Has its ups and downs
Outnumber the downs
But not in Nottingham”
Personally, I have to say that having such a big and nice outcrop in the middle of the city is a big ‘up’ to me.
Nottingham, a medieval fortress in northern England. Unfortunately, there is not much left of Nottingham Castle, which was destroyed and replaced by a Georgian mansion. This was really a bad discovery for me, grown up with the Disney cartoon and the Robin Hood Prince of Thieves movies. However, the foundations of the original castle, 40 meters high cliffs of sandstone – a rock consisting of consolidated sand – are still there and continue to resist to the test of time, witnessing the beautiful geology of the area.
These rocks are part of a geological formation known as the Chester Formation and previously called Nottingham Castle Formation. They are very old, dating back to the Early Triassic (about 245 – 250 million years ago). They also show one of my favorite sedimentary features: cross-bedding.
I pretty much dislike definitions of sedimentary rocks you find on the web stating that ‘they are made of flat, parallel layers’. See the sandstones of Castle Rock? They are everything but plane-parallel beds! Layers and delicate laminae in the sand are inclined, they show a curved outline, and they cross at about 20 – 30 ° angles with each others. These sandstones were deposited in an environment where sand was pushed around and being accumulated in inclined structures, like the sand dunes of the desert of the previous post. But the sandstones of Nottingham are not from an ancient desert. This time we are looking at an ancient river, and, more specifically, at a braided river, consisting of a lot of anastomosing channels intersecting with each other and covering a large alluvial plain.
Rivers do not stand forever in the same place. In the geological time (we are talking about millions of years), their channels migrate laterally, dig the earth beneath them, then deposit new sediment. At the end, what remains are these inclined layers of sand, each of them representing the story of birth, lateral migration, and deactivation of a river channel over thousands of years.
Even though these ancient rivers existed millions of years ago and are not visible anymore, there are many structures preserved in the rocks of Nottingham still witnessing fluvial transport of sediments.
In Nottingham, you can find many beautiful outcrops just by walking on the sidewalk, along the streets. Many border parks or small gardens. I like that they are not covered by walls or nets like in many other cities. It is a great example of ‘Urban Geology’.
These urban outcrops allow us to have a closer look at the sandstones. Layers are inclined and sometimes scoop-shaped and they cross each other, separated by planar erosional surfaces, a classic example of cross-bedding. Many of these sandstone layers contain objects that are much larger than sand grains.
Here are some closer views.
These are pebbles of various rocks, mostly fragments of quartzite – a metamorphic rock rich in quartz – and volcanic rocks. They are very useful to understand better the origin of the sandstones. How? Well, prepare yourself to the wide quantity of information a geologist can extract from a pebble.
First, the presence of cobbles in the sandstones tell us that they were not transported by wind. Water is strong enough to transport gravel-sized particles and even boulders, whereas wind generally transports mostly sand. Second, the shape of the cobbles: they are all well rounded and smoothed. Water transport and erosion typically rounds grains by dragging and moving them around against other grains or the riverbed. Finally, the composition of the pebbles: mostly rocks coming from outcrops located hundreds of kilometers away, plus some sedimentary rocks coming from Devonian and Carboniferous formations cropping out in Britain.
These pebbles largely comes from northern France, which was dismantled and eroded by large rivers that flowed towards the North, supplying large floodplains. The cross-bedding and other structures within these rocks, which are found all over the British Isles, also indicate a main direction of the flow from south to north. At the time, 250 million years ago, the supercontinent Pangea was still around and much of the globe was covered by these enormous floodplains and crossed by the scars of the Variscan Orogeny, which left tall mountains that supplied the rivers with sediments. Can you imagine Nottingham in a desert-like river environment with reptiles roaming everywhere? Better come back to the present.
Even if they are very old, the sandstones of this ancient river were never buried much underneath other sediments, and they are still pretty soft! So soft that the locals have carved them for centuries for all sort of purposes, using them as cellars, warehouses, cheap houses, tanning, and even shelters during world war II.
It is nice to visit the tunnels. Even if most of the stuff inside is not original, it is still impressive that people were able to carve underground spaces, sometimes with columns, arches, and chambers. It is also very nice to look at cross-bedding from the inside!
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