Geology Reference
In-Depth Information
By 1840 it was recognised that rocks were laid down, one on top
of the other, in an orderly fashion. At any one time in the geo-
logical past particular groups of animals had existed, or a dis-
tinctive species dominated, and geologists used the fossils these
creatures left behind as the means by which they could position
a rock stratigraphically, relative to the younger one above it or
the older one below it. In the Jurassic for example, along with
all the dinosaurs, ammonites had evolved very quickly from one
species to another, so ammonites are used to 'zone' the Jurassic,
making it quite easy to accurately pin-point what part of the
Jurassic a particular fossil is from. Eventually all the fossil-bear-
ing rocks of Britain were arranged into a continuous sequential
column, based on the fossils they contained, which was divid-
ed up into geological 'periods', frequently referred to as geo-
logical 'ages'.
A significant boundary within this geological column was
recognised at the base of the Cambrian, a geological period
named after the Latin word for Wales where these ancient rocks
are typically exposed. The earliest fossils with hard shells date
from the Cambrian period and it was during this time that the
'Cambrian Explosion' occurred as life on the planet became fully
established, bursting into an enormous variety of weird and
wonderful forms. This 'Base Cambrian' boundary was often
interpreted by early geologists as the time at which the Earth
had cooled su~ciently from a molten state for life on Earth to
begin because, below the Cambrian, Precambrian rocks were
thought to be almost completely devoid of fossils and frequently
appeared to have been solidified from molten material. Now,
however, we know that many Precambrian rocks contain soft-
bodied fossils, clearly indicating that life existed long before the
Cambrian, and that the Precambrian represents a period of time
eight times as long as all the other periods put together.
