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Most of us don’t think about it but life on earth as we know it, simply wouldn’t exist in without erosion, it is constantly happening all around us tiny piece by tiny piece. The beautiful beaches we relax on, the soil we grow our food in, the power trails we walk and even the kitty litter our pets use all in one way or another trace back to erosion.
Today we would like to draw you attention to the beauty of Mechanical erosion near Broome’s spectacular Entrance Point.
Firstly we need to give you some information about Sea Caves and Natural Arches.
A natural arch, is a natural rock formation where an arch has formed with an opening underneath. Natural arches commonly form where inland cliffs, coastal cliffs, fins or stacks are subject to erosion from the sea.
Most natural arches are formed from narrow fins and sea stacks composed of sandstone or limestone with steep, often vertical, cliff faces. The formations become narrower due to erosion over geologic time scales. The softer rock erodes away creating rock shelters, or alcoves, on opposite sides of the formation beneath the relatively harder rock, or caprock, above it. The alcoves erode further into the formation eventually meeting underneath the harder caprock layer, creating an arch. The erosional processes exploit weaknesses in the softer rock layers making cracks larger and removing material more quickly than the caprock; however, the caprock itself continues to erode after an arch has formed, which will ultimately lead to collapse.
On coasts two different types of arches can form depending on the geology. On discordant coastlines rock types run at 90° to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland. When these arches eventually collapse, they form stacksand stumps.
On concordant coastlines rock types run parallel to the coastline, with weak rock such as shale protected by stronger rock such as limestone. The wave action along concordant coastlines breaks through the strong rock and then erodes the weak rock very quickly.
Sea caves are formed by the power of the ocean attacking zones of weakness in coastal rocks. The weak zone is often a fault, or fractured zone formed during slippage. Another type of weak zone is formed where dissimilar types of rocks are interbedded and one is weaker than the other. Typically this is a dike, or intrusive vein of more easily eroded rock found within a stronger host rock. Yet a third instance is in sedimentary rocks where a layer of softer rock is interbedded between harder layers.
The cave may begin as a very narrow crack into which waves can penetrate and exert tremendous force, cracking the rock from within by both the weight of the water and by compression of air. Sand and rock carried by waves produce additional erosive power on the cave's walls.
There are two lovely examples here in the Broome Sandstone that have both formed in a very similar manner.
To log this cache we require you to visit both the posted Coordinates and WP2, then message us with the following answers to the best of your ability;
1. Compare the features you can see at the Posted Coordinates and Wp2, which one is the cave and which one is the arch?
2. The Cave has a feature in its ceiling, what is it and what does this feature suggest?
3. Three instances of Sea caves were mentioned, which one does this cave fit into?
4. Is the arch concordant or discordant?
5. A photo of your team or GPS near GZ with your log and answers. (Optional)
You are welcome to log your find straight away to keep your TB's and Stats in order but please message us with your answers within 1 week of your log. Cacher’s who do not fulfil the Earth Cache requirement will have their logs deleted without notice.
Source: Wikipedia, Amanda Morris, Alice Richardson, goodearthgraphics.com.
