Geoscience Reference
In-Depth Information
John Michell, in his paper on earthquakes, also discussed the nature
and extent of various geological units in England. These he tabulated
and named in a manuscript dating from 1788; the list was published
posthumously by the mineral surveyor John Farey in 1810. However,
it contained stratigraphical nomenclature that is largely unrecognised
today.
Clarification and general acceptance of stratigraphical units
were to come later, by the 1840s. Certainly within Britain, as field
work gathered pace and results were disseminated, geologists began to
recognise the clear differences between the red sandstones that occur
in north Devon, which were informally called the Old Red Sandstone,
and the finer-grained silts, slates and greywackes that occur on the
other side of the Bristol Channel in south Wales. They also began to
recognise the relationship between varying lithologies and realised
that, for example, the Coal Measures and coarse grits, called the
Millstone Grit on account of their use as quernstones and millstones,
that occur in Lancashire and Yorkshire directly overlie the grey crys-
talline limestones familarly seen in the Pennines. They recognised
that the green chloritic sands seen near Cambridge, naturally termed
the Greensand, are older than the chalk so splendidly and dramatically
exposed along the Dover coastline.
The vertical spatial relationships on a local level were quite
easily unravelled, but geologists went further and began to determine
the relationships between rock successions much further afield. The
sandstones of Devon were recognised to overlie the silty greywackes
of south central Wales, but were overlain themselves by the crystal-
line limestones found in Yorkshire and elsewhere. Through detailed
observation and description of British geology, a number of geological
Periods were proposed in the early 1800s, all of which represented a
portion of geological time, and all based upon distinct lithological and
palaeontological grounds. As similar work was carried out in Europe
and in Russia, other distinctive lithological units were described and
ascribed to newly named geological Periods - terminology that began
to replace the older more descriptive terms such as Old Red Sandstone,
