Fossils on Friday – Crossbedding & Horizontal deposition.

Welcome back to Fossils on Friday!

Over the last few weeks since we started up again, we’ve been focussing on the red sandstone from both the USA and the UK, and comparing them both. Then last week, I introduced a new concept – the idea of crossbedding and horizontal deposition. But what is this, and what can we learn from it? Over the next couple of weeks, before we wrap up our adventure into the red sandstone, we will explore this important concept in geology, and what it means regarding how the rocks formed.

Many of you may have seen a picture or diagram similar to the one below. This is called a ‘Geologic Column’.

An example of a geologic column with fossils.

Now the idea of a geologic column was first thought up way back in the 1600s. Basically, the idea is that when sediment settles into rock layers, the bottom layer forms first, then the next layer, then the next. In other words, the bottom layer got there first, and the top layer got there last. In fact, some of you may even have done the experiment where you get a glass of water, put in some rocks, pebbles, sand and mud, shaken it all up, then watched it all settle into layers – bottom layer first, top layer last.

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The ‘Sediment in Jar’ experiment

Now, many people believe that the jar experiment shown above is an example of how all rock layers in the world formed – bottom layer first, top layer last. And the experiment would be a wonderful example if the world was a glass of water – but its not! As it turns out, it is a very BAD example of how layers form…..

You see, the only way you can create rock layers is if you have sediments in water. But the sediments are heavier than water – so the only way to move these sediments into place is by flowing water – water moving horizontally. The speed that the water is flowing will determine the total amount and the maximum size of the sediment being carried.

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A flowing river carrying sediment is a good example of horizontal flow.

Think of when you go to the beach, and paddle in the sea. A big wave comes in, and covers your feet! As the wave goes back out, you feel thousands of little grains of sand hit your feet. The water moving sideways, horizontally, is pulling all those grains of sand along with it – until they hit your toes! Those grains of sand are not dropping down from above, like in the glass of water, but are flowing horizontally along with the water flow.

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Just like at the beach, just like with a river, everything we observe says that sediments travel sideways with the water to be deposited, they don’t settle straight down. This is important, especially when considering how rock layers form. Think about it: If the bottom layer got there first, then that is the oldest layer. And the top layer is the youngest. That means that, if sediment takes many, many years to settle, then the bottom layer must be much older, even millions of years older, than the top layer. But all this is reliant on the bottom layer getting there first, which is almost impossible in the real world!

Now, next week we will be looking at some unique experiments that I have been a part of to really show you how this all works. But to leave you with a tantalising taste of what’s to come, the video below shows one of the experiments in slow motion. See if you can guess how it works!

See you next week!