Lyme Regis (and a passing visit to Charmouth)

In this second part to the Dorset Fieldwork I will show you all the world famous Lyme Regis Lias outcrops. These were incredibly interesting both geologically and palaeontologically. Our trip leader, Professor David Martill found something very exciting when making our way back to the coach.

We visited the beach to the west of the town of Lyme Regis, Monmouth Beach.

Lyme Regis (with a quick stop at Charmouth):

Map of West Dorset showing the location of Monmouth Beach.
(Source: Google Maps)

The alternating succession of shales and argillaceous
limestones at Monmouth Beach. (Source:
https://www.shutterstock.com/video/clip-5416676-stock-
footage-the-blue-lias-beds-and-limestone-pavement-on-
monmouth-beach-cliffs-lyme-regis-on-the-jurassic.html)

The limestone ammonite pavement, or ammonite
graveyard. (Source: https://chandlerscottage.co.uk/1
125x750-lyme-regis-0772/)

In the morning after a rather lengthy drive from Swanage, we arrived in Charmouth. Although we didn't actually go onto the beach to see the cliffs up close, we got a pretty good view of the geology. I have also visited this site a number of times and will post the fossils I found in my collection series.

When standing at the Charmouth Heritage Centre, if you were to look to the west towards Lyme Regis those dark clay cliffs are called Black Ven. To the east you have Stonebarrow and the Golden Cap.

Charmouth is a Jurassic succession being deposited during the Sinnemurian to Pliensbachian ages of the lower Jurassic, approximately 190 Ma.

Black Ven is highly fossiliferous with ammonites being the most common fossil you will find. If you are very lucky you can also find crinoids, Ichthyosaur and Plesiosaur remains. A complete dinosaur, Scelidosaurus, has also been found here and is on display in the Charmouth Heritage Centre. Another must see is in the fossil shop, where they have on display a large Temnodontosaurus skull.

This is also one of the sites that Mary Anning collected from, finding marine reptiles and the pterosaur, Dimorphodon macronyx, later described by Sir Richard Owen.

After this brief talk we then drove a bit further west into Lyme Regis and walked down onto Monmouth Beach. Here you will notice, similar to Kimmeridge Bay, a repeating pattern in the cliffs between shales and argillaceous limestones, another example of Milankovitch Cyclicity.

But the really interesting thing to see here is in the wave cut platform that is formed by one of these bands of limestone. It is covered in hundreds of ammonites, the majority are of the genus Coroniceras. This was a single event where all of these ammonites died at the same time, this was gradual, possibly they returned to the same site seasonally. Ammonites won't be the only fossil you find here, you can also spot nautiloids, crinoids and gastropods. A complete Plesiosaur was even found in the pavement some years back.

If you wet the limestone around an ammonite you will find dark marks in the rock. These are fossilised burrows. Some are branching 'Y' shapes, this is formed by a shrimp, the name given to these burrows is Thalassinoides. Others will be two circles next to each other (Diplocraterion), straight burrows (Planolites), and some very fine burrows. These very small burrows are called Chondrites and show that the water had become anoxic for a period.

On the walk to the next locality, Professor Andy Gale found a large nautilus, preserved in three dimensions, a very nice find indeed. It was picked up by a course mate on the way back to the coach.

The next site was geologically of interest. This was the boundary between the limestone dominated White Lias, to the shale of the Blue Lias. The interesting part is, lithologically speaking the boundary is very clear, you can see a pale limestone become a dark shale. You would be forgiven for thinking that this is simply the boundary between the Rhaetian of the Triassic and the Hettangian of the Jurassic.

But to date the rock to confirm this is very difficult. The use of lavas of similar ages are regularly used to radiometrically date rock, however, there are no lavas to use. So dating could be done with the fossils, using relative dating. The change in the ammonites present between the White and Blue Lias represents a biological boundary. This is higher in the succession than the lithological boundary.

On the way back, Professor Dave Martill came across some Ichthyosaur remains in a limestone band of the Blue Lias. Unfortunately I did not see it but there was enough articulated material to warrant him going back to recover it.