The Lewisian and Torridonian:
 

The making of the Lewisian Gneiss.

Mull and Iona’s oldest rocks, the Lewisian Gneisses or Lewisian Complex are also Britain’s oldest rocks. The oldest of these metamorphic rocks, which were once sediments and igneous rocks, are 2,700 MY.  Lewisian gneisses form the basement rocks of NW Scotland. These rocks underlie Mull and outcrop on Iona. These metamorphic rocks must have existed before as sedimentary and igneous rocks which were subsequently changed during not just one, but many rock, and tectonic cycles. Some of  the Lewisian Gneiss was once volcanic ash, and sand, deposited around a volcanic island chain possibly 3000 MY ago. Other gneisses were formed from oceanic crust. The Lewisian was subjected to as many as six tectonic cycles before being buried and preserved to this day. Gneiss is an extremely high grade of metamorphic rock and some Lewisian gneiss is ‘granite gneiss’ - almost an igneous rock.  Although Lewisian gneiss on Iona has been dated at around 2000 MY this is not the age of the original rocks which were much older.

Next sandstones were deposited on the Lewisian Gneiss.
 

The Torridonian  

It is thought that by 1000 MY ago the Lewisian rocks of the Hebridean craton had been uplifted to about their present level. NW Scotland and the Hebrides at that time lay in the southern hemisphere on the south eastern margin of an ancient continent, called Laurentia, of which Greenland formed a large part. When rocks are uplifted and form continental masses they are subjected to weathering and erosion processes, (denudation), and this happened 1000 MY ago during the time when Torridonian sandstone was deposited. Then the rugged Lewisian landscape was gradually buried under pebbles and red sands. 200 MY later (800 MY ago), the main mass of the Torridonian sandstone, 7 kilometres thick, was deposited in a vast barren plain threaded by rivers rising in Greenland’s area of the continent, which was periodically flooded by a very shallow sea.
Scotland and Mull’s area at that time lay south of the Tropic of Capricorn. Today the Torridonian rocks have mostly been eroded away. There is a 500m thick sequence of breccias and sandstones unconformably overlying the Lewisian basement on Iona. It is thought to be part of the imposing relict hills which are found in Torridon, NW Scotland, where they sit on top of the old Pre-Torridonian erosion surface of Lewisian gneiss, but definite proof is lacking.

Simplified calendar of events for those times.

1,000 MY ago    -        Torridonian - Uplift, prolonged erosion forming sediments, stabilisation of the crust.

1,800 MY ago    -        Deep burial, metamorphism, granites.

2,200 MY ago    -        Igneous dykes injected.

2,300 MY ago    -        Uplift, squeezing, reheating, recrystallisation.

2,700 MY ago    -        Deep burial and metamorphism.

3,000 MY ago    -        Early sediments and igneous rocks.

The Lewisian complex of Iona consists of former sediments and igneous rocks which were metamorphosed into garnet-biotite granulite, metasomatised marble rich in serpentine with associated compact green rocks, orthogneisses:- anorthosite feldspar rock (‘white rock’),
hornblende schist and eclogite, and granite gneiss with bands of hornblende schist. This wide variety of metamorphic rocks reflects the varied nature of the original sediment.

Rock types which can be found on the Isle of Iona:- Various Lewisian gneisses and ‘Iona’ marble, Torridonian sandstone, flags, dark shales and conglomerate. Exposures are best seen around the coast. The famous serpentinised green and white Iona marble can be found washed up as pebbles on south-western beaches, whilst what remains of the narrow vein of a serpentinised forsterite-tremolite marble can be found in the SE.