When Burton falls

Burton Bradstock by Dr Tim Barrows on Sketchfab

One of the most spectacular stretches of the Jurassic Coast of southern England is the cliff face of Burton Bradstock. The cliffs will be familiar to viewers of Broadchurch which was filmed at nearby West Bay. The lower section is made of the Bridport Sand, which weathers to a characteristic yellow-orange, and above that is the Inferior Oolite. Both are Jurassic in age and marine in origin. Although fossils are sparse in the sand, the oolite (named after egg like nodules) is rich in ammonites, clams, brachiopods, sea urchins, sponges and fish fossils. Local amateur palaeontologists patiently wait for cliff collapses (“Burton to fall”) to bring the fossil rich beds down to within reach. This happens once or twice a year without warning and is very dangerous. In 2012 a woman was killed walking beneath the cliffs when the rock unexpectedly gave way.

Looking for fossils at these cliffs several years ago, I wondered whether there would be an easy way to record the shape of the cliff to document whether there had been a rock fall. Documenting cliff retreat is of considerable importance for understanding causes of coastal erosion, sediment generation, and also for assessing the risks for people walking above or below the cliffs. I took a series of photos for structure from motion (above). Within the hour I had enough data for a sub centimetre model of the cliff face. Repeating this procedure in the future and comparing the original model would show where the cliff had retreated. The main problem is that there is only one direction to photograph from the beach, so neither the beach or the top of the cliff is included in the model. The reverse problem is experienced by aerial LiDAR (light detection and ranging) from a light aircraft, which is the standard monitoring tehnique along the coast, which can only obtain the 2D view downwards. However, combining the 2 approaches would provide a full 3D view of the cliff. Moreover, structure from motion is much cheaper than land-based terrestrial laser scanning and is the way of the future for surveying large stretches of the coast quickly at low cost.