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Ussher’s 1654 calculation remained the mainstream view until the 18th century saw the birth of a new science, geology – the study of how the earth was shaped. It became obvious to the practitioners of this nascent science that the processes and phenomena they observed must be acting on timescales much larger than the few millennia allowed by biblical literalism. The deposition of rocks, the uplift and folding of strata and mountains, the erosion of valleys and cliffs; all these spoke of slow processes operating over long periods. The emerging evidence of fossils, with their record of strange forms now vanished from the Earth, also suggested a long passage of time. Indeed, to geologists such as Charles Lyell, author of the seminal Principles of Geology, it seemed likely that natural processes of rock formation and erosion had been occurring for an effectively incalculable length of time; if not for a limitless period, certainly of the order of billions of years.

Meanwhile, another group of scientists was approaching the problem of the age of the Earth from a different angle. Naturalists were becoming increasingly convinced that species of plants and animals had changed over time through some form of evolution, but that this transformative process operated extremely slowly, and therefore constituted its own brand of evidence for the great age of the planet. The expanses of geological time opened up by Lyell were a key plank of Darwin’s argument in his 1859 publication On the Origin of Species, in which he cautioned: ‘He who can read Sir Charles Lyell’s grand work on the Principles of Geology and yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume.’ To illustrate just how vast these periods had been, Darwin included a rough estimate he had made of the length of time it must have taken for the ocean to erode the Weald (a geological feature in the south-east of England), putting it at around 300 million years.