Rock of Ages!

Time is Money… and a whole lot more!

They say that time is money, but on the Jurassic Coast it can be a lot more than that. With a handy six million years you can end up with an iconic building stone, a beguiling petrified forest, a persistent geological mystery and a fossil treasure trove.

Rock of Ages
There is no doubt that Portland Limestone is one of the most iconic buildings stones in the world. Its pale, creamy colour and fine-grained texture have made it desirable for carving, building, grand architecture and monuments since Roman times. Listing all the places it has been used around the world would take pages but highlights include St Paul’s Cathedral and the Cenotaph in London, and the UN headquarters in New York. On the other hand, listing all the places where Portland Limestone can be found takes just one word – Dorset. Aha, no, not ‘Portland’ as you might guess. Portland Limestone is actually also quarried in Purbeck, at the eastern end of Dorset. But the finest building stone comes from the Isle of Portland, near Weymouth. This most famous of limestones was laid down in a shallow tropical sea around 146 million years ago, as the Jurassic Period drew to a close. Sea levels had been falling for some time and it wasn’t long before the tropical sea disappeared altogether, replaced by coastal forests and lagoons.

The West Weares of Portland expose the Portland Limestone layers in spectacular fashion.

A fossil breeze
Once the sea retreated at the end of the Jurassic Period a soil developed and a forest grew. The tress would have mostly been primitive conifers. The environment was so changeable that it wasn’t long before a lagoon flooded the forest and killed the trees. Some were fossilised where they stood, the base of the trunks encased in hard limestone shapes called ‘burrs’. These were actually created by colonies of algae and are properly called thrombolites or stromatolites – satisfying words to roll around in your mouth. Samples of fossilised tree stumps have been taken whilst they were still in situ, that is in the original position where they once grew million years ago. Looking at the samples under the microscope it’s possible to see tree rings and even the wood texture preserved. Armed with the right measurements, it’s been possible to work out the prevailing wind direction when the trees were alive… a 145 million year old breeze, preserved in rock.

An Unsolved Mystery
Over the course of the next five million years the environment changed rapidly between swamps, lakes and lagoons, each depositing its own layer of rock. These layers are collectively called the Purbeck Limestone Group and to say that they are varied is an understatement. Some scientists have spent their lives studying these rocks, trying to make sense of them. One mystery that refuses to go away is the exact position of the boundary between the Jurassic and Cretaceous periods. Its somewhere in the Purbeck Limestone Group, but no one has been able to say precisely where. Fossils are a very useful in these instances as they are often used as markers for geological boundaries. A bit like page numbers in the great book of time. However, you need the right fossils… and guess what? The Purbeck Limestone Group simply doesn’t have them. Other evidence has narrowed the search so we’re able to say the boundary is within the oldest layers, but until fresh data or new analyses are available we just have to make do with waving an arm vaguely at the right layers and saying “oh the boundary? … it’s there… ish”.

Layers of the Purbeck Limestone group exposed at Stair Hole near Lulworth. The Jurassic-Cretaceous boundary is in there somewhere!

 

 Fossil Treasure Trove
The most important exposure of the Purbeck Limestone Group is found in Durlston Bay. Not exactly a safe place to visit, but nonetheless one of the most important sources of early Cretaceous reptile and mammal fossils in the world. Remains of crocodiles, turtles, primitive lizards, dinosaurs and pterosaurs have all been found there, each contributing an autobiography to the great library of life on Earth. Dazzling repositories of fossil evidence, like Durlston Bay, are key to how we piece together the evolution of animals through time. That was highlighted this year by the discovery there of two new species of primitive mammal. The finds were of tiny fossil teeth, only a few mm in size; a good example of a specialist find. The researchers found them in bulk samples of clay that were washed, sieved then searched under a microscope.

 

The fossils hunting part of this discovery happened in a lab. The teeth were found to come from a very special class of mammals called Theria. As Therians evolved two important ‘infraclasses’ emerged – the Metatherians, which includes marsupials, and Eutherians, which includes all placental mammals. Most mammals today belong to one of these infraclasses. Humans are Eutherians. These newly discovered specimens show features that suggest they might have come from animals in the Eutherian infraclass. Fossils of Eutherian mammals aren’t unusual… in much younger deposits. The fact that they have been found in rocks almost 145 million years old is astounding! In fact they are the oldest fossil evidence of the type of mammals from which humans are directly descended. This isn’t the first extraordinary find from the Jurassic Coast, and with 186 million years exposed across 95 miles of coast, you can bet it won’t be the last.

 

This new species of crocodile was recovered from Durlston Bay in 2007.