I wonder how much geological time is represented by those layers?
Edit: From Stonehammer UNESCO global Geopark.
Doonbristy Rock
This layered stack, also called the rock of Dan Bristy or Dun Briste, is found along Ireland’s north coastline. Rocks like these are the product of ancient geologic processes combined with the force of the modern ocean.
The layers are sedimentary, part of the Downpatrick Formation. That unit formed during the early Carboniferous, about 350 million years ago, in the ocean off of a passive margin boundary. Passive margins form where continents meet oceans if there are no active plate boundaries (subduction zones or transform faults) nearby. Instead, sediments erode from nearby continents, move offshore, and deposit in mostly flat-lying layers at the bottom of the sea.
The lower layers alternate between fine-grained shales and sandy limestones, indicating marine conditions. Each layer represents some change in depositional conditions and could have taken thousands of years to accumulate. The finer grained shales form in deeper, quiet water, settling out with little to no current to stir up the grains. The sandy limestones are made of the shells of crinoid and coral organisms that can be used to tell the approximate age of the rocks. These organisms likely lived in shallower waters where more organisms could live and reefs could grow. They’ve been broken up before they reached this spot, indicating that this area was probably still deeper water than where corals could live.
The different layers therefore represent different sediment supplies as the ocean depth changed. Deeper water led to only quiet water shales being deposited, while shallower water led to increased supplies of coarse material from shallow levels. Near the halfway point there is also an eroded surface and a slump, probably the remnant of a sediment-carrying underwater channel formed as water levels moved in and out.
The upper portion of the sequence shows thicker limestone beds, indicating that sea level shallowed as the rocks higher in the sequence formed. Some of these are cross bedded and some have been churned up by organisms that live in the sediments on the ocean floor, indicating shallow water conditions where currents could move the sand grains around. Once they were deposited, they were buried for hundreds of millions of years, during which time the slow processes of chemistry took the loose sediments and sealed them together as strong rocks.
Today, these rocks are exposed offshore as a sea stack. Sea stacks form as a result of the relentless attack of the ocean waves on the shoreline. When the ocean waves find a weak spot in the exposed rocks, such as a small crack or a section of rock not as strongly lithified, it erodes that spot more quickly. Once the ocean has begun eroding a spot, waves can focus on the edges of that site and widen it, leaving only headlands that jut out into the water.
The waves don’t just let rocks jut outwards; they attack them, eroding at the sides and the back. Sometimes the focused waves will create a cave, other times they will carve an arch, other times they will just leave a solitary stack of sediments.
This rock was likely once attached to the mainland directly; one reference suggests it broke off in 1393. In fact, groups have landed on this rock via helicopter and found the remnants of ancient structures possibly indicating someone could have lived on this site.
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Loveyourwifenow t1_jaez7ic wrote
I wonder how much geological time is represented by those layers?
Edit: From Stonehammer UNESCO global Geopark.
Doonbristy Rock This layered stack, also called the rock of Dan Bristy or Dun Briste, is found along Ireland’s north coastline. Rocks like these are the product of ancient geologic processes combined with the force of the modern ocean. The layers are sedimentary, part of the Downpatrick Formation. That unit formed during the early Carboniferous, about 350 million years ago, in the ocean off of a passive margin boundary. Passive margins form where continents meet oceans if there are no active plate boundaries (subduction zones or transform faults) nearby. Instead, sediments erode from nearby continents, move offshore, and deposit in mostly flat-lying layers at the bottom of the sea.
The lower layers alternate between fine-grained shales and sandy limestones, indicating marine conditions. Each layer represents some change in depositional conditions and could have taken thousands of years to accumulate. The finer grained shales form in deeper, quiet water, settling out with little to no current to stir up the grains. The sandy limestones are made of the shells of crinoid and coral organisms that can be used to tell the approximate age of the rocks. These organisms likely lived in shallower waters where more organisms could live and reefs could grow. They’ve been broken up before they reached this spot, indicating that this area was probably still deeper water than where corals could live.
The different layers therefore represent different sediment supplies as the ocean depth changed. Deeper water led to only quiet water shales being deposited, while shallower water led to increased supplies of coarse material from shallow levels. Near the halfway point there is also an eroded surface and a slump, probably the remnant of a sediment-carrying underwater channel formed as water levels moved in and out.
The upper portion of the sequence shows thicker limestone beds, indicating that sea level shallowed as the rocks higher in the sequence formed. Some of these are cross bedded and some have been churned up by organisms that live in the sediments on the ocean floor, indicating shallow water conditions where currents could move the sand grains around. Once they were deposited, they were buried for hundreds of millions of years, during which time the slow processes of chemistry took the loose sediments and sealed them together as strong rocks. Today, these rocks are exposed offshore as a sea stack. Sea stacks form as a result of the relentless attack of the ocean waves on the shoreline. When the ocean waves find a weak spot in the exposed rocks, such as a small crack or a section of rock not as strongly lithified, it erodes that spot more quickly. Once the ocean has begun eroding a spot, waves can focus on the edges of that site and widen it, leaving only headlands that jut out into the water.
The waves don’t just let rocks jut outwards; they attack them, eroding at the sides and the back. Sometimes the focused waves will create a cave, other times they will carve an arch, other times they will just leave a solitary stack of sediments.
This rock was likely once attached to the mainland directly; one reference suggests it broke off in 1393. In fact, groups have landed on this rock via helicopter and found the remnants of ancient structures possibly indicating someone could have lived on this site.