Ancient beginnings

The rocks west of the Strathy River have fascinated geologists for a long time. Their age and origin have been subject to debate and even today this still rages on.
The most ancient rocks in the Brittish Isles are known as the Lewisian and outcrop on the Outer Hebrides and along the west coast of the Northern Highlands.
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The Moine Supergroup in the Highlands lie between the Great Glen Fault (Inverness to Fort William) and the Moine Thrust (Durness - Skye. These Neoproterozoic sequences were deposited between c. 1000Ma and c. 870 Ma. These have been changed due to extreme pressures and temperatures and have been squeezed into great folds. The rocks of the Strathy Complex croups out within one of these folds, the Swordly Nappe. The outcrop of the complex roughly correlated to a large magnetic anomaly that extents 9km offshore. The meta-igneous (ie originated from igneous rocks) of the complex are both lithologically and geochemically different from both the other Moinian and basement rocks. The complex is bounded by brittle faults and by a ductile thrust (low angle fault) in the west.
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The rocks are dominated by K-feldspar free, silicious grey gneisses with subordinate hornblende gneiss, rare ultramafic units, garnet/staurolite/sillimanite gneiss and marble. Later cross cutting is dominated by trondhjemitic pegmatites and amphiboloites. The rocks have suffered three phases of deformation and amphibolite facies metamorphism. To the non-geologist this simply means that the rocks are hard and fairly complicated!
Latest research points to the rocks of the Strathy Complex having been deposited in a region where there was active volcanoes (arc-related) next to a warm sea (marble). The rocks are probably sub-Moine and thus can be considered to be Archean, yet they are distinct from all the other outcrops of Archean rocks in Scotland. Some authors have suggested that the Strathy Complex was related to the Grenvillian mountain building event (1100-1000Ma) that took place on the Rondianian Supercontinent.