The rocks of the Dunoon area are admirably exposed along the shores of the East Bay, at Bullwood, Innellan and Toward.
Further north they outcrop in shores at Kirn, Hunter's Quay, Loch Eck side and in the neighbouring valleys of Glen Lean and Glenmassan.
North of Innellan the rocks are metamorphic clates, schists and phyllites - grey-green rocks often intensely folded and crumpled, glistening with the minerals mica and quartz.
All these rocks, which extend from Ireland to Aberdeenshire, are called 'Dalradian', and formed about 600 million years ago as muds in an ancient ocean which then separated England from Scotland.
This ocean closed 500 million years ago, the muddy sediments were squeezed, folded and heated in the building of the Caledonian Mountains, converting the muddy shales into the phyllites and schists of today.
Thus all these local rocks are the roots of that ancient mountain chain and display the characteristics of compression ten miles down in the earth's crust.
At Innellan Pier (which has recently been demolished) the rocks change suddenly, from grey schists to brown-red sandstones.
Here is the Highland Boundary Fault, a large crack in the earth's crust which 450 million years ago formed the northern edge of the sinking Midland Valley of Scotland.
Erosion wearing away the Caledonian Mountains deposited iron-coated sands and pebbles in this rift valley, where they now form the Old Red Sandstone.
The sandstone is perhaps the best known rock in the Firth of Clyde, as it forms the beaches and shores on either side of the estuary at Toward, in Bute and Arran and across the Firth from Inverkip to Largs.
The sandstone is well seen at Innellan and all the way to Toward Point and Castle, displaying beds of brown sandstone and pudding-stone.
At Toward Castle the Highland Boundary Fault runs out into Rothesay Bay, bringing in the schists which form the dramatic scenery of Loch Striven.
