Castle Rocks: Dr Oliver Wakefield Digs Into the Geology of Nottingham Castle

Probably one of the most iconic sites in the city, Nottingham Castle is well known worldwide as the mythical centre of the fight between Robin Hood and the Sheriff of Nottingham. Despite this acclaim, the very foundation that the castle sits upon is often overlooked, by which, of course, I mean the rocky cliff of ‘Castle Rock’. Yet, like all rocks, its geology offers some fascinating insights into the city’s past. 

The city of Nottingham stands on an outcrop of Triassic sandstone that forms low hills on the north side of the River Trent. Castle Rock is made of the same sedimentary rock that is located directly beneath most of the city. Indeed, it is this rock, part of a group called the Sherwood Sandstone Group, which hosts the myriad of tunnels and caves for which the city is so well known.  

This sandstone records a period of history which would have looked very different from our modern understanding of Nottingham. This is because the sandstone was originally deposited by a huge river that flowed all the way from Northern France. This river would have been very wide, likely thousands of metres, and it flowed over a barren, hot, desert-like landscape. 

You might not notice as you’re posing for your next picture with Robin Hood, or marvelling at the ancient Gatehouse, but the evidence of this ancient river is recorded in the very rocks we now see in places like Castle Rock, itself a major geological landmark in the city, with its bare sandstone cliff rising to around 38 metres to the castle terrace. 

A quick glance upon the cliff and you will probably see a yellowish rock. But look a little closer, and you’ll find all manner of subtle colour changes from yellows to deep reds and oranges, and also strange curved layers. These layers are interesting as they run contrary to the general thinking that sedimentary rocks are made of horizontal layers like you’d get in a good cake. In fact, if you spend even a couple of minutes looking at Castle Rock itself, it’s relatively difficult to see any horizontal layers in the cliff at all.

Instead, these layers are all curved, some look like interlocking ‘U’s while others exhibit more subtle lines. They are all actually the same thing; examples of huge fossilised ripples that were flowing within the channel of our ancient river. The technical name for these curved layers is ‘crossbedding’. You might already be familiar with seeing sandbars or small ripples in rivers; these are the same but much bigger.   

Satellite images of any of the big rivers around the world often show sandbars in almost every river channel, and it’s very likely that our ancient Nottingham river looked very much like these.

It’s well worth a visit to Castle Rock just to spot all of the beautiful curved layers in the cliff on a sunny day. Alongside the crossbedding there are lots of pebbles scattered throughout the cliff.  Some are small and coin-sized, while others are, as a guest on a tour once pointed out, more potato-sized! They range from milky white quartz to dark, almost black, igneous pebbles, and chocolate-brown mudstone pebbles. These pebbles can, in places, be grey-green instead, a colour change caused by water present in the ground. They are actually very soft and often smeared into a combination of curious shapes. A quick trip into the famous Ye Olde Trip to Jerusalem pub and you can see them with ease. 

One of the hidden gems in all the sandstone outcrops is the fascinating Grade II-listed Park Tunnel; a cavernous tunnel originally designed as a grand thoroughfare for the horse-drawn carriages of an exclusive Victorian residential development, but was allegedly created at a gradient too steep to safely use. Despite this rather mysterious engineering blunder it is without doubt the most pristine sandstone outcrop in all the city, where you will find all manner of wonderful features in the walls and ceilings, and evidence of this ancient river is clearly displayed in the intricately curved layers of sandstone.

Further in from the entrance, you might even spot the big ‘U’ shaped layers I mentioned earlier, which cut deeply into underlying layers, thickening considerably as they do so. About midway into the tunnel, the layers are perfectly aligned either side of a crack, or ‘joint’. It is very likely that the crack formed when the tunnel was originally dug, however the rest of the sandstone is sufficiently strong enough to make up for it.

It is well worth taking the stairs at the exit of the tunnel up onto The Ropewalk, with its vantage point over the Park Estate, where you can see clearly the impact of the sandstone and how it shaped this part of the city. 

Research tells us that Nottingham owes its origin to its combination of geology and geography, which resulted in the presence of the sandstone cliff and enabled the Normans to take advantage of its strategic importance by building a castle on its cliff-bounded summit. It is the geotechnical properties of the sandstone that allowed rock shelters and caves to be easily excavated. 

Added to that, Nottingham is an area rich with an abundance of natural geological resources, so that as the city grew and industries developed, it was supplied by clean water, sandstone, limestone and coal, all within easy transportation distance of the city. By the early twentieth century Nottingham was a major industrial centre of the East Midlands.

Today, our geological understanding of the city continues to be hugely important, not only because building construction in Nottingham continues to take the presence of caves into consideration. As populations grow in the future, solutions founded in geology will be needed to protect our environment, natural resources and the health of the population. All cities, like Nottingham, will face new challenges as they continue to develop and to meet the challenges of a changing climate. Tackling them has to start with a sound understanding of the rocks beneath our feet.

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