Recording an extraordinary Dinosaur trackway at Purbeck, Dorset
Rose Ferraby, Dominic Powlesland
In 1996 during quarrying work in an area then operated by Keates Quarry, Purbeck a remarkable set of dinosaur footprints were discovered which were to turn out to be the one of the most extensive exposures of dinosaur footprints in Britain. This trackway of footprints was discovered around 140 million years after they were originally tramped into the mud. They are the prints of Sauropods, either a brachiosaurid or a tatanosaurid (Wright 1997:185). The prints are comprised of circular craters in what was originally mud, created over a period of time as the sediment dried out on a beach next to a freshwater lagoon. During that time, a number of different individuals, including a juvenile, walked over this patch. The prints were then sealed by the swift deposition of sediment over the top – perhaps the result of a storm – where the mud was transformed into stone which ensured their preservation.Many dinosaur footprints have been found both on the coast of Purbeck and in the quarries, but these sauropod tracks are the only place where a great number can be seen still in the bedrock, where they have lain for millions of years.
The footprints are found within the Purbeck Freestone, one of the many different limestones that form the Purbeck beds. The late Jurassic and Cretaceous periods were marked in Purbeck by subtly changing environments. As a result, it is a geological landscape formed of many layers of stone, whose characters vary greatly both between and across them. As well as this variation in geological formation, subsequent processes have tipped, rippled and eroded the stone, making the Purbeck beds a real challenge to understand. As well as the geologists, the quarrymen here have a very detailed and tacit understanding of this geology. The limestones in Purbeck have made it a site for intensive quarrying since the Roman period ( if not before). In the process of quarrying, many geological discoveries have been made, from Beckles mammal pit at Durlston in 1857 to the more recent discovery of fossilised trees and turtles at the Haysom’s Quarry.
These footprints were first noticed when Kevin Keates was quarrying the Freestone, a limestone valued for its ability to be worked in any direction. The Upper and Lower Freestones are separated by a bedding plane and it is here that the footprints were preserved, on the surface of the Lower Freestone. After the Keates had stripped off the Upper Freestone, the top surface of the Lower Freestone was left exposed. It was then that Trev Haysom, on a winter walk, noticed depressions which he thought were footprints and alerted Kevin Keates to the discovery. The low sunlight defined the prints, and having seen them in other nearby quarries, Trev was able to identify them as dinosaur footprints. Geologists were brought in, and it was decided to preserve the site. Quarrying ceased and the footprints were temporarily re-buried with a protective covering of fine gravel and long term responsibility for the site handed to the National Trust, who own the land.
In 2013, the decision was made to uncover the footprints once more whilst the National Trust and the Jurassic Coast World Heritage Site Team worked together to develop a long term management strategy. The whole surface covering an area 34 x 24 metres at its greatest extent was meticulously cleaned by National Trust volunteers but this raised the question of exactly how to document, preserve and display them?
It was decided that of high priority was to gain a detailed record of the prints, since it was uncertain how they might weather in the exposed Purbeck landscape. Digital Photogrammetry or 3D imaging (3Di) was chosen as a method which would not only allow a detailed record of the footprints to be created but would also record the topography of the whole of the limestone surface. Recording in this way would ensure that these unique features could be viewed by the wider public and researchers alike, using an internet browser on a computer connected to the internet anywhere in the world. The method also allows for the creation of standalone 3D models which could be stored in 3D format in Adobe PDF files which can be freely downloaded by schools and other interested folks who do not have live access to the internet.
To record the footprints an Olympus OM-D EM-1 digital camera was mounted on a 6m high photo pole, the camera being controlled using a remote control device or a mobile phone with live view so that the person controlling the camera could control most of the camera functions and fire the shutter from the ground. More than 1500 photographs were taken in standard jpeg and RAW formats – the jpeg images employ compression techniques to average the values of some pixels to reduce the file size whereas the RAW files record an uncompressed image directly from the sensor in the camera. In order for the software (Agisoft Photoscan Pro) to calculate the camera positions build the 3D model an image overlap of 60% is preferred in all directions. It was difficult to be certain on the northern edge of the degree of photo coverage and overlap and ascend set of photographs was created at a more oblique angle.
The primary photographs were taken at an angle of about 10-20 degrees from the vertical. In fact problems arising from the curved nature of the surface being recorded meant that the more oblique images were mostly unsuitable for precise modelling and were not used to produce the final model. It took most of a day to collect the photographs but several weeks of image editing, masking and processing on two very high powered computers ( an Apple IMAC 5K retina and a purpose built windows games machine with 48Gb of RAM and top of the range Intel I7 processor and gaming graphics card ) to create the finished model. The reason for using what would normally be termed a gaming machine is that the software can use the immense power of the gaming graphics card to undertake a lot of the most intensive 3D graphics calculations.
Once the images were examined and the primary model complied Fraser Sturt from the University of Southampton visited the site with Rose Ferraby with a print of the full model, marked up with key survey points each of which was recorded using a high precision GPS. Using these points the model was finally processed and set in true geographic space to form a permanent scaled record that can now be used to monitor change over time as this magnificent story of past stampings from 140 million years ago is available for the public to visit and walk amongst the prints.
The results of this project can now be viewed by all using an internet browser (currently this is best done using Firefox, although internet browser capabilities change all the time) on a computer and even on a smart phone.
Baby footprints within an adult footprint
In addition to showing the entire pavement a second model (see above) was complied to show what is believed to be a single footprint 90cm across with small c.10cm footprints of a possible infant dinosaur running cross one edge. In another area a fossil grove has been interpreted as the impression made by a tail hitting or dragged through the mud or alternatively a piece of vegetation pressed into the mud and thus leaving the fossil impression.
The full model can be viewed by selecting the arrow in one of the two panels below which include both High and Low resolution models. If the High resolution causes your computer to creak the select the low Res model
High Resolution Model
Low Resolution Model
The footprints are now recorded in their present condition for posterity. It is hoped that the same process can be repeated again to see how they are weathering in a few years time. The footprints will be open for the public to view later on this year starting in May or June. The opening of the footprints and arrangements for pubic access will be announced on the Jurassic Coast World Heritage Site website with detailed information regarding how to get to the site.
Visitors should be aware that although the discoveries were made when the site was under the management of Keates Quarry, following the discovery the site was been handed back to the National Trust who have been restoring the land around it to secure the site for safe public access. Keates Quarry is an operating stone quarry, located some distance from the footprints and access to the footprints cannot be gained through the quarry. Once the Site is opened to the public the 3D models and other resources resulting from this exciting and challenging recording project will all be available on the Jurassic Coast Website.
Wright, J.L. 1997. Keates’Quarry Dinosaur Footprint Sute, Intermarine Member, Purbeck Limestone Group (Berriasian), UK. Proceedings of the Dorset Natural History and Archaeological Sociaty 119: 185 – 211
Downloads and other models
During the process of creating the final record of the footprints a number of models were created all those placed online using Sketchfab can be found by following the link below
You can download the PDF files and a Google KMZ file for display in Google Earth by following the links below
This work was made possible owing to the collaboration and assistance of all below to all of whom thanks are due
The Landscape Research Centre
The National Trust
Sam Scriven and the Jurassic Coast World Heritage Site Team supported by Natural England
The Jurassic Coast Trust
Keates Quarry, Purbeck
Fraser Sturt (University of Southampton)