Geologic dating techniques
By the early 1800's it was generally accepted that the Earth had a long history. The uniformatarians (Hutton 1788, Lyell 1830) pictured the Earth as being indefinitely old.The catastrophists (Cuvier 1812, de Beaumont 1852, Buckland 1836) accepted that the Earth was old; they disagreed with the kind of change and the rate of change that had occurred over that long history.
The great debate was won by the uniformitarians, so much so that the degree of gradualism was overstated and the importance of catastrophes was unduly minimized.In 1637 Descartes produced a cosmogony that was highly influential for more than a century. It was not in their estimates of the age of the Earth - Descartes retained the biblical date.Ussher accepted the Biblical account at face value, relying on the Biblical genealogies and on extant historical records.He implicitly assumed that the world was created much as it is now.Descartes, however, attempted to discern a physical history of the Earth.In short, Genesis was an allegory and not literal history.
The story of this great change in the conception of the history of Earth is not a simple one.
His account was plausible by the immature standards of the Science of his times; however it quite definitely did not match the Biblical account of a completed creation in six days.
In the 1700's belief in a 6000 year old Earth crumbled.
Attempts to calculate the age of the Earth from physical considerations yielded estimates that ranged from 75,000 years (Buffon, 1774) to several billion years (de Maillet, Buffon).
The physical models were open to question and, in retrospect, were naive. It became quite clear that many areas of the Earth had alternated between being land and being covered by seas, that there had been extensive slow sedimentation, that the mountains had not been created in situ as is but rather had a long history of slow deformation, and that long periods of erosion had shaped the Earth everywhere.
If you asked the same question of an educated European in AD 1900 you would have received a quite different answer.