Torridonian

The sequence was first mapped as a unit by John MacCulloch and was initially assumed to be part of the Old Red Sandstone. The first name used specifically for this sequence was "Torridon Sandstone" introduced in 1866 by James Nicol. By 1892 the term was shortened to "Torridonian" by the Geological Survey. In 1893 the survey had subdivided the Torridonian into four units, the Diabaig, Applecross, Aultbea and Cailleach Head groups (which are now the names of formations within the Torridon Group). At about the same time Lower Cambrian fossils were found in the sequence above the unconformity, suggesting that the Torridonian was of Precambrian age. Variations in thickness and lithology were interpreted to mean that both the Stoer and Sleat/Torridon Groups were deposited in a rift setting. Evidence from seismic reflection data in the Minch suggested that the Minch Fault was active throughout the deposition of the Torridon Group. This is consistent with the generally westerly derived pebbly material throughout the thickness of the Applecross Formation, suggesting a constantly rejuvenated sediment source in that direction. ==Revised stratigraphy==

Going back to the late 19th century, geologists had speculated that at least the lower part of the Moine sequence might be a lateral equivalent of the Torridonian, based on similarities in lithology, sedimentary structures and thickness. The application of detrital zircon geochronology to the Torridonian and Moine sequences has allowed this proposed correlation to be tested. The Morar Group] of the Moine has a very similar spectrum of detrital zircon ages to the Torridon Group. Both have a population of youngest zircons that give ages that match the Grenville Orogeny. In contrast, the Glenfinnan Group has a youngest zircon population that matches the Renlandian Orogeny, an event that is now known to have affected the Morar Group. The detrital zircon and other geochronological data show that there is major break in the Moine stratigraphy at the base of the Glenfinnan Group and that it can, therefore, no longer be regarded as a supergroup. In the revised stratigraphic framework for the Proterozoic sedimentary rocks of the Highlands: the Stoer Group is part of a separate sequence whose deposition predated the Grenville Orogeny and has no equivalents in Scotland; the Torridon Group (together with the Sleat Group, the Iona and Tarskavaig groups and three groups on Shetland) are correlated with the Morar Group forming the Wester Ross Supergroup; after the Renlandian Orogeny, the Glenfinnan, Loch Eil and Badenoch groups were deposited and together form the younger Loch Ness Supergroup. ==References==