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Snowball Earth Deposits, Islay

Inner Hebrides, Scotland

""Unsung Heroes

First described by James Thomson in 1871, the Port Askaig Formation on Islay comprises the first rocks to be described of glacial origin from the Neoproterozoic era (then lowest Cambrian).

Snowball EarthThomson recognised that the rocks contained clasts of rock not seen anywhere else in Scotland. Consequently he proposed the clasts were remnants of an ancient continent transported to Scotland by ice that had, “yielded to the ceasless gnawing tooth of time”. We now know the source of these clasts can be found in North America.

Glacial rocks similar in age (~717-660 Million Years) to those first found on Islay are now found all over the world and in 1992 Joe Kirschvink proposed the phrase "Snowball Earth" to describe the state of the Earth system at this time.

The hypothesis that Earth entirely froze over for periods in the Neoproterozoic was formally laid out in a seminal work led by Paul Hoffman in 1998. This tumultuous period in Earth history is now a major focus of research efforts.

Text: Ross Peter Anderson
Snowball Earth DepositsNASA Earth and Space Science Fellow, Yale University

‘If . . . we compare the embedded boulders of granite [in the schist] with the granites found in situ throughout the Highlands, we feel the necessity of tracing them to another source, and hope we do not overstep the bounds of prudent speculation in suggesting that those erratics are the reassorted materials of some great Northern Continent that has yielded to the ceaseless gnawing tooth of time, leaving scattered fragments as wreckage of its former greatness, and that the material of which the mass is composed have in time, deeper than we have hitherto suspected, been transported by the agency of ice.’

James Thomson, F.G.S. 1871. On the stratified rocks of Islay. Report of the 41st Meeting of the British Association for the Advancement of Science, Edinburgh, John Murray, London, 110 – 111.

100 Great Geosites

Images (top to bottom)

  • Stromatolites in the "cap carbonate" (Bonahaven Formation) © Ross Peter Anderson
  • 'Snowball Earth' © Neethis, (source Wikimedia CommonsPublic Domain
  • Clasts within Port Askaig Formation © Ross Peter Anderson

Earth Science Week

ESW no date badge 2018
October 13 - 21
Theme: 'Earth Science in our lives'