Geology Reference
In-Depth Information
shelf of red sandstone dipping out to sea at a jaunty angle and truncating the underlying
vertical beds of gray sandstone.
Hutton's unconformity at Siccar Point showing the inclined beds of the Silurian Old Red Sandstone truncating vertical
beds of Devonian graywacke sandstone
(
by Alan Witschonke based on a photograph by the author
).
When Hutton discovered this outcrop in 1788, it confirmed his suspicion that mountains
could be recycled into sand and remade into new rock. I had the advantage of having
my colleagues from the University of Edinburgh explain how the gray rock, four-to-eight-
inch-thick beds of sandstone separated by thin layers of mudstone, recorded erosion of the
mountains that formed the geologic suture from the closing of the ancestral Atlantic Ocean.
This collision united England and Scotland 425 million years ago during the Silurian Peri-
od, several hundred million years before the days of the dinosaurs. The upper formation,
the Old Red Sandstone, formed when the younger Caledonian mountains eroded 345 mil-
lion years ago in the Devonian Period, with the resulting sand deposited in what is now
modern Scotland. The other half of the sandstone derived from erosion of the Caledonian
mountains lies across the Atlantic, in New England, as the Catskill Formation in New York
and Maine. The present far-flung distribution of the two halves of the red sandstone records
