A GEOLOGICAL WALK AROUND LULWORTH COVE
Last weekend Amy and I decided to head down to Dorset to explore and do some star gazing. Well it was really cloudy so we didn’t see a single star, but we still had a great time! We started off by having a look around the Isle of Portland but soon headed back to the mainland to Lulworth Cove.
We’d been to Lulworth Cove together a few years ago and really loved the area. It’s a stunning landscape and is part of England’s only UNESCO site of “Outstanding Universal Value”. A large part of what helped the Dorset and East Devon Coast site to achieve this status is its incredible geology. This includes towering cliffs and ancient fossils.
However, last time we were there I didn’t take the time to learn anything about the geology and I regretted this. So this time, we had a good look at the rocks as we walked around Lulworth Cove. We even bought a printed guide to help us along the way. I thought it would make a nice blog post to share what we found out about the geology of Lulworth Cove!
FORMATION OF LULWORTH COVE
First of all, please forgive me for my “basic” language… I’m not a geologist, but will try my best to explain the geology of Lulworth Cove as I understand it! Here we go….
So, running along the section of coast where Lulworth Cove sits, there are five “strips” of rock guarding the land from the sea. I believe that although these rocks are visible to us at ground level, even forming cliffs in many places, they also reach far below the ground as well! These strips of rock are arranged from the sea to the land as follows: Portland Stone, Purbeck Beds, Wealden Beds, Greensand and finally Chalk. Check out the drawing below if you need help visualising this.
Well, these five rocks haven’t always been arranged like they are today. In fact they all formed under water at varying points over the last 150 million years. As far as I understand, each rock type formed roughly on top of each other with Portland Stone at the bottom and Chalk on top.
However, well after the formation of these five rock types, around 65 million years ago the African tectonic plate crashed into the European tectonic plate causing both the Alps and the Pyrenees mountain ranges to form! Around the same time, this gigantic tectonic plate collision caused these five rock types to push up. The colossal tectonic plate activity twisted these rocks that were originally laying one on top of the other into their current position resting one in front of the other. The oldest rock (Portland Stone) which was originally on the bottom of the pile is now closest to the sea and the youngest rock (Chalk) is now the furthest inland.
So, there the rock formations roughly stood for a few million years with the tough Portland Stone guarding the coast from erosion. It wasn’t until around 10,000 years ago when the most recent ice age was drawing to a close that Lulworth Cove began to take on its “cove” shape.
Either through the action of frozen glaciers or flowing rivers the five rocks were worn away. Starting inland and working through the Chalk, Greensand, Wealden Beds, Purbeck Beds and Portland Stone in turn. This meant that the river or glacier began to flow all the way in to the sea.
Once all five rocks had been breached they were exposed to weathering from both inland rivers and the sea. Pebbles would crash against the rocks with every wave and ice would crack open hairline fractures gradually widening the newly formed valley.
However, because each of the five rocks had varying degrees of resistance to weathering, they did not wear away at the same rate. The softer Chalk, Greensand and Wealden Beds eroded much faster than the tough Portland Stone and Purbeck Beds. This caused the young valley to widen into a cove while the Portland Stone and Purbeck Beds remained relatively untouched.
A WALK AROUND THE COVE
After a quick bite to eat in the car, we walk down to the beach. We turn left along the beach to see if we can see the five main rocks of Lulworth Cove for our selves.
The first section that we came to is the most imposing! Giant white cliffs of chalk covered in green vegetation (wherever it isn’t too steep plants grow). This is the same chalk that makes up Dover’s famous White Cliffs and which covers much of northern France. I did notice however that it seemed quite a bit tougher and less white than the chalk here in Thanet. Much of it had a slightly orange tint which might be from contamination with iron.
The chalk is the youngest rock on display in Lulworth Cove having formed less than 100 million years ago (perhaps around 97 million years ago). The sediment of chalk is actually made up of the tiny calcite shells of a type of plankton called Coccolithophores. There aren’t very many fossils to be found in the chalk but flint can be found in layers.
You can seen from the piles of chalk at the bottom of the cliffs that it is still being eroded to this day. However, each time a new rockfall occurs, it actually protects the bottom of the cliff from further erosion. This slows down the rate of decay significantly, but once the fallen rock has worn down fully the erosion of the cliff will continue.
As we walk further along the beach the chalk cliffs lower somewhat and an obvious change in rock type occurs. Instead of the bright white cliffs we find a dark stone with a greenish hue. On closer inspection I find that the rock is very soft. A firm rub with a finger will scrape some of the sandy sediment off. It’s easy to see where it gets its name from…. it’s green… and sandy.
After a little reading I find out that the sand takes its colour from a mineral called glauconite. Due to the crumbly texture of this rock, cliff falls occur quite frequently in areas of the cove where greensand is present. They are the second youngest rock at Lulworth Cove after the chalk and formed around 125 million years ago. “125 million years young”, right?
Although I didn’t notice any fossils, there are supposed to be quite a lot of fossilised sea urchins and bivalve shells to be found here.
The Wealden Beds are really distinctive due to their bright orange colouring. This colour is caused by the great amount of iron present in this layer.
In Lulworth Cove the Wealden Beds can be seen as low crumbly cliffs with grass growing over them. Similar to the Greensand, the texture is quite soft, especially in areas that have become very wet. This rock is thought to have formed around 140 million year ago at the bottom of a huge freshwater lake.
We found a few bits of fossilised wood emerging from some Wealden rock that was slowly eroding. However, the most common fossils in this rock are supposed to be from the shells of snails and the outer coats of fleas.
Although the rock was generally quite soft and crumbly I also noticed quite a few hard “nodules”. To me they they look and feel a lot like pure iron.
As we continue walking along the beach we begin to get closer to the protective front section of the cove. But before we reach the world renowned Portland Stone we reach the Purbeck Beds. Apparently the Purbeck Beds formed in a large lagoon. Changeable sea levels and variations in temperature during the deposition stage led to variable layers forming. Some layers were much tougher than others and as a consequence they all weather at different rates.
The multiple layers can be seen really clearly due to the tectonic plate collision that has raised and twisted them onto their side. What I think is especially cool is that you can clearly see the effect that the water has had on the Purbeck Beds. The sections that are closest the water (and actually in the water) are worn right down so that the water covers them.
Throughout the Purbeck Beds we found loads of fossils. These ranged from petrified wood to layers of shells and a possible Ammonite imprint!
After clambering over the ragged strata of the Purbeck Beds we eventually get to the outermost Portland Stone. It’s not only the toughest of the rocks at Lulworth Cove, it’s also the oldest at around 150 million years old!
Portland Stone is a fine grained limestone and in my eyes has a yellowish hue to it when freshly broken. However, much of it has a much darker surface which I think might be down to the effects of exposure to air or maybe from algae growing upon it.
You’ve probably seen Portland Stone before as it’s one of the most loved building materials around.
Although we didn’t find any fossils in the Portland Stone, I’ve heard that you can find Ammonites in it. These are just about the most easily recognised fossil. A few miles away in Lyme Regis there are loads of ammonites to be found.
Well, hopefully even though my understanding of the geology in this amazing place is very basic, you’ve managed to get a better idea of what there is to see. I highly recommend that you visit Lulwoth Cove if you haven’t been before. Walk along the beach and admire what’s been 150 million years in the making!
All the best, Joe.
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