The Moines and Dalradian:

Whilst the Torridonian sands were being deposited on top of the Lewisian gneisses on the south-western side of  Laurentia, across the adjacent ocean other sediments were being deposited. These sediments later became intensely folded, heated and metamorphosed as the ocean closed at the end of another tectonic cycle (Known as the Caledonian orogeny or mountain building). These other rocks today form the Moine and Dalradian Schists. Both Moine and Dalradian are found on Mull. They are comprised of psammites, schists and gneisses which in Scotland total 25 kilometres thick. The Moine rocks have undergone several tectonic cycles spanning 600MY. Some of this was in Pre-Cambrian times, but some occurred later during the Caledonian mountain building period. The Caledonian orogeny, - named because Scotland is the type area, occurred between 500 and 400 MY ago at the end of Lower Palaeozoic times. The Moine and Dalradian rocks were sediments worn from an old continent. The Moines were sands and clays about 1000 MY ago. The Dalradian which succeeds them, were younger sediments of sands and clay which were deposited into an ocean during Cambrian and Ordovician times.

The Moines:- These are about 10 kilometres thick and occur in the NW Highlands. The Moines rest on the old Lewisian basement. They consist of a monotonous sequence of banded light and dark grey rocks (schists and psammites) which were once reddish sand and mud deposited in large current-swept offshore deltas probably fed by Torridonian age rivers. After severe disruption by folding about 730 MY ago the Moine deltas gave way to shallow marine conditions which initiated the beginning of the Dalradian. Mull is underlain by an unknown area and thickness of Moine rocks which outcrop in the Ross of Mull as Moine biotite, and garnet mica schists, and at Gribun as metamorphosed sandstones or psammites. These psammites have current bedded structures preserved in them which are evidence for shallow water deposition.

(Psammites = Quartzites)

The Dalradian:- This is about 15 kilometres thick and forms most of the Grampian Highlands as well as the mountains of Donegal, Mayo and Connemara. The Dalradian marine environment was shallow and swept by sea currents. There were sandy shoals, calcareous lagoons and local deeps with muddy floors. At one stage, around 670 MY ago, a great ice sheet moved over the sea floor depositing a distinctive boulder bed, an outcrop of which can be seen on the Garvelloch isles. The upper part of the Dalradian is actually Cambrian and Ordovician in age. It consists of gritty and slaty rocks deposited in a highly unstable environment of underwater volcanoes and steep submarine slopes swept by turbidity currents. Mull is underlain by an unknown thickness and area of Dalradian rocks which outcrop as an inlier in a breached anticline at Loch Don.

Rock types of the Moine and Dalradian found on Mull:

Moines:- Garnetiferous mica schist and gneiss, quartzite, pebbly arkose gneiss and quartzo-feldspathic granulite, and hornblende schist.

Dalradian:- Black limestone and grey phyllite. (Limestone on Lismore.)

Mull’s story of the Moines and Dalradian continues:

The rocks of the Moines and the Dalradian accumulated in an area known as the ‘Highland Trough’.  A long way to the S.E. where the basement rocks of England and Wales existed lay another deep sediment collecting area known as the Mona Trough. 500 MY ago the Caledonian orogeny started, when the Iapetus Ocean, in the middle of its tectonic cycle, began to close. As closure proceeded much rock in the Highland trough was folded in huge recumbant folds and pushed on top of younger rocks which had formed on the continental shelves. The orogeny ended about 400 MY ago. (This explains why, in NW Scotland Moine meta-sediments aged 1000 MY can be found on top of  younger Cambrian sediments such as the Durness limestone.) During the Caledonian orogeny, deeply buried sediments such as the Moines and Dalradian were regionally metamorphosed, whilst Caledonian granites were intruded and these contact metamorphosed the surrounding rocks. This is demonstrated in the Ross of Mull, where Moine biotite-garnet schists have formed from clays, some of which were later contact metamorphosed. At Bendoran Cottage, the Ross of Mull granite has thermally baked the already regionally metamorphosed Moine rocks into contact schists with high temperature minerals such as  Sillimanite, Andalusite and Cordierite.