A couple of months ago, I was writing some educational materials for the Portland Quarries Nature Park on behalf of the Dorset Wildlife Trust. One of my main tasks was to help teachers and children learn about some curious but scientifically important fossils that can be seen in King Barrow Quarry on Portland. When I showed the picture of the fossil to my partner at home and asked him to guess what it might be, I got some very interesting and intriguing replies that ranged from dinosaur footprints to archaeological fire pits. Since he is an engineer, I can forgive those creative interpretations but it did get me thinking about difficult it is to extrapolate a strange rock formation to a living organism or a process that occurred hundreds of millions of years ago.
What is this mystery fossil on Portland? (Image courtesy of Dorset RIGS)
In 1785 James Hutton (charmingly known as the father of geology) began to theorise about such questions. He suggested that the earth was much older and that processes occurring in the present were the same processes that had operated in the past, and would be the processes that operate in the future. This concept became known as uniformitarianism and can be summarised by the phrase “the present is the key to the past.” The concept was controversial at the time as it challenged the theory of catastrophism, which purported that only violent disasters (such as floods) could modify the surface of the earth. In 1830, Sir Charles Lyell published the Principles of Geology which popularised Hutton’s theories and set about an avalanche of change resulting in the eventual publication of the Origin of Species by Charles Darwin.
So back to our mystery fossil….can you guess what it is yet? Using our understanding of Uniformitarianism, these doughnut shaped fossils are actually fossil burrs and look remarkably similar to these strange features actively forming in Lake Thetis, Australia today. The fossils on Portland are the remains of what was once a forest on Portland 140 million years ago. The strange rounded rock features are actually fossilised algae (known as Stromatolites) and in image below you can see the algae growing and living in a lake in Australia.
Stromatolites forming in Lake Thetis, Australia. These formations are the similar to the features that can be seen at the Fossil Forest in King Barrow Quarry on Portland.
So how did the Fossil Forest on Portland form? The forest was periodically inundated with water as it was on the edge of a lagoon. Over time, algae started to build up around the tree roots. Combined with sediment being washed in by the lagoon, the algae grew in thick rings round the tree. Eventually this environment was too challenging for the trees and they died and toppled over. As the sea levels fell further and the water retreated for good, the algae also died and the whole forest floor became covered by mud and then buried by layers of sediment. Through fossilisation, the algal burrs (as the concentric forms came to be known) and the logs turned to stone. With continued erosion the forest floor was once again exposed with fossils ready for us to see today.
Illustration to show how the Fossil Forest on Portland formed (Copyright: Jurassic Coast Trust)
So next time you’re wondering what that mystery fossil might be, think like a scientist and not as an engineer.
Written By Anjana Ford- Programme Manager
