THE GROUND BENEATH OUR FEET or A NIBBLE AT THE CRUST OF THE EARTH

Professor David Dineley, Trustee, on 4 November 1999

Bath is situated on rocks belonging to the mid-part of the Jurassic System in the stratigraphic column. Thus the local outcrops are part of a famous group of strata that stretches from the Yorkshire coast to that of Dorset and extends under much of eastern and southern England. The German naturalist Alexander von Humboldt first defined this group of highly fossiliferous beds in the Jura mountains of eastern France in 1795, but it was the British civil engineer William Smith who, a few years later, published the first map of the geology of England and Wales and on it showed the disposition of the different Jurassic formations of the Bath area. Since then rocks of the same age, identified by the fossils they contain, have been found on each of the continents around the world.

By studying the rocks themselves, their fossils and relationships to other formations, pictures emerge of the world in Jurassic times, some 195 -255 million years ago. The region that was to become southern Britain was then a shallow tropical sea, teeming with life and dotted with islands which were also rich in flora and fauna. The seas contained all manner of invertebrates, fishes and marine reptiles. Over the land and sea flew pterosaurs; small, very early mammals were present on the islands. The scene was not very dissimilar to the Bahamas today.

The strand line was continually moving, sometimes advancing onto the land, at other times retreating seawards. Jurassic times began with a steady advance of the shoreline which continued in a hesitant way almost to the end of the period when a sharp uplift occurred and only the Hampshire area remained submerged. In the area of the North Sea large sand deltas had been formed and to the south-east lay the marine deep of the Tethys Ocean. The Atlantic Ocean was just beginning to open with a rift spreading between the continents of Laurentia and Eurasia.

In the southern hemisphere the old supercontinent of Gondwana was about to split into the present southern continents. All of these crustal movements and perhaps also the behaviour of the world's oceans were driven by the mechanism of plate tectonics. As the continents shifted so the volume of the ocean basins changed. These changes to the crust of the Earth and its oceans were ultimately powered by the internal processes deep below. These chains of events transformed and transported the old Jurassic sea floor sediments at Bath to become the limestone uplands that are the ground beneath our feet.

David Dineley