Geoscience Reference
In-Depth Information
veteran of the Peninsular Campaigns, before becoming a late convert
to geology, thanks to his wife's influence, following his discharge from
the army. Like many army engineers, he was well positioned to take
up the geological challenge Wales offered. He was organised and meti-
culous, and embarked on his work with the zeal of a general attempt-
ing to get to Moscow before winter.
It was clear that the deformed slaty rocks of northern Wales,
which are so beautifully exposed in the Snowdonia region, were
older than the highly fossiliferous siltstones and overlying lime-
stones found in the Welsh Borderlands. Sedgwick named his tract
'Cambrian', on the suggestion of Murchison, who gave his lower
succession the name 'Silurian', after the tribe the Silures. However,
as they began to publish the fruits of their field work in papers and in
topics both Sedgwick and Murchison began to expand the frontiers of
their territories so that they began to overlap. The precise attribution
of the uppermost Cambrian and lowermost Silurian became a battle-
field played out for many years by the two protagonists. On my study
wall in front of me, barely two feet from my desk, is a hand-coloured
geological map of England and Wales, published by the Society for the
Diffusion of Useful Knowledge in 1843. It is both a beautiful work of
art and a testament to the ferocious wrangling that occupied the two
men for many years. The map was arranged by Murchison, and unsur-
prisingly when I read the key that explained the colouration, I found
that his Silurian was divided into two: the Upper Silurian contained
the 'Ludlow rocks and the Wenlock Limestone' found near Dudley in
the west Midlands, while the Lower Silurian comprised the 'Caradoc
sandstone, Llandeilo Flags & Cambrian Slates'. Sedgwick's Cambrian
had been completely annexed by Murchison within eight years of its
initial definition! It wasn't until 1879 after the death of the pugilists
that this matter was finally settled by Charles Lapworth (1842-1920),
the Birmingham professor, who inserted the Ordovician Period
between the Cambrian and the Silurian, largely defined on his studies
of the planktonic colonial organisms called graptolites that in their
fossil form resemble pencil marks drawn on slate.
