The (Long Term) History of St Albans – what’s beneath our feet?

Posted: January 8, 2012 by Mr Bilton in Geology
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Imagine you had a time machine. You park it out on the main lawn and then programme it to go backwards in time. Unfortunately you doze off, and when you wake up 112,000,000 (112 million) years have gone by (that’s 40,880,000,000 days). What would the world look like? Luckily, there’s a way to find out. The rocks beneath the school record millions of years of history, recording evidence of the environments and organisms that have occupied the very spot that our school now fills.

So, what exactly would you find?

The drawing below shows an approximation of the geology beneath the school (the ages are approximate and it’s not to scale).

Approximate stratigraphy beneath the school

The lowest rocks shown in this cross section (and there are deeper, older ones, not shown) are called the Gault Clay. They were formed about 112 million years ago. That was during the Cretaceous period. On the land dinosaurs were going about their business, but at that time most of southern Britain was underwater, covered by a sub-tropical sea.

Postion of continents in the Cretaceous - Britain is shown by the dot

The water was relatively calm, which is why fine sediments like clay could be deposited. In some places, the current was a little stronger and sandy deposits were formed (these became the Upper Greensand).

Marine life in the Cretaceous

If you sat in your time machine, deep beneath the waves, you would have seen a rather impressive array of now-extinct animals – molluscs like ammonites and belemnites, and large carnivorous marine reptiles like ichthyosaurs and plesiosaurs.

A sub-tropical sea - how southern England looked in the Cretaceous period

You set your time machine to tick forwards, slowly, watching things change around you. Around 99 million years ago the seas become deeper and now no material reaches the sea floor from the distant coastline.
Instead, the skeletal remains of algae form a steady ‘rain’ of particles, slowly settling onto the sea floor. These tiny plates of calcium carbonates are produced by algae near the surface, and are called coccoliths.

Coccoliths - tiny plates produced by planktonic algae

Their remains will eventually form chalk, and in Hertfordshire the chalk is approximately 220m thick. Considering that each coccolith plate is 0.0025 mm in size, that’s an awful lot of algae needed to produce a huge amount of chalk (incidentally, the chalk that forms the White Cliffs of Dover is of the same age).
At the end of the Cretaceous movements of tectonic plates formed the Alps in Europe, and the rocks in the entire region were folded, tilted and changed. Southern Britain didn’t escape unaffected, and the sea floor was shoved up until it became land. Then, erosion and weathering worked on the surface, removing some of the rock that had formed on the sea floor.
This continued for millions of years, with little material being deposited except by occasional rivers and floods. However, 400,000 years ago Britain (and the rest of the Northern Hemisphere) was in the clutches of a major ice age (called the Anglian Glaciation).

The extent of the ice sheet during the Anglian Glaciation, 400,000 years ago

At this time an ice sheet covered most of Britain, and eventually, as the ice melted and retreated, dumping sediment, meltwater streams left huge amounts of material over the landscape.
These form the most recent deposits beneath our school. Slowly, a layer of soil built up over these sediments, and then in 1922 our school was built, on top of all that history!
So the next time you’re sitting on the front lawn, spare a thought for what’s beneath you, and where St Albans has been over the last 112 million years.
Want to read more? Information on the Geology of Hertfordshire

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