Smiling Syncline 😁 EarthCache

Introduction

We've been doing quite a bit of Earthcaching lately and Mrs M and Junior have enjoyed visiting numerous locations with interesting things to see. I remember standing in a Zonal Quarry looking at some tilted rock strata with junior, trying to explain how they were actually on a gradual curve and would turn around and come back up again - but that the curve was underground and might be miles long, which made it difficult to visualise easily.

Fast-forward to a month later and we're dashing down a beach-side path toward the Kent Estuary Tidal Bore and I notice something that brings a smile to my face - and make a mental note to check it out on the way back to the car later .

The reason I was smiling was that I'd noticed in the sea cliff of Great Scar limestone a big, white smile . Okay - maybe not a smile but certainly a smile shape. The limestone layers which had orignally been laid down horizontally in a warm, shallow sea approximately 326 to 343 million years ago had been compressed into a nice, flowing curve and we could see all of it in that short space and I was finally able to show Junior what we'd been talking about a month previous, standing in that quarry, looking at those tilting beds of Millstone and Kinder Scout Grit.

So for this Earthcache we enter the realms of structural geology - the study of the deformation of rocks and the effects of this movement.

Logging Tasks

IN ORDER TO COMPLETE THESE LOGGING TASKS PLEASE EMAIL YOUR ANSWERS TO US VIA OUR GEOCACHING PROFILE BEFORE SUBMITTING YOUR LOG. PLEASE DO NOT INCLUDE ANSWERS IN YOUR ONLINE LOG. YOU CAN GO AHEAD AND LOG YOUR FIND AS SOON AS YOU HAVE SENT YOUR ANSWERS IN ACCORDANCE WITH GROUNDSPEAK GUIDELINES. LOGS WITHOUT ADEQUATE LOGGING TASK EVIDENCE MAY SUBSEQUENTLY BE DELETED.

1. Looking at the folded layers of limestone which make up the syncline and remembering that they were laid down horizontally, which stress do you think reshaped the rock as you see it today? Was it tension, compression or shearing stress?
   
   
2. What is the maximum angle of curvature away from the horizontal of the syncline's limbs? I measured this using a clinometer app on my smartphone - there are plenty to choose from. Or you could make an inexpensive clinometer from basic materials that would do the job just as well
   
   
3. Considering the curvature of the rock, would you say the syncline is an example of brittle behaviour, ductile behaviour or a mixture of the two? What observations did you make to arrive at your conclusion?
   
   
4. Taking into account your responses to all of the previous logging tasks, do you think this rock was always at the surface, as it is now, or do you think it might have been deeper / had more rock on top of it? How did you come to this conclusion?
   
   
5. Do you think the deformation of the rock that has taken place is elastic or plastic deformation? How did you come to this conclusion?
   
   
6. 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.
   
   

Steno's Principle of Original Horizontality

As recently as (in geological terms) 1669 a Danish scientist by the name of Niels Stensen, better known by his Latinised name of Nicolas Steno put forward three defining principles of the science of stratigraphy - the branch of geology which studies rock layers (strata) and layering (stratification), primarily used in the study of sedimentary and layered volcanic rocks. These laws or principles, known as Steno's Laws or Principles are still in use today

. . . strata either perpendicular to the horizon or inclined to it, were at one time parallel to the horizon.

Steno reasoned strongly tilted rocks did not start that way - that all rock layers (strata) started off essentially horizontal, but were affected by later events—either upheaval by volcanic disturbances or collapse from beneath by cave-ins. Today we know that some strata start out tilted, but nevertheless this principle enables us to easily detect unnatural degrees of tilt and infer that they have been disturbed since their formation. And we know of many more causes, from tectonics to intrusions, that can tilt and fold rocks.

It stands to reason that because of the way sedimentary rocks are formed, under the force of gravity, that the layers in them would all be flat / horizontal, and many sedimentary rock deposits do indeed look precisely that way - so what would cause them to be otherwise? What would cause them to look like the rocks you will see in the process of completing this EarthCache?

Stresses in the Earth's Crust

The Earth's crust changes shape in response to different types of stress which arise from phenomena such as the movement of individual rock plates that make up the crust. Three types of stress are:

• Tension
• Compression
• Shearing

Before we look at those three different types of stress let's imagine a series of rock layers (strata) which haven't been subjected to any stresses and let's assume that they were laid down horizontally, just as Steno said they should be:

Here we see a stratigraphic section - a sequence of sedimentary layers stacked one atop the other.

