Geology Reference
In-Depth Information
some indication of the time taken to form that period, but
one also had independent constraints on that time, then it
should be possible to combine time and thickness to build a
geological time scale.
The problems inherent in such a method were well recog-
nised by Holmes and he pointed out that any estimate of
maximum thickness was likely
'to be full of innumerable “gaps”
representing intervals of erosion'
, and thus much of the true
thickness of the rocks could well be missing. He also acknowl-
edged that there may well come a time when even Nier's precise
data were superseded by superior methods. Nevertheless, in
1947 there appeared to be no other way of providing a scale for
the duration of geological time, other than by using the isotope
dates available as 'way points' in the total thickness of sedi-
ments. So, having collected as much accurate data as he could
find on sediment thickness from around the world, and having
spent a great deal of time evaluating the 'most probable age'
represented by each of the five samples selected from Nier's
data, he decided the approach worth trying while
'remembering
that neither the maximum thicknesses so far reported nor the
most probable ages adopted as control-points are likely to be
final'
.
It was an incredibly simple idea. Taking a large piece of
squared paper he first drew to scale the total thickness of rock
known from each geological period, until he had the whole geo-
logical column from the present day down to the Base Cambrian
represented by the thickness of rock it contained. Against this
he then plotted the 'most probable ages' derived from Nier's five
data points in their 'most likely' geological positions, and drew
a curve from top to bottom through the fixed control points.
Suddenly, in a few short minutes, after half a century of hard
work, there, lying on the table in front of him was a geological
time scale. The whole of his life's work had been directed to this
one exhilarating moment. The dream had finally come true. A
'true' age could be obtained for any rock with a known geologi-
