, formed when an arch collapses alongside a coastal headland.
Stacks are formed over time by wind and water, processes of coastal geomorphology.
Headlands are exposed to the full force of erosion by the sea. The force of the sea crashing against the rock, or hydraulic action, attacks and widens cracks in the rock to form small caves.
These caves gradually enlarge and cut all the way through to the other side of the headland to form an arch. As the erosion continues over millions of years, the force of the water weakens cracks in the roof of the arch, causing them to later collapse, which forms the free-standing stacks. By definition, sea stacks are wholly surrounded by the sea at high tide.
Sea stacks will then erode further to become stumps before collapsing completely and being lost into the sea.
Another method of hydraulic action is wave-cut notches. A wave-cut notch is formed by erosional processes such as abrasion and hydraulic action. They usually appear as a dent in the cliff or stack, visible at the level of high tide. As the notch increases in size, the cliff becomes unstable and collapses, leading to the retreat of the cliff face or increased instability of the stack, due to it's top-heavy weight.
Many other factors can assist in the formation of stacks, including wind, rain, marine organisms, and grinding by sand and pebbles.
The southern border of Freshwater parish is fringed by a long, high ridge of chalk that extends east from the Needles through Freshwater Bay to Culver Cliff near Bembridge. Chalk is a variety of limestone composed mainly of the mineral, calcium carbonate, and is made up from the shells of tiny marine animals known as foraminifera and from the calcareous remains of marine algae known as coccoliths. Stag and Mermaid Rocks, and the headland that survives around them, represent the hardest sections of that chalk.
As a sedimentary rock, chalk is formed under the sea, on the sea bed, from the products of dead organisms sinking to the sea floor and then mixing with mud. These sediments then become cemented and over millions of years, lithify into sedimentary rocks. Sedimentary rocks are often bedded, which means they are in definite layers (beds) of a single rock type with a break above and below called a bedding plane. Each bedding plane represents an interval when deposition ceased, (causing the 'lines' we can see today.) In the headland and sea stacks at Freshwater Bay, we can see beds that are several metres thick. These massive beds probably developed in a shallow, warm water environment where sediment was laid down from water carrying a large volume of suspended sediment. These currents then experienced a rapid decrease in velocity and the suspended load dropped suddenly.
Freshwater Bay itself has been created by the same sort of process, as over millions of years the power of the sea has been exploiting and eroding a weakness in the chalk, then widening and deepening it to form the small, semi-circular bay.
These sea stacks, and the headland around them, are highly vulnerable to further hydraulic action. Driven by the prevailing south-west winds, the waves can be extremely powerful and storms regularly throw shingle up onto the promenade. In the future, the Stag and Mermaid stacks will eventually also disappear due to these unremitting processes of nature.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
- Look at the angle of the bedding planes (layers) in the cliff on the headland. From the horizontal angle, which angle do they go in, and how thick is each of the bedding planes?
- Look at the bedding planes at the top of Mermaid and Stag Rocks. Do the same planes continue in each stack, or are they misaligned?
- Look at Stag Rock. Describe the visible processes of hydraulic action on the stack.
- Look at the side edges of the Mermaid Rock. Explain how the position of these edges give us clues about how the stack was formed.
- Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache, but please do not give any answers away.
