The geological timescale
The enormous length of geological time is very difficult to grasp because we are used to dealing with the very short period of time associated with human experience and history. One way of grasping this problem is to convert millions of years to miles. Thus the total span of human evolution is 6 million years, which is equivalent to the distance between Great Malvern and the outskirts of Worcester. However the age of the Triassic rocks which form the foundations of many of our homes in Malvern are about 230 million years old – equivalent of the distance between Malvern and Newcastle!
This lecture considers two aspects of the age of rocks in Britain. Firstly the relative age of rocks and then their absolute age.
One of the great projects of early 19th century geologists was to work out the stratigraphy of the sedimentary rocks in Britain. The work had already commenced on the continent where the broad subdivisions of Primary, Secondary, Tertiary and Quaternary had been established by the end of the 18th century. Geological mapping in England and Wales by William Smith, whose map was published in 1815, and subsequently George Greenough and his colleagues created an understanding of the sequence of rocks we now class as Late Palaeozoic and Mesozoic. However, the rocks older than Old Red Sandstone, which underlie much of SW England, Wales and the Lake District were poorly understood and were classed as the Transition Series. Two eminent geologists, Roderick Murchison and Adam Sedgwick, embarked on the task of mapping the Transition rocks in Wales with Murchison studying the younger part of the sequence in the Welsh Borderland whilst Sedgwick focused on the rocks in North Wales. Murchison had the easier task as his rocks were only slightly deformed and were fossiliferous. His work led him to declare that his field area was underlain by rocks which had a characteristic fauna and he grouped them together as belonging to a system which he termed the Silurian. Soon after Sedgwick announced that the rocks in his field area belonged to a distinctive system which he called the Cambrian. Unfortunately for Sedgwick his rocks contained few fossils and it was some years before his claim was recognised by the geological community, with his erstwhile friend Murchison leading the opposition. Later in the 19th century Charles Lapworth, working in the Southern Uplands, discovered that graptolites were excellent fossils for subdividing the lower Palaeozoic and he introduced the Ordovician System, effectively ending the bitter argument. Thus by the close of the 19th century the relative ages of the rocks in England and Wales were well established but their precise ages were not known.
The first decade of the 20th century witnessed spectacular breakthroughs in physics, most notably the discovery of radioactivity by Marie and Pierre Curie and the structure of the atom by Ernest Rutherford. Their work soon led to the realization that the radioactive decay of certain elements could be used as a basis for determining the absolute ages of rocks. Rutherford obtained the first result and a little later the remarkable Arthur Holmes, whilst still an undergraduate, produced results from a range of rocks in Britain. Further developments in the precision of results came with the development of the mass spectrometer, which can measure the quantities of individual isotopes.
Whitman Hill Quarry, near Storridge, can be used as a case history to illustrate how the dating method has been applied in a local context. Here the geology is essentially a sequence of gently dipping limestones of Silurian age. At intervals within the sequence there are narrow clay bands which are formed by the weathering of volcanic ash which drifted across the area from time to time during deposition of the limestone and thus formed part of the sedimentary succession. The mineralogy of the weathered volcanic ash is predominantly clay minerals but also includes the mineral zircon, which resists weathering and can easily be separated from the sticky clay. Zircon contains measurable quantities of uranium and lead which have led to a result of 425 million years for the volcanic ash, and thus for the associated limestones too.
I wonder what Murchison would have had to say about that?
Richard Edwards