Book - Sustainable Aggregates

PRECAMBRIAN — volcanoes, seas and the dawn of life 5
Pyroclastic flow cascades down the mountain
side of the Soufrière Hills volcano on Montserrat.
example of how the Precambrian volcano
of Charnwood Forest would have erupted.
The volcanic debris formed layers of
sediment in the sea which hardened over
time to form rocks that we know as tuffs
(Walk 4). One of the most spectacular
finds in these rocks has been the discovery
of primitive marine organisms. These
impressions are amongst the oldest
known fossils in the world and many,
such as Charniodiscus concentricus and
Charnia masoni, have been named after the
Charnwood Forest area.
Late in the Precambrian, the volcanic
magma chambers cooled very slowly
at depth, forming intrusions of coarse
grained igneous rocks — the North and
South Charnwood Diorites. The South
Charnwood Diorites were originally
named Markfieldite, after the village of
Markfield where they occur.
Eventually, subduction stopped, the
volcanoes ceased erupting and erosion
began to wear them down. A subdued,
low-lying landscape resulted, enabling the
Old John Tower in Bradgate Park with
well-bedded tuffs in the foreground.
Charnia masoni,
the first fossil to be
found in Charnwood
Forest.
Reproduced by permission of the Bradgate Park Trust