The area between Stainforth, Austwick and Helwith Bridge, holds many an area of geological interest. Within the area are some of the oldest rocks in Yorkshire. These are known as the Basement Rocks. These were formed during the Ordovician and Silurian periods, when the area was under a deep ocean known as the Iapteus Ocean. The ocean was between two ancient continents, and sediment was laid down on the ocean floor, forming sandstones and siltstones. As the two ancient continents moved together the layers of sedimentary rocks were subjected to immense pressure and heat, and their once horizontal layers were pushed upwards and deformed. Then during the period known as the Caledonian Oregeny, the geology was further altered. The rocks that form the area, are made up of sandstones and siltstones.
We are here, to learn about and examine a type of siltstone known as Brathay Formation Siltstone. It is one of the Basement Rocks. It outcrops here on the east side of Upper Ribblesdale, and also in Crummackdale. It is found in greater quantities in the area around Ambleside in the Lake District.
For the purpose of the EarthCache, you need to be at the location indicated in the photograph, more specifically where the hand of Orinoco rests.
A visitor from West Yorkshire, a geocacher known as Orinoco, on a geological tour visiting the EarthCache location.
Brathay Formation Siltstone, is a Hemipelagite. So, what you may ask is a Hemipelagite?
A hemipelagite is a type of rock which is very fine grained, so it has very small particles, and was formed in a deep marine environment, though near the edge of a continental shelf. It has a combination of biogenic and terrigenous materials, and is formed from the fine sediment which has come from both the land and sea.
Biogenic materials are made up by organic matter, such as small particles of dead sea creatures, whilst terrigenous materials are made up of clay, sand or silt. So basically, the siltstone is a mixture of different materials.
Brathay Formation Siltstone, is part of a group of rocks known as the Windermere Supergroup, though more specifically it is part of a subgroup known as the Tranearth group. During this period sediment was deposited in a marine basin, though was anoxix, meaning there was lack of oxygen.
What is special about Brathay Formation Siltstone, is that it has a different appearance according to the time of year it was deposited. So, here you really are looking at a geological time record.
There are darker layers, and lighter layers. These layers are known as laminations. The darker layers represent the warmer months, when algael bloom occured, and as a result more organic material was deposited. The lighter layers represent the cooler months when more sediment was deposited.
This being an EarthCache, in order to log it, I ask that you complete the above tasks. Please send the answers 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 Brathay Formation Siltstone, namely the area of Orinocos hand. Which has the thicker laminations, the lighter colour or darker colour?
2. What is the rationale for your answer if you think about the marine environment at the time of deposition?
3. What type of rock is Brathay Formation Siltstone?
4. Please have your photograph taken, or that of your device in the area, but please do not reveal any answers. Even better, please pose like Orinoco...DO AN ORINOCO.