The layers here are shown in different colors, suggesting layers of different types of rock but stratigraphic units are often made up of individual layers of just one rock type too. The rock on which this Earthcache is based is made up of individual layers of carboniferous limestone.

The sediments have been laid down horizontally under the force of gravity and at this point have not been subject to any of the stresses which can be found in the Earth's crust.

Tension

According to the scientific theory of plate tectonics the rock plates which make up the Earth's lithosphere (the crust and upper mantle) move around - although much too slowly to be seen with the naked eye .

When two of these plates move away from each other the rock between them is pulled in two directions at the same time, creating tension in the rock. That tension causes the rock between the plates to stretch out and become thinner in the middle. The blue arrows in the diagram represent the divergent forces, pulling the rock simultaneously in two different directions, resulting in the thinning in the middle section.

Compression

Compression arises when tectonic plates move toward each other. The mass of the moving plates is so great that the convergent compression forces are strong enough to squeeze the rock between them until it either folds or breaks.

Whether the rock folds or breaks under compression is not dictated by the compression alone. The other factors which can impact on this outcome are important to the logging tasks for this Earthcache and will be covered further down the page.

Shearing

Stresses which are offset from one another and push a mass of rock in two opposite directions at the same time are known as shear stresses.

Shearing is a type of stress that differs from tension and compression. Tension pulls the layers in the rock and causes them to become thinner in the middle. Compression squeezes the layers in the rock and causes them to thicken or to fold or to break.

Shearing on the other hand can cause masses of rock to slip past each other as they are pushed or pulled in opposite directions by opposing forces.

Folded Rocks - Anticlines and Synclines

When I looked at the curved white limestone at GZ I thought it looked like a big smile - so that's how I described it but geologists use much more scientific words to describe upward and downward folds in rock strata

The axial plane is an imaginary line which bisects the curved rock unit (or cuts it down the middle, if you prefer). The curved layers of rock either side of the axial plane are called limbs. In simple terms*, a curve where the limbs run down toward the axis is typically known as a syncline and the opposite case - a curve where the limbs run up toward the axis is typically known as an anticline.

An easy way to remember which is which as that Anticlines form an A shape and Synclines form the bottom of an S shape. Or if you prefer you could use my method - S is for Syncline and S is for Smile

*By definition an anticline has the oldest layers of rock at its centre or core and a syncline has the youngest layer of rocks at its centre or core. If a unit of rock is inverted (turned upside-down) before being folded that would result in synclines with limbs which run up toward the axis and anticlines with limbs which run down toward the axis - but the means to work out whether or not the layers of rock at the published coordinates have been inverted since they were laid down is beyond the scope of this Earthcache - so we are going to assume that the layers have not been inverted and thus that the smile shape is a syncline

Properties of Materials - Brittle vs Ductile

We tend to think of rock being a pretty hard material, which it is, but at the temperatures and pressures found at the Earth's surface it is also brittle and will tend to break rather than bend when subjected to large amounts of stress. At greater depth though, deep down below the Earth's surface it's a different story. As depth increases, temperature and pressure increase and under greater temperature and pressure rock tends to become more ductile - more able and more likely to bend than break.

Properties of Materials - Elastic vs Plastic

At the temperatures and pressures found at the Earth's surface rock tends to be brittle - but if subjected to sufficient stress rock will still bend - but only so far - and will tend to spring back to its original shape if the stress is released, just like a piece of elastic - which is why this type of deformation of the rock is known as elastic deformation.

Elastic deformation will only allow the rock to bend so far before it breaks. The point at which the stress causes the rock to break is known as the yield point or the elastic limit.

Deeper in the Earth's crust, where temperatures and pressures are higher and the rock becomes more ductile it can bend beyond the elastic limit without breaking. At this degree of bend the deformation is known as plastic deformation. Rock which has been subjected to plastic deformation beyond the elastic limit tends to retain its deformed shape rather than spring back to its original shape - the deformation becomes permanent.

That was a lot of words and terminology so let's summarise that with a simple diagram:

If you've made it this far and can apply everything you've learned to the smiling syncline at the published coordinates then you're ready to tackle the Logging Tasks - good luck!