... The Sea Arch here
Formation of the cliffs 25 million years ago New Zealand was just a chain of islands, about a fifth of today’s land area, and was sinking. But a new tectonic plate boundary was developing through New Zealand. This gave us an uplifting experience, which accelerated once the Alpine Fault formed 15 million years ago. (Even so, only 5% of Zealandia is above water today.) As land appeared above the sea it began to erode. Between 25 to 15 million years ago thick layers of sand were deposited in the seas off East Otago, together with shells of tiny marine organisms. This formed 230 metres of calcerous sandstone – our Caversham Sandstone of which these cliffs are formed
The coastal erosion that forms sea arches is responsible for a variety of landforms. Caves, tunnels, columns sea arches and sea stacks are some of the landforms created by the action of waves on sections of rock that are more easily eroded than the rock around it.
A Sea Arch is a rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame.
Sea arches form when waves are deflected to the sides by a point on the headland, or where waves attack a plane of weakness which cross cuts a promontory. Where there are two different types of rock and the bottom layer of rock is softer than the top, waves can erode a plane of weakness over time to form a small crack or cave. This erosion continues on both sides and once it has worn all the way through the rock forms a small arch. The stronger rock spanning the opening at the top is known as the keystone.
Sea arches do not last very long on a geological time scale. Most collapse within a few decades to a few centuries. When the center collapses, the stacks of rocks remaining on the sides are known as sea stacks.
A Sea Stack is a geological landform consisting of a steep and often nearly vertical column of rock in the sea near the coast formed by erosion by waves, wind and water. Once the keystone of a Sea Arch collapses the stacks of rock remain on the sides.
Fortunately, time and conditions have lead to a beautiful landscape here at Tunnel Beach. Have fun as you explore this unique part of the coastline. You will need to explore the whole area to be able to answer the questions. Happy EarthCaching
From promontory to sea caves to sea arch to sea stacks to rocks
To log this geocache Please send your answers to the following questions to me through my profile BEFORE you log your find. You don't need to wait for me to reply, I will contact you if there is a problem.
1. At the published coordinates estimate the length of the wall of rock that protrudes beyond the end of the Arch, how far is this?
2. Feel the rock at the barrier and describe it. Eg. Is it rough, smooth, hard, soft, what size are the grains, what colours can you see here etc. From what you’ve described here do you think this rock is hard wearing and the features here will last a long time, or is it fragile and will erode quite fast?
3. Of the five features shown in the diagram above ( promontory, caves, sea Arch, sea stacks and rocks) which can you find here at this general location.
4. Having gone down through the man made tunnel do you think the small cave to the left at the bottom of the steps and ramp is man made or natural?
5. Please provide a photo with you and/or your caching name somewhere near GZ
Many thanks to Daywalk for her help and allowing me to poach some of the info from her cache pages