In geology people tend to look for the larger features, as they are easier to see, though sometimes smaller things can catch the eye, and then lead to interest and understanding. In some locations, looking at the rocks which make up the anatomy of a wall can give interest, and also learning about geology.ย
We are here to look at an aspect of geology on a macro level. You would benefit from having a camera๐ท or mobile phone๐คณ that can take an image on a macro level, or a magnifying glass๐. The below photograph shows where to look. DO NOT MOVE ANY STONES.ย
The geology of the area.ย
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.
So, what are we here to see?
You would think that so long ago, that there would be no evidence of life from then. A preserved trace of a lifeform, be it animal or plant is known as a fossil.ย It is hard to find Silurian period fossils at times in the area, as they can be quite elusive, also the massive sedimentary forces that occurred and deep ocean environment can mean that at times any evidence is dissapeared into the eons of time. Though at times, a stop for a rest, and just look at what there is, can come up trumps. Here, we have a fossil from the Silurian period.ย
Why is this fossil important?
It is what is known as an index fossil. Index fossils are the remains of organisms that only lived within a certain time period. When found in a rock, they can help date the time period that the rock originated, and thus help to understand the geology that we see.ย
We are here to look at a Graptolite fossil.ย
Graptolites are now extinct, but they lived in the Silurian period, and also the preceeding Ordovician period. They lived in the deep ocean, and as time progressed so did graptolites. They were what is known as zooplankton, and a colonial organism, that initiatilly lived in a collection of tubes, out of which they had branches which they used to filter feed. As geological time progressed, so did the graptolite , with forms evolving to being free drifting in the ocean, and also swimming. Identification of different types of graptolites helps to date rocks in the geological timescale.ย They can also give an indication of water temperature and depth of deposition, so are an important markerย in geological understanding.ย
Anatomy ofย graptolites, and different types.ย
The name graptolite originates from ancient Greek, and means writing on the rock. It can appear like someone has taken a pencil and drawn on the rock.
Graptolite fossils are small, and the psrt of its anatomy that tends to be found is itsย ย branches or branch.ย These were called stipe, or stipes if there was more than one. On the stipes were small cones, which look like teeth, these are called thecae.ย
If a stipe had thecae on one side of the stipe it is described as uniserial, and called biserial if one both sides.ย
ย A uniserial stipe.
A biserial stipe.
Theย literature states that the rocks containย a sparse amount ofย graptolites, though these are indicative of the nilssoniโscanicus zones of 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. Some of the calcareous nodules in the siltstones contain fossilย fragments. Graptolites which tend to be found in the locality are called Cucullograptus scanicus, and Neodiversograptus nilssoni.ย
What are calcareous nodules?
Calcareous nodules are a common site in Horton Formation Siltstones, and in the Upper Ribblesdale locality, they tend to occur in the bottom layers.ย 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.ย
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 describe the graptolite fossil in terms of whether it is uniserial or biserial.ย ย
2. Is it contained in a calcareous nodule or outwith it?
3. What geological layer would you say this piece of Horton Formation Siltstone came from? Is it Top, middle or the base layer?
4. What is your rationale for your answer to question 3?