The geology of the area is varied and interesting, though there are items of a variety of sizes. This is one of them. You need to look at the rock indicated in the below photograph. There is no need to move any rocks.
The geology of the area.
The rocks that you can see where originally laid down in a deep ocean, during the Silurian period. The ocean, known as the Lapteus Ocean, was situated south of the equator, between two continents known as Avalonia and Laurentia. Sediment from these continents was washed down into the Lapteus Ocean, where it settled as sandstone and Siltstone. Lighter sediment was carried further out to sea and deposited as mudstone on the deep ocean floor. Movement of continental plates caused earthquakes, and this caused the heavier sediment of sandstones and mudstones to be carried at high speed by the water into the deeper ocean.
What are we here to look at?
We are here to look at a calcareous nodule. Calcareous nodules are a common site in Horton Formation Siltstones, and in the Upper Ribblesdale locality, they tend to occur in the bottom layers. They can be rounded or oval in shape, and tend to be grey in colour, and are bedded with in the Horton Formation Siltstones. They are formed of carbonate material. They were deposited during the Gorstian Stage of the Ludlow Series. These were substages of the Silurian period. Towards the base of the rocks of this period, which are Horton Formation Siltstones, there tends to be calcareous nodules, which occur in layers.
Why are we looking at this calcareous nodule?
We are here to look at, and learn about Internal Zonation.
Internal Zonation.
Internal zonation relates to the development of a nodule, basically what it is made off, and how it developed. We know that the background geology is sediment which was laid down in a deep marine environment, so how did the calcareous nodule start?
The literature states that the nodules are made up of carbonate material, so we will just concentrate on that progress.
The progress of nodule formation.
Nodules don't just appear out of nowhere, they need a nucleus to form. A nucleus is the start of a geological body such as a nodule, and could be described as the centre. It has to come from somewhere.
The following can be the source of a nucleus in a nodule:
Plant debris.
These could be branches, twigs, leaves, seeds and rootlets. The plant might be a plant that lives in a marine environment, or part of debris washed into ocean environment from the land.
Shell fragment(s).
These could present as whole shells, or fragments. Underwater currents can move items such as shells, and they can be fragmented as a result of being smashed against other items, such as other shells and rocks.
Fragments of rock.
There will have been geology millions of years old when the Horton Formation Siltstones were originally laid down. After all, the Iapteus Ocean was between two continents, with mountains, rivers, and hills. All of which would have had rocks as part of their structure. As happens today, rocks get eroded, they break up and fragments get moved, by events such as floods and landslides, the same happened during the Silurian and Ordovician periods. There could be angular fragments of rock forming the nucleus, or pebbles, some which could have sedimentation lines in them.
Precipitation and Accretion.
When sediment is deposited in a marine environment it can be saturated with water. This water can be saturated with minerals such as calcium carbonate, with precipitation the minerals come out of the water and form a solid mass. These minerals then form around the nucleus. Am example of water saturated with minerals is in the limestone areas of the Dales, due to a high calcium carbobate content, eventually this will precipitate out of the water and form limescale on the lining of pipes, and in kettles.
Accretion is the forming of layers of minerals, such as calcium carbonate, around the nucleus. These layers can be many.
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. Please examine the specified nodule, please describe which type of nucleus source has occured here.
2. What is your rationale for your answer to question 1?
3. Can you see any evidence of accretion, if so, what?
4. Please have your photograph taken, or that of your device on the nearby stile.