Geology Reference
In-Depth Information
absolute age in years, in the same way that you and I know how
old we are in terms of years. This distinction between the geo-
logical age and the dated or 'absolute' age of a rock is an impor-
tant one to bear in mind.
Lyell tried not to be too specific about the age of any one geo-
logical event, but Darwin rushed head-long into attempting a
calculation. In the first edition of
The Origin of Species
Darwin took
as an example of the enormity of time required for geological
processes, the erosion of the Weald, a large valley that today
stretches between the North and South Downs in the south of
England, but which at one time had had a great dome of rocks
above it: 'It is an admirable lesson to stand on the North Downs
and to look at the distant South Downs' Darwin wrote, 'one can
safely picture to oneself the great dome of rocks which must
have covered up the Weald within so limited a period as since
the latter part of the Chalk formation'. Using rather sketchy
figures for the amount of rock that had been present in the dome
and making the assumption that the sea 'would eat into cli¬s
500 feet in height at the rate of one inch in a century', Darwin
boldly calculated that 'At this rate the denudation of the Weald
must have required 306,662,400 years; or say three hundred
million years.'
The implications of this calculation for the total age of the
Earth were stupendous. If it had taken three hundred million
years to erode the Weald dome down to the level we see it today,
how much longer had it taken to deposit the Chalk in the first
place before it could be eroded? And as the Chalk formation of
the Weald was deposited near the top of the geological column,
what did that say about the age for the miles of rock below it?
There was indeed 'no prospect of an end'. It caused a furore,
with one of Darwin's critics, James Croll, suggesting that he had
been led astray by the geologists who provided him with vast
