Logging Requirements:
Please send your answers to the contact info in my profile/Message Manager.
- Was the arch discordant or concordant? (viewable from mid-tide through low-tide)
- Which of the erosion processes present in many “wave action” arches do you see at from GZ? (read the Description section "Wave Action")
- Do you think that the incredibly high vertical tides of the Bay of Fundy INCREASED or DECREASED the rate at which the arch became a stack/knob? Why?
- Which knob is larger (left or right)?
Before October 20, 2015
After October 20, 2015
Geology: The Islands:
The islands and the municipal (and nearby provincial) park are composed of basalt resting on top of Triassic age sandstone.
Arches:
Simply put, arches are formed by the “selective removal of rock.” Erosion is the major process by which this stone is removed.
Erosion of arches happen on two levels – macroscopically and microscopically. Both methods effectively create arches, though at very different time scales. How does it work? Well, rocks of any type (except pure crystals) are a complex matrix of small interlocking solid particles. Most of the particles are microscopic pieces of a variety of mineral crystals, called grains. Some of these crystalline grains fuse under heat and pressure (metamorphic) and thus act as a cement for the parts of the rock that are non-crystalline or larger sized grains.
Macroscopic erosion happens when the rock matrix experiences a catastrophic process that fracture or joint the rock. Other factors such as rain, wind, chemicals, or temperature change will continue to erode the rock (often microscopically), splitting it into distinct macroscopic pieces that then are moved by gravity (they fall – “falling rocks signs”) or water pressure (like at this location).
Arches on coastlines are formed very differently than those inland/in rivers or streams (like at Natural Bridge, Virginia or Arches National Park in Utah). Specifically, there are two ways that coastal arches are formed. You will need to figure out which TYPE you are viewing from GZ.
Discordant Arch Formation: the rocks run at 90 degrees to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when the caves break through the headland. This type of arch eventually collapses creating stumps or rock stacks.
Concordant Arch Formation: the rocks run parallel to the coastline, with weak rock capped or surrounded (initially) by more resistant rocks. The wave action beats the ocean/sea side of the concordant coastline until it breaks through the rocks. Where the rocks are weakest, an arch forms.
Process of Arch Formation (the details):
There are lots of ways that erosion and specifically arches can happen. Arches happen in both coastal and inland environments, but require water in most cases to form.
Wave action.
Waves make erosion happen very quickly (relatively). Erosional processes that happen along the coast include – chemical exfoliation, differential erosion, and cavity merger. LOOK TO SEE WHICH ONE(S) YOU CAN IDENTIFY HERE!
Chemical exfoliation:
Chemical exfoliation happens when water that is in constant contact with the rock that dissoves the lattice of fine crystalline grains that have acted as cement for the larger grains. Basically, the dissolution releases the rock and allows gravity and the waves to wash away the rock. This process creates potoles, caves, or smaller depressions.
Differential erosion:
Differential erosion happens when erosion happens at different rates. It happens for several reasons 1) when the grain and cementing properties of rock surfaces differ. 2) When different types of rock (the formation includes rock layers from different geological members) As a result, arches can appear where the matrix is weaker or where the wave action breaks through “thin spots” in the harder/resistant rock at a faster rate than other locations in the exposure.
Cavity Merger:
Differential erosion and chemical exfoliation acting on the surfaces of a rock exposure frequently cause concave recesses in the rock. These concave recesses eventually can become cavities (caves) and potentially become connected. If a lintel (harder “cap”) remains, then an arch is formed. If there is no lintel then stacks are created.
References:
Natural Arch – Wikipedia -- https://en.wikipedia.org/wiki/Natural_arch
Natural Arch Formation http://www.naturalarches.org/archinfo/formation.htm