We came here following the tourist sign that said ‘Blowhole’, we have seen a few now, all-different types, this would have to be the smallest one we have seen, if indeed we were looking at the right hole. Anyway while we were here we noticed another geological feature that we have seen several times before.
Todays Earth Science focuses on this feature.
What is an Arch? A natural arch, natural bridge or, less commonly, a rock 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, rivers or weathering.
Most natural arches are formed from narrow fins and sea stacks composed of sandstone or limestone with steep, often vertical, cliff faces. In this case it has formed in Limestone, we wont bore you with the details about Limestone, there are plenty of great EC’s along this coastline explaining it in detail.
The formations become narrower due to erosion over geologic time scales. The softer rock stratum erodes away creating rock shelters, or alcoves, on opposite sides of the formation beneath the relatively harder stratum, or caprock, above it. The alcoves erode further into the formation eventually meeting underneath the harder caprock layer, thus 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.
The choice between bridge and arch is somewhat arbitrary. The Natural Arch and Bridge Society identify a bridge as a subtype of arch that is primarily water-formed. By contrast, the Dictionary of Geological Terms defines a natural bridge as a "natural arch that spans a valley of erosion."
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 stacks and 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.
To log this Earth Cache we require you to visit GZ to make some observations, consider the information given, then message us with the following answers to the best of your ability;
1. From the posted coordinates look South East, can you see the Arch, what type of Arch is it, Discordant or Concordant?
2. Do you think the Arch getting wider faster than it's getting higher? What factors influenced your answer?
3. A photo of your team or GPS near the Arch, ( please dont show the Arch in your Pic) upload as many photos as you like!
4. Bonus points to anyone who can find the nearby blowhole and post a photo of it!
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted without notice.
Source: Wikipedia, Department of Parks and Wildlife
