Introduction
You might be surprised to learn, as you stand at Rivington Pike, that the rocks beneath your feet - and the rock used to construct the pike tower itself - were formed around 312 million years ago, during the Carboniferous Period when the area that is now Britain was just a little way north of the Equator and was part of a large landmass consisting of what is now North America and continental Europe.
The rock beneath your feet is Margery Flags and the rock used to build the tower is known as The Rough Rock. Both are types of sandstone which is a sedimentary rock but these two sandstones are quite different in certain ways which are clues to variations in the conditions in which they were formed.
Logging Tasks
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- In some detail, describe the Margery Flags including such properties as colour(s), particle size(s), surface texture(s) and any visible layering (pay particular attention to exposed edges).
- In some detail, describe the Rough Rock including such properties as colour(s), particle size(s), surface texture(s) and any visible layering.
- Comparing the Margery Flags and the Rough Rock - what is the smallest and largest particle size in each?
- Based on your observations and comparisons, which of these sandstones was laid down in a higher energy environment and what makes you think that?
- Some of the slabs of Margery Flag are buried in the ground so that only their top face is showing. There are some slabs though which are exposed edge-on so you can see how thick the slab is - I've provided a waypoint to help you find them. Please measure one of these slabs and tell me how thick it is.
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
How are sedimentary rocks formed?
Basically, sedimentary rocks are formed when sediments are deposited and then compressed and cemented (lithified) into solid rock. Those sediments are fragments of other rocks which have been broken down by the processes of weathering and erosion and then transported by some mechanism such as wind, water or gravity before being deposited and then lithified into new rocks.
How were these rocks formed?
In order to answer that question we need to think about the conditions at this location in the Carboniferous Period.
Large river systems drained from an extensive mountainous region, carrying quantities of sands and gravels which formed huge deltas divided by many channels The environment was not unlike the Mississippi delta today.
The rate of the flow of water and sediments through these river deltas and channels would dictate the size and type of sediments that were laid down and thus which types of stone were formed.
Simply put, faster, high energy water flows can carry larger, heavier sedimentary particles. As the rate of water slows down so does its energy and ability to carry the sedimentary particles and so sedimentary particles begin to be deposited - larger, heavier particles first and then progressively smaller particles, as the water continues to slow down.
This means of course that the particle sizes in the Margery Flags and in the Rough Rock tell us about the speed / energy of the water they were carried in and deposited by - higher energy = larger particles and lower energy = smaller particles.