Conglomerate or Concrete? EarthCache

Introduction

I love shingle beaches 😍. I can happily spend ages perusing the pebbles, trying to identify the various rock types and looking for typical examples of each.

One rock type that I have always struggled with though is conglomerate - basically because I've been unable to decide with any confidence that the fragment of rock I'm looking at, which has a similar appearance to conglomerate, isn't just man-made concrete. And that's mainly because conglomerate and concrete are similar in a number of ways.

I concluded that the only way to ever be certain that I was looking at conglomerate was to find some in situ, where it had been deposited / formed, rather than as a loose fragment, found on a beach.

Imagine my delight when one day, out on a walk, I happened upon a lovely example quite by accident! .

This EarthCache takes you to a nice, accessible exposure of Sedbergh Conglomerate.

The EarthCache location is not a shingle beach - it's actually the bank of a narrow stream which, at least on the day I visited, was flowing pretty quickly over boulders and cascades and, in combination with the bank's steep angles, makes for a location where you'll need to take care with your footing. Sturdy, waterproof footwear with good grip is recommended.

Logging Tasks

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Based on your observations at the cache location and the information on the cache page, please tell me:

1. The MATRIX element of the conglomerate is predominantly one colour, which is especially visible in the vertical banking that's being worn away by the small waterfall / cascade. What colour is it?
   
   
2. To what degree would you say this conglomerate is matrix supported or clast supported? Is it the same everywhere or does it vary depending on where you look?
   
   
3. Using the Wentworth scale, what's the Wentworth grade size of the FRAMEWORK clasts here?
   
   
4. 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.
   
   

Background

Let's start with some basics:

• A clast is a fragment of rock - any rock
  
  
• Clasts come in a huge range of sizes, from large boulders right down to grains so tiny they can barely be seen with the naked eye
  
  
• Clastic sedimentary rock is rock made up of collections of individual clasts, cemented together into a single, solid mass
  
  
• Conglomerate is a clastic sedimentary rock typically made up of clasts of two distinct size groupings:
  
  
  • Framework clasts - larger than 2mm in size
    
    
  • Matrix clasts - smaller than 2mm in size
    
    
• Framework clasts in conglomerate are typically rounded to some degree - soft edges rather than sharp edges
  
  
• Through a natural process known as lithification this whole assemblage of clasts becomes solid rock as a result of being compacted and then stuck together by the growth of mineral crystals
  
  

In summary, a collection of larger, rounded clasts, surrounded by smaller clasts, squeezed together under pressure then stuck together into one solid mass by mineral crystals = conglomerate

Up to this point we've described the recycled fragments of rock which get stuck together to make new sedimentary rocks as clasts but these fragments are also commonly called grains or sediments and these words are commonly used interchangeably.

Sedbergh Conglomerate

The small town of Sedbergh, nestled at the foot of the Howgill Fells, sits on top of an area of conglomerate which consists of framework clasts of Silurian age (around 420 to 440 million years) surrounded by and cemented together with matrix clasts from weathered granite.

This particular conglomerate forms the base of a series of rock layers laid down during the Carboniferous Period which lasted from around 360 million to 299 million years ago. If you do the math you should realise that what we have here is older rocks and their component parts being recycled to make newer rocks - a process which is happening constantly on Planet Earth.

Back in the Carboniferous the world looked quite different from how it looks today. The various individual plates of rock which make up the Earth's crust and thus the individual continents that we recognise were moving towards each other until they eventually converged into a single super-continent known as Pangea. What we know today as Great Britain, a comparatively small island surrounded by sea, ended up land-locked, somewhere in the middle of Pangea, very close to the equator and thus subject to a hot, arid desert climate subject to frequent flash floods which left deposits which later became the sedimentary rock which we see at the cache location.

The colour of the Sedbergh Conglomerate today is typical of sedimentary rocks with a history of hot desert conditions.

Rock Classification

Scientists have developed such an extensive array of ways to describe and classify distinct rock types and formations that amateur geologists and other mere mortals can be easily forgiven for being quite bewildered by the complexity. Not that this is a bad thing - it's very useful for quite practical things like identifying which rocks might contain useful or valuable stuff like gold, diamonds, oil, stuff that can be used to make fertiliser or medicines, and fundamental basics essential to life such as drinkable water.

And of course, as we extend our quest for knowledge out into outer space these classifications, and scientists understanding of how these rocks came to be, become useful for understanding how other planets were formed and what resources - including life supporting ones - might exist there and where to look for them.

But fear not - for this EarthCache you need only concern yourself with a couple of fairly simple classification types .

Grain Size

Sedimentary rocks form when sediments are deposited and lithified and they can be classified based on the size of their grains.

Geologists determine grain sizes in the field using printed cards called comparators, which usually have a millimeter scale, phi scale, and angularity chart. They are especially useful for larger sediment grains.

One scale of measurement commonly used to classify sediments / clasts / grains by size is the Wentworth scale, published in 1922 by Chester K. Wentworth. Wentworth's grades and sizes were later supplemented by William Krumbein's phi (from the greek letter φ) or logarithmic scale, which transforms the millimeter number by taking the negative of its logarithm in base 2 to yield simple whole numbers.

Here's a simplified version of the Wentworth scale, including the φ grades, which you can use to classify the clasts in the conglomerate at the cache location:

Matrix Supported or Clast Supported?

If you've followed everything so far you'll know that the small rock fragments that make up clastic sedimentary rocks are called clasts - regardless of size - and that in conglomerate, clasts smaller than 2mm in size are known as matrix clasts and clasts larger than 2mm in size are known as framework clasts.

So what do we mean by matrix supported or clast supported when the whole rock is made of clasts? Surely those two terms mean the same thing? Well, no, they don't - in this context the meaning is slightly different and, as a picture paints a thousand words, I think a picture here will be the simplest way to clarify things:

So it seems that when geologists describe a rock as being matrix supported or clast supported, what they mean by clast is framework clast. In other words, for this purpose we need to think of the large grains in the rock as clasts and the tiny grains in the rock as just matrix .

In the case of conglomerate, the rock is considered matrix supported when the clasts constitute less than 15% of it's volume. Anything above that and the conglomerate is classed as clast supported or grain supported if you prefer .

In the above image the conglomerate on the left would be considered a paraconglomerate because the clasts are not in contact with each other and are completely surrounded by matrix. The conglomerate on the right of the diagram though features clasts which are in contact with each other and the matrix is merely filling in the remaining gaps, so that conglomerate would be considered an orthoconglomerate.

If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there .

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