Geoscience Reference
In-Depth Information
about 458; but these primary and secondary strata were inclined in
almost opposite directions ... From this situation of those two
different masses of strata, it is evidently impossible that either of
them could have been formed originally in that position.
The Arran unconformity, or 'Hutton's unconformity' as it is now
popularly known, displays calcareous sandstones that overlie altered
and ancient schist (a highly metamorphosed rock type). Hutton did
not have to wait long before he found similar examples elsewhere. In
the autumn he was walking along the banks of the River Jedd close by
Jedburgh in the Borders (40miles southeast of Edinburgh) and he found
another unconformity, this time more obvious. Here nearly vertical
beds of a Greywacke (a gritty sedimentary rock) of Silurian age are
overlain by horizontal beds of Old Red Sandstone of Upper Devonian
age. The actual time between the two is now thought to be least
40 million years. It was this example, rather than that on Arran, that
Hutton illustrated in his expanded theory in 1795. In 1788 he was to
be found near Cockburnspath near St Abb's Head on the east coast of
Scotland, and the weather that day was so poor that he and Sir James
Hall were unable to take out the boat they had planned to hire. Instead
of viewing a panorama of the coastline they had to content themselves
with picking their way along it on foot, and there at Siccar Point they
discovered another unconformity (Figure 6.5 ). As with the Jedburg
example Silurian Greywacke is overlain by Old Red Sandstone. Of
all three of Hutton's unconformities, the Siccar Point example is the
best known, and has, as I said earlier, made its way across the Atlantic
as a fibreglass cast. Many of the sites visited were sketched by
Hutton's companions, either Hall, or John Clerk of Eldin who exe-
cuted the majority of the drawings. For a long time these drawings
were 'lost' or rather their whereabouts were unknown to the geologi-
cal community. In 1968 they were relocated at Penicuik, home of Lord
Eldin, and in 1978 a portfolio containing many of themwas published
by the Scottish Academic Press together with an accompanying book
authored by Gordon Craig, Donald McIntyre and Charles Waterston.
Search WWH ::




Custom Search