Mull’s ancient landscape is worn down:

When the Caledonian mountains formed the ‘Old Red Sandstone’ continent, marking the joining of two continental masses at the end of a tectonic cycle, another ocean began opening to the south. This was the Rheic ocean and rocks laid down in Devon (the type area of the Devonian) were shallow marine sediments. By the beginning of the Lower Carboniferous period  most of the area of Britain except the higher land of the north gradually became invaded by a shallow sea. Britain lay just south of the equator and coral reef limestones were deposited in the warm seas.

Inch Kenneth, Mesozoic Sediments

Sediment from the denuding northern mountains was taken down by rivers into this shallow continental shelf area and later, during the Upper Carboniferous, when Britain lay in equatorial latitudes, large delta’s formed which were covered in dense vegetation. Slowly these sank forming coal swamps.  Oscillations in sea level due to tectonic disturbance elsewhere created many coal seams. In Mull any rocks of Carboniferous age are either covered or were removed by denudation, but across the Sound of Mull, at Inninmore Bay on Morvern there are Carboniferous shales and sandstones and a thin coal seam. The Carboniferous ended when the tectonic cycle which created the Rheic Ocean ended, creating a new mountainous land mass, the Variscan mountains, today the Pennines, South Wales, Devon and Cornwall. The British Isles, most of which was now land, had drifted further north and now lay in northern desert latitudes with environments similar to today’s Sahara. Desert sandstones were deposited on land, those closest to Mull can be seen on the Isle of Arran, whilst salty sediments were deposited in the shallow salt seas. Mull during these times was subjected to millions of years of denudation during which the arid landscape made of  Lewisian, Moinian, Dalradian and Devonian rocks were progressively worn down.

The Triassic:

The Triassic, the first period of the Mesozoic, was marked by the transgression of  another shallow sea which spread slowly back over worn down land areas. Mull had continued to move north and at that time had a latitude and an environment similar to the Persian Gulf . Some sediments show features of lagoonal environments close to deserts. Characteristically in Mull the Trias rests with a marked unconformity on the underlying lower basement rocks as is shown by the Moine psammites at Gribun, and represent sediments of an alluvial fan. They are followed by marine sediments. The conglomerates contain rounded fragments of the underlying rocks which rivers passed over, including the Moine schists and psammites. Almost horizontal Triassic conglomerates and sandstones rest on top of uneven and steeply dipping Moines. The seashore cliffs at Gribun are both an example of an angular unconformity and a buried landscape.

The cornstones occur in beds up to 20' thick, or as nodules in the marls and sandstones. They are chemically precipitated calcareous rocks and are very similar to concretionary limestones forming at the present day in the superficial deposits of many tropical countries. A marked desert feature of one particular bed are the polygonal dessication cracks which have been infilled by a later more resistant sediment. These beds represent deposition in shallow lagoonal conditions subjected to periods when the sediment dried out in high temperature conditions. The cornstones at Gribun are developed in fine grained alluvial sediments.

The Rhaetic (formerly considered to be the lowest member of the Jurassic) forms the top of the Trias. In Mull it can be seen in the stream sections at Gribun. It consists of dark sandy
limestones containing fossils of  Bivalve Molluscs.

The Jurassic:

A shallow sea had spread over much of the area of the British Isles including Mull by the beginning of the Jurassic. Tropical rivers brought fine grained sediments, sands and muds from islands and peninsulas, into that sea. Mull had land areas close by which supplied these sediments, but in the south, away from land areas, organic carbonate deposits were formed in the warm shallow seas. Extensive limestone development in Mull was hindered by deposition of sands and muds, though calcareous concretions are found and there are limestones developed within the Inferior Oolite at Port Donain. In the area of the North Sea, volcanic activity at times accompanied phases of faulting and uplift of the crust, and these earth-movements controlled and at times interrupted deposition, which may account for why Mull has only rocks of Lower Jurassic age.

Mull’s Jurassic rocks:

Exposures of  Jurassic rocks can be seen at Carsaig Bay, the Wilderness, Laggan Deer Forest,  Loch Spelve, Loch Don, and Torosay. The Jurassic in Britain consists of a lower marine series, the Lias and Inferior Oolite, followed by the Great Estuarine series and then the Oxford Clay, Corallian and Kimmeridge Clay. Much of the Lower Jurassic is present on Mull